2131 lines
91 KiB
HTML
2131 lines
91 KiB
HTML
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<!DOCTYPE html>
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<html lang="en">
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<head>
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<meta charset="UTF-8">
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<meta name="viewport" content="width=device-width, initial-scale=1.0">
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<meta name="generator" content="Asciidoctor 1.5.8">
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<meta name="author" content="Christian Couder">
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<title>Fighting regressions with git bisect</title>
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<link rel="stylesheet" href="https://fonts.googleapis.com/css?family=Open+Sans:300,300italic,400,400italic,600,600italic%7CNoto+Serif:400,400italic,700,700italic%7CDroid+Sans+Mono:400,700">
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.admonitionblock td.content>.title,.audioblock>.title,.exampleblock>.title,.imageblock>.title,.listingblock>.title,.literalblock>.title,.stemblock>.title,.openblock>.title,.paragraph>.title,.quoteblock>.title,table.tableblock>.title,.verseblock>.title,.videoblock>.title,.dlist>.title,.olist>.title,.ulist>.title,.qlist>.title,.hdlist>.title{text-rendering:optimizeLegibility;text-align:left;font-family:"Noto Serif","DejaVu Serif",serif;font-size:1rem;font-style:italic}
|
||
|
|
table.tableblock.fit-content>caption.title{white-space:nowrap;width:0}
|
||
|
|
.paragraph.lead>p,#preamble>.sectionbody>[class="paragraph"]:first-of-type p{font-size:1.21875em;line-height:1.6;color:rgba(0,0,0,.85)}
|
||
|
|
table.tableblock #preamble>.sectionbody>[class="paragraph"]:first-of-type p{font-size:inherit}
|
||
|
|
.admonitionblock>table{border-collapse:separate;border:0;background:none;width:100%}
|
||
|
|
.admonitionblock>table td.icon{text-align:center;width:80px}
|
||
|
|
.admonitionblock>table td.icon img{max-width:none}
|
||
|
|
.admonitionblock>table td.icon .title{font-weight:bold;font-family:"Open Sans","DejaVu Sans",sans-serif;text-transform:uppercase}
|
||
|
|
.admonitionblock>table td.content{padding-left:1.125em;padding-right:1.25em;border-left:1px solid #dddddf;color:rgba(0,0,0,.6)}
|
||
|
|
.admonitionblock>table td.content>:last-child>:last-child{margin-bottom:0}
|
||
|
|
.exampleblock>.content{border-style:solid;border-width:1px;border-color:#e6e6e6;margin-bottom:1.25em;padding:1.25em;background:#fff;-webkit-border-radius:4px;border-radius:4px}
|
||
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.exampleblock>.content>:first-child{margin-top:0}
|
||
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|
.exampleblock>.content>:last-child{margin-bottom:0}
|
||
|
|
.sidebarblock{border-style:solid;border-width:1px;border-color:#e0e0dc;margin-bottom:1.25em;padding:1.25em;background:#f8f8f7;-webkit-border-radius:4px;border-radius:4px}
|
||
|
|
.sidebarblock>:first-child{margin-top:0}
|
||
|
|
.sidebarblock>:last-child{margin-bottom:0}
|
||
|
|
.sidebarblock>.content>.title{color:#7a2518;margin-top:0;text-align:center}
|
||
|
|
.exampleblock>.content>:last-child>:last-child,.exampleblock>.content .olist>ol>li:last-child>:last-child,.exampleblock>.content .ulist>ul>li:last-child>:last-child,.exampleblock>.content .qlist>ol>li:last-child>:last-child,.sidebarblock>.content>:last-child>:last-child,.sidebarblock>.content .olist>ol>li:last-child>:last-child,.sidebarblock>.content .ulist>ul>li:last-child>:last-child,.sidebarblock>.content .qlist>ol>li:last-child>:last-child{margin-bottom:0}
|
||
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.literalblock pre,.listingblock pre:not(.highlight),.listingblock pre[class="highlight"],.listingblock pre[class^="highlight "],.listingblock pre.CodeRay,.listingblock pre.prettyprint{background:#f7f7f8}
|
||
|
|
.sidebarblock .literalblock pre,.sidebarblock .listingblock pre:not(.highlight),.sidebarblock .listingblock pre[class="highlight"],.sidebarblock .listingblock pre[class^="highlight "],.sidebarblock .listingblock pre.CodeRay,.sidebarblock .listingblock pre.prettyprint{background:#f2f1f1}
|
||
|
|
.literalblock pre,.literalblock pre[class],.listingblock pre,.listingblock pre[class]{-webkit-border-radius:4px;border-radius:4px;word-wrap:break-word;overflow-x:auto;padding:1em;font-size:.8125em}
|
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@media screen and (min-width:768px){.literalblock pre,.literalblock pre[class],.listingblock pre,.listingblock pre[class]{font-size:.90625em}}
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@media screen and (min-width:1280px){.literalblock pre,.literalblock pre[class],.listingblock pre,.listingblock pre[class]{font-size:1em}}
|
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.literalblock pre.nowrap,.literalblock pre.nowrap pre,.listingblock pre.nowrap,.listingblock pre.nowrap pre{white-space:pre;word-wrap:normal}
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.literalblock.output pre{color:#f7f7f8;background-color:rgba(0,0,0,.9)}
|
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.listingblock pre.highlightjs{padding:0}
|
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.listingblock pre.highlightjs>code{padding:1em;-webkit-border-radius:4px;border-radius:4px}
|
||
|
|
.listingblock pre.prettyprint{border-width:0}
|
||
|
|
.listingblock>.content{position:relative}
|
||
|
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.listingblock code[data-lang]::before{display:none;content:attr(data-lang);position:absolute;font-size:.75em;top:.425rem;right:.5rem;line-height:1;text-transform:uppercase;color:#999}
|
||
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.listingblock:hover code[data-lang]::before{display:block}
|
||
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.listingblock.terminal pre .command::before{content:attr(data-prompt);padding-right:.5em;color:#999}
|
||
|
|
.listingblock.terminal pre .command:not([data-prompt])::before{content:"$"}
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||
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table.pyhltable{border-collapse:separate;border:0;margin-bottom:0;background:none}
|
||
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table.pyhltable td{vertical-align:top;padding-top:0;padding-bottom:0;line-height:1.45}
|
||
|
|
table.pyhltable td.code{padding-left:.75em;padding-right:0}
|
||
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pre.pygments .lineno,table.pyhltable td:not(.code){color:#999;padding-left:0;padding-right:.5em;border-right:1px solid #dddddf}
|
||
|
|
pre.pygments .lineno{display:inline-block;margin-right:.25em}
|
||
|
|
table.pyhltable .linenodiv{background:none!important;padding-right:0!important}
|
||
|
|
.quoteblock{margin:0 1em 1.25em 1.5em;display:table}
|
||
|
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.quoteblock>.title{margin-left:-1.5em;margin-bottom:.75em}
|
||
|
|
.quoteblock blockquote,.quoteblock p{color:rgba(0,0,0,.85);font-size:1.15rem;line-height:1.75;word-spacing:.1em;letter-spacing:0;font-style:italic;text-align:justify}
|
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.quoteblock blockquote{margin:0;padding:0;border:0}
|
||
|
|
.quoteblock blockquote::before{content:"\201c";float:left;font-size:2.75em;font-weight:bold;line-height:.6em;margin-left:-.6em;color:#7a2518;text-shadow:0 1px 2px rgba(0,0,0,.1)}
|
||
|
|
.quoteblock blockquote>.paragraph:last-child p{margin-bottom:0}
|
||
|
|
.quoteblock .attribution{margin-top:.75em;margin-right:.5ex;text-align:right}
|
||
|
|
.verseblock{margin:0 1em 1.25em}
|
||
|
|
.verseblock pre{font-family:"Open Sans","DejaVu Sans",sans;font-size:1.15rem;color:rgba(0,0,0,.85);font-weight:300;text-rendering:optimizeLegibility}
|
||
|
|
.verseblock pre strong{font-weight:400}
|
||
|
|
.verseblock .attribution{margin-top:1.25rem;margin-left:.5ex}
|
||
|
|
.quoteblock .attribution,.verseblock .attribution{font-size:.9375em;line-height:1.45;font-style:italic}
|
||
|
|
.quoteblock .attribution br,.verseblock .attribution br{display:none}
|
||
|
|
.quoteblock .attribution cite,.verseblock .attribution cite{display:block;letter-spacing:-.025em;color:rgba(0,0,0,.6)}
|
||
|
|
.quoteblock.abstract blockquote::before,.quoteblock.excerpt blockquote::before,.quoteblock .quoteblock blockquote::before{display:none}
|
||
|
|
.quoteblock.abstract blockquote,.quoteblock.abstract p,.quoteblock.excerpt blockquote,.quoteblock.excerpt p,.quoteblock .quoteblock blockquote,.quoteblock .quoteblock p{line-height:1.6;word-spacing:0}
|
||
|
|
.quoteblock.abstract{margin:0 1em 1.25em;display:block}
|
||
|
|
.quoteblock.abstract>.title{margin:0 0 .375em;font-size:1.15em;text-align:center}
|
||
|
|
.quoteblock.excerpt,.quoteblock .quoteblock{margin:0 0 1.25em;padding:0 0 .25em 1em;border-left:.25em solid #dddddf}
|
||
|
|
.quoteblock.excerpt blockquote,.quoteblock.excerpt p,.quoteblock .quoteblock blockquote,.quoteblock .quoteblock p{color:inherit;font-size:1.0625rem}
|
||
|
|
.quoteblock.excerpt .attribution,.quoteblock .quoteblock .attribution{color:inherit;text-align:left;margin-right:0}
|
||
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|
table.tableblock{max-width:100%;border-collapse:separate}
|
||
|
|
p.tableblock:last-child{margin-bottom:0}
|
||
|
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td.tableblock>.content{margin-bottom:-1.25em}
|
||
|
|
table.tableblock,th.tableblock,td.tableblock{border:0 solid #dedede}
|
||
|
|
table.grid-all>thead>tr>.tableblock,table.grid-all>tbody>tr>.tableblock{border-width:0 1px 1px 0}
|
||
|
|
table.grid-all>tfoot>tr>.tableblock{border-width:1px 1px 0 0}
|
||
|
|
table.grid-cols>*>tr>.tableblock{border-width:0 1px 0 0}
|
||
|
|
table.grid-rows>thead>tr>.tableblock,table.grid-rows>tbody>tr>.tableblock{border-width:0 0 1px}
|
||
|
|
table.grid-rows>tfoot>tr>.tableblock{border-width:1px 0 0}
|
||
|
|
table.grid-all>*>tr>.tableblock:last-child,table.grid-cols>*>tr>.tableblock:last-child{border-right-width:0}
|
||
|
|
table.grid-all>tbody>tr:last-child>.tableblock,table.grid-all>thead:last-child>tr>.tableblock,table.grid-rows>tbody>tr:last-child>.tableblock,table.grid-rows>thead:last-child>tr>.tableblock{border-bottom-width:0}
|
||
|
|
table.frame-all{border-width:1px}
|
||
|
|
table.frame-sides{border-width:0 1px}
|
||
|
|
table.frame-topbot,table.frame-ends{border-width:1px 0}
|
||
|
|
table.stripes-all tr,table.stripes-odd tr:nth-of-type(odd){background:#f8f8f7}
|
||
|
|
table.stripes-none tr,table.stripes-odd tr:nth-of-type(even){background:none}
|
||
|
|
th.halign-left,td.halign-left{text-align:left}
|
||
|
|
th.halign-right,td.halign-right{text-align:right}
|
||
|
|
th.halign-center,td.halign-center{text-align:center}
|
||
|
|
th.valign-top,td.valign-top{vertical-align:top}
|
||
|
|
th.valign-bottom,td.valign-bottom{vertical-align:bottom}
|
||
|
|
th.valign-middle,td.valign-middle{vertical-align:middle}
|
||
|
|
table thead th,table tfoot th{font-weight:bold}
|
||
|
|
tbody tr th{display:table-cell;line-height:1.6;background:#f7f8f7}
|
||
|
|
tbody tr th,tbody tr th p,tfoot tr th,tfoot tr th p{color:rgba(0,0,0,.8);font-weight:bold}
|
||
|
|
p.tableblock>code:only-child{background:none;padding:0}
|
||
|
|
p.tableblock{font-size:1em}
|
||
|
|
td>div.verse{white-space:pre}
|
||
|
|
ol{margin-left:1.75em}
|
||
|
|
ul li ol{margin-left:1.5em}
|
||
|
|
dl dd{margin-left:1.125em}
|
||
|
|
dl dd:last-child,dl dd:last-child>:last-child{margin-bottom:0}
|
||
|
|
ol>li p,ul>li p,ul dd,ol dd,.olist .olist,.ulist .ulist,.ulist .olist,.olist .ulist{margin-bottom:.625em}
|
||
|
|
ul.checklist,ul.none,ol.none,ul.no-bullet,ol.no-bullet,ol.unnumbered,ul.unstyled,ol.unstyled{list-style-type:none}
|
||
|
|
ul.no-bullet,ol.no-bullet,ol.unnumbered{margin-left:.625em}
|
||
|
|
ul.unstyled,ol.unstyled{margin-left:0}
|
||
|
|
ul.checklist{margin-left:.625em}
|
||
|
|
ul.checklist li>p:first-child>.fa-square-o:first-child,ul.checklist li>p:first-child>.fa-check-square-o:first-child{width:1.25em;font-size:.8em;position:relative;bottom:.125em}
|
||
|
|
ul.checklist li>p:first-child>input[type="checkbox"]:first-child{margin-right:.25em}
|
||
|
|
ul.inline{display:-ms-flexbox;display:-webkit-box;display:flex;-ms-flex-flow:row wrap;-webkit-flex-flow:row wrap;flex-flow:row wrap;list-style:none;margin:0 0 .625em -1.25em}
|
||
|
|
ul.inline>li{margin-left:1.25em}
|
||
|
|
.unstyled dl dt{font-weight:400;font-style:normal}
|
||
|
|
ol.arabic{list-style-type:decimal}
|
||
|
|
ol.decimal{list-style-type:decimal-leading-zero}
|
||
|
|
ol.loweralpha{list-style-type:lower-alpha}
|
||
|
|
ol.upperalpha{list-style-type:upper-alpha}
|
||
|
|
ol.lowerroman{list-style-type:lower-roman}
|
||
|
|
ol.upperroman{list-style-type:upper-roman}
|
||
|
|
ol.lowergreek{list-style-type:lower-greek}
|
||
|
|
.hdlist>table,.colist>table{border:0;background:none}
|
||
|
|
.hdlist>table>tbody>tr,.colist>table>tbody>tr{background:none}
|
||
|
|
td.hdlist1,td.hdlist2{vertical-align:top;padding:0 .625em}
|
||
|
|
td.hdlist1{font-weight:bold;padding-bottom:1.25em}
|
||
|
|
.literalblock+.colist,.listingblock+.colist{margin-top:-.5em}
|
||
|
|
.colist td:not([class]):first-child{padding:.4em .75em 0;line-height:1;vertical-align:top}
|
||
|
|
.colist td:not([class]):first-child img{max-width:none}
|
||
|
|
.colist td:not([class]):last-child{padding:.25em 0}
|
||
|
|
.thumb,.th{line-height:0;display:inline-block;border:solid 4px #fff;-webkit-box-shadow:0 0 0 1px #ddd;box-shadow:0 0 0 1px #ddd}
|
||
|
|
.imageblock.left{margin:.25em .625em 1.25em 0}
|
||
|
|
.imageblock.right{margin:.25em 0 1.25em .625em}
|
||
|
|
.imageblock>.title{margin-bottom:0}
|
||
|
|
.imageblock.thumb,.imageblock.th{border-width:6px}
|
||
|
|
.imageblock.thumb>.title,.imageblock.th>.title{padding:0 .125em}
|
||
|
|
.image.left,.image.right{margin-top:.25em;margin-bottom:.25em;display:inline-block;line-height:0}
|
||
|
|
.image.left{margin-right:.625em}
|
||
|
|
.image.right{margin-left:.625em}
|
||
|
|
a.image{text-decoration:none;display:inline-block}
|
||
|
|
a.image object{pointer-events:none}
|
||
|
|
sup.footnote,sup.footnoteref{font-size:.875em;position:static;vertical-align:super}
|
||
|
|
sup.footnote a,sup.footnoteref a{text-decoration:none}
|
||
|
|
sup.footnote a:active,sup.footnoteref a:active{text-decoration:underline}
|
||
|
|
#footnotes{padding-top:.75em;padding-bottom:.75em;margin-bottom:.625em}
|
||
|
|
#footnotes hr{width:20%;min-width:6.25em;margin:-.25em 0 .75em;border-width:1px 0 0}
|
||
|
|
#footnotes .footnote{padding:0 .375em 0 .225em;line-height:1.3334;font-size:.875em;margin-left:1.2em;margin-bottom:.2em}
|
||
|
|
#footnotes .footnote a:first-of-type{font-weight:bold;text-decoration:none;margin-left:-1.05em}
|
||
|
|
#footnotes .footnote:last-of-type{margin-bottom:0}
|
||
|
|
#content #footnotes{margin-top:-.625em;margin-bottom:0;padding:.75em 0}
|
||
|
|
.gist .file-data>table{border:0;background:#fff;width:100%;margin-bottom:0}
|
||
|
|
.gist .file-data>table td.line-data{width:99%}
|
||
|
|
div.unbreakable{page-break-inside:avoid}
|
||
|
|
.big{font-size:larger}
|
||
|
|
.small{font-size:smaller}
|
||
|
|
.underline{text-decoration:underline}
|
||
|
|
.overline{text-decoration:overline}
|
||
|
|
.line-through{text-decoration:line-through}
|
||
|
|
.aqua{color:#00bfbf}
|
||
|
|
.aqua-background{background-color:#00fafa}
|
||
|
|
.black{color:#000}
|
||
|
|
.black-background{background-color:#000}
|
||
|
|
.blue{color:#0000bf}
|
||
|
|
.blue-background{background-color:#0000fa}
|
||
|
|
.fuchsia{color:#bf00bf}
|
||
|
|
.fuchsia-background{background-color:#fa00fa}
|
||
|
|
.gray{color:#606060}
|
||
|
|
.gray-background{background-color:#7d7d7d}
|
||
|
|
.green{color:#006000}
|
||
|
|
.green-background{background-color:#007d00}
|
||
|
|
.lime{color:#00bf00}
|
||
|
|
.lime-background{background-color:#00fa00}
|
||
|
|
.maroon{color:#600000}
|
||
|
|
.maroon-background{background-color:#7d0000}
|
||
|
|
.navy{color:#000060}
|
||
|
|
.navy-background{background-color:#00007d}
|
||
|
|
.olive{color:#606000}
|
||
|
|
.olive-background{background-color:#7d7d00}
|
||
|
|
.purple{color:#600060}
|
||
|
|
.purple-background{background-color:#7d007d}
|
||
|
|
.red{color:#bf0000}
|
||
|
|
.red-background{background-color:#fa0000}
|
||
|
|
.silver{color:#909090}
|
||
|
|
.silver-background{background-color:#bcbcbc}
|
||
|
|
.teal{color:#006060}
|
||
|
|
.teal-background{background-color:#007d7d}
|
||
|
|
.white{color:#bfbfbf}
|
||
|
|
.white-background{background-color:#fafafa}
|
||
|
|
.yellow{color:#bfbf00}
|
||
|
|
.yellow-background{background-color:#fafa00}
|
||
|
|
span.icon>.fa{cursor:default}
|
||
|
|
a span.icon>.fa{cursor:inherit}
|
||
|
|
.admonitionblock td.icon [class^="fa icon-"]{font-size:2.5em;text-shadow:1px 1px 2px rgba(0,0,0,.5);cursor:default}
|
||
|
|
.admonitionblock td.icon .icon-note::before{content:"\f05a";color:#19407c}
|
||
|
|
.admonitionblock td.icon .icon-tip::before{content:"\f0eb";text-shadow:1px 1px 2px rgba(155,155,0,.8);color:#111}
|
||
|
|
.admonitionblock td.icon .icon-warning::before{content:"\f071";color:#bf6900}
|
||
|
|
.admonitionblock td.icon .icon-caution::before{content:"\f06d";color:#bf3400}
|
||
|
|
.admonitionblock td.icon .icon-important::before{content:"\f06a";color:#bf0000}
|
||
|
|
.conum[data-value]{display:inline-block;color:#fff!important;background-color:rgba(0,0,0,.8);-webkit-border-radius:100px;border-radius:100px;text-align:center;font-size:.75em;width:1.67em;height:1.67em;line-height:1.67em;font-family:"Open Sans","DejaVu Sans",sans-serif;font-style:normal;font-weight:bold}
|
||
|
|
.conum[data-value] *{color:#fff!important}
|
||
|
|
.conum[data-value]+b{display:none}
|
||
|
|
.conum[data-value]::after{content:attr(data-value)}
|
||
|
|
pre .conum[data-value]{position:relative;top:-.125em}
|
||
|
|
b.conum *{color:inherit!important}
|
||
|
|
.conum:not([data-value]):empty{display:none}
|
||
|
|
dt,th.tableblock,td.content,div.footnote{text-rendering:optimizeLegibility}
|
||
|
|
h1,h2,p,td.content,span.alt{letter-spacing:-.01em}
|
||
|
|
p strong,td.content strong,div.footnote strong{letter-spacing:-.005em}
|
||
|
|
p,blockquote,dt,td.content,span.alt{font-size:1.0625rem}
|
||
|
|
p{margin-bottom:1.25rem}
|
||
|
|
.sidebarblock p,.sidebarblock dt,.sidebarblock td.content,p.tableblock{font-size:1em}
|
||
|
|
.exampleblock>.content{background-color:#fffef7;border-color:#e0e0dc;-webkit-box-shadow:0 1px 4px #e0e0dc;box-shadow:0 1px 4px #e0e0dc}
|
||
|
|
.print-only{display:none!important}
|
||
|
|
@page{margin:1.25cm .75cm}
|
||
|
|
@media print{*{-webkit-box-shadow:none!important;box-shadow:none!important;text-shadow:none!important}
|
||
|
|
html{font-size:80%}
|
||
|
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thead{display:table-header-group}
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||
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p,blockquote,dt,td.content{font-size:1em;orphans:3;widows:3}
|
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h2,h3,#toctitle,.sidebarblock>.content>.title{page-break-after:avoid}
|
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|
|
#toc,.sidebarblock,.exampleblock>.content{background:none!important}
|
||
|
|
#toc{border-bottom:1px solid #dddddf!important;padding-bottom:0!important}
|
||
|
|
body.book #header{text-align:center}
|
||
|
|
body.book #header>h1:first-child{border:0!important;margin:2.5em 0 1em}
|
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|
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body.book #header .details{border:0!important;display:block;padding:0!important}
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body.book #header .details span:first-child{margin-left:0!important}
|
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body.book #header .details br{display:block}
|
||
|
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body.book #header .details br+span::before{content:none!important}
|
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|
|
body.book #toc{border:0!important;text-align:left!important;padding:0!important;margin:0!important}
|
||
|
|
body.book #toc,body.book #preamble,body.book h1.sect0,body.book .sect1>h2{page-break-before:always}
|
||
|
|
.listingblock code[data-lang]::before{display:block}
|
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#footer{padding:0 .9375em}
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.hide-on-print{display:none!important}
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.print-only{display:block!important}
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.hide-for-print{display:none!important}
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.show-for-print{display:inherit!important}}
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@media print,amzn-kf8{#header>h1:first-child{margin-top:1.25rem}
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.sect1{padding:0!important}
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.sect1+.sect1{border:0}
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#footer-text{color:rgba(0,0,0,.6);font-size:.9em}}
|
||
|
|
@media amzn-kf8{#header,#content,#footnotes,#footer{padding:0}}
|
||
|
|
</style>
|
||
|
|
</head>
|
||
|
|
<body class="article">
|
||
|
|
<div id="header">
|
||
|
|
<h1>Fighting regressions with git bisect</h1>
|
||
|
|
<div class="details">
|
||
|
|
<span id="author" class="author">Christian Couder</span><br>
|
||
|
|
<span id="email" class="email"><a href="mailto:chriscool@tuxfamily.org">chriscool@tuxfamily.org</a></span><br>
|
||
|
|
</div>
|
||
|
|
</div>
|
||
|
|
<div id="content">
|
||
|
|
<div class="sect1">
|
||
|
|
<h2 id="_abstract">Abstract</h2>
|
||
|
|
<div class="sectionbody">
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>"git bisect" enables software users and developers to easily find the
|
||
|
|
commit that introduced a regression. We show why it is important to
|
||
|
|
have good tools to fight regressions. We describe how "git bisect"
|
||
|
|
works from the outside and the algorithms it uses inside. Then we
|
||
|
|
explain how to take advantage of "git bisect" to improve current
|
||
|
|
practices. And we discuss how "git bisect" could improve in the
|
||
|
|
future.</p>
|
||
|
|
</div>
|
||
|
|
</div>
|
||
|
|
</div>
|
||
|
|
<div class="sect1">
|
||
|
|
<h2 id="_introduction_to_git_bisect">Introduction to "git bisect"</h2>
|
||
|
|
<div class="sectionbody">
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>Git is a Distributed Version Control system (DVCS) created by Linus
|
||
|
|
Torvalds and maintained by Junio Hamano.</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>In Git like in many other Version Control Systems (VCS), the different
|
||
|
|
states of the data that is managed by the system are called
|
||
|
|
commits. And, as VCS are mostly used to manage software source code,
|
||
|
|
sometimes "interesting" changes of behavior in the software are
|
||
|
|
introduced in some commits.</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>In fact people are specially interested in commits that introduce a
|
||
|
|
"bad" behavior, called a bug or a regression. They are interested in
|
||
|
|
these commits because a commit (hopefully) contains a very small set
|
||
|
|
of source code changes. And it’s much easier to understand and
|
||
|
|
properly fix a problem when you only need to check a very small set of
|
||
|
|
changes, than when you don’t know where look in the first place.</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>So to help people find commits that introduce a "bad" behavior, the
|
||
|
|
"git bisect" set of commands was invented. And it follows of course
|
||
|
|
that in "git bisect" parlance, commits where the "interesting
|
||
|
|
behavior" is present are called "bad" commits, while other commits are
|
||
|
|
called "good" commits. And a commit that introduce the behavior we are
|
||
|
|
interested in is called a "first bad commit". Note that there could be
|
||
|
|
more than one "first bad commit" in the commit space we are searching.</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>So "git bisect" is designed to help find a "first bad commit". And to
|
||
|
|
be as efficient as possible, it tries to perform a binary search.</p>
|
||
|
|
</div>
|
||
|
|
</div>
|
||
|
|
</div>
|
||
|
|
<div class="sect1">
|
||
|
|
<h2 id="_fighting_regressions_overview">Fighting regressions overview</h2>
|
||
|
|
<div class="sectionbody">
|
||
|
|
<div class="sect2">
|
||
|
|
<h3 id="_regressions_a_big_problem">Regressions: a big problem</h3>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>Regressions are a big problem in the software industry. But it’s
|
||
|
|
difficult to put some real numbers behind that claim.</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>There are some numbers about bugs in general, like a NIST study in
|
||
|
|
2002 <a href="#1">[1]</a> that said:</p>
|
||
|
|
</div>
|
||
|
|
<div class="quoteblock">
|
||
|
|
<blockquote>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>Software bugs, or errors, are so prevalent and so detrimental that
|
||
|
|
they cost the U.S. economy an estimated $59.5 billion annually, or
|
||
|
|
about 0.6 percent of the gross domestic product, according to a newly
|
||
|
|
released study commissioned by the Department of Commerce’s National
|
||
|
|
Institute of Standards and Technology (NIST). At the national level,
|
||
|
|
over half of the costs are borne by software users and the remainder
|
||
|
|
by software developers/vendors. The study also found that, although
|
||
|
|
all errors cannot be removed, more than a third of these costs, or an
|
||
|
|
estimated $22.2 billion, could be eliminated by an improved testing
|
||
|
|
infrastructure that enables earlier and more effective identification
|
||
|
|
and removal of software defects. These are the savings associated with
|
||
|
|
finding an increased percentage (but not 100 percent) of errors closer
|
||
|
|
to the development stages in which they are introduced. Currently,
|
||
|
|
over half of all errors are not found until "downstream" in the
|
||
|
|
development process or during post-sale software use.</p>
|
||
|
|
</div>
|
||
|
|
</blockquote>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>And then:</p>
|
||
|
|
</div>
|
||
|
|
<div class="quoteblock">
|
||
|
|
<blockquote>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>Software developers already spend approximately 80 percent of
|
||
|
|
development costs on identifying and correcting defects, and yet few
|
||
|
|
products of any type other than software are shipped with such high
|
||
|
|
levels of errors.</p>
|
||
|
|
</div>
|
||
|
|
</blockquote>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>Eventually the conclusion started with:</p>
|
||
|
|
</div>
|
||
|
|
<div class="quoteblock">
|
||
|
|
<blockquote>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>The path to higher software quality is significantly improved software
|
||
|
|
testing.</p>
|
||
|
|
</div>
|
||
|
|
</blockquote>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>There are other estimates saying that 80% of the cost related to
|
||
|
|
software is about maintenance <a href="#2">[2]</a>.</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>Though, according to Wikipedia <a href="#3">[3]</a>:</p>
|
||
|
|
</div>
|
||
|
|
<div class="quoteblock">
|
||
|
|
<blockquote>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>A common perception of maintenance is that it is merely fixing
|
||
|
|
bugs. However, studies and surveys over the years have indicated that
|
||
|
|
the majority, over 80%, of the maintenance effort is used for
|
||
|
|
non-corrective actions (Pigosky 1997). This perception is perpetuated
|
||
|
|
by users submitting problem reports that in reality are functionality
|
||
|
|
enhancements to the system.</p>
|
||
|
|
</div>
|
||
|
|
</blockquote>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>But we can guess that improving on existing software is very costly
|
||
|
|
because you have to watch out for regressions. At least this would
|
||
|
|
make the above studies consistent among themselves.</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>Of course some kind of software is developed, then used during some
|
||
|
|
time without being improved on much, and then finally thrown away. In
|
||
|
|
this case, of course, regressions may not be a big problem. But on the
|
||
|
|
other hand, there is a lot of big software that is continually
|
||
|
|
developed and maintained during years or even tens of years by a lot
|
||
|
|
of people. And as there are often many people who depend (sometimes
|
||
|
|
critically) on such software, regressions are a really big problem.</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>One such software is the Linux kernel. And if we look at the Linux
|
||
|
|
kernel, we can see that a lot of time and effort is spent to fight
|
||
|
|
regressions. The release cycle start with a 2 weeks long merge
|
||
|
|
window. Then the first release candidate (rc) version is tagged. And
|
||
|
|
after that about 7 or 8 more rc versions will appear with around one
|
||
|
|
week between each of them, before the final release.</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>The time between the first rc release and the final release is
|
||
|
|
supposed to be used to test rc versions and fight bugs and especially
|
||
|
|
regressions. And this time is more than 80% of the release cycle
|
||
|
|
time. But this is not the end of the fight yet, as of course it
|
||
|
|
continues after the release.</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>And then this is what Ingo Molnar (a well known Linux kernel
|
||
|
|
developer) says about his use of git bisect:</p>
|
||
|
|
</div>
|
||
|
|
<div class="quoteblock">
|
||
|
|
<blockquote>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>I most actively use it during the merge window (when a lot of trees
|
||
|
|
get merged upstream and when the influx of bugs is the highest) - and
|
||
|
|
yes, there have been cases that i used it multiple times a day. My
|
||
|
|
average is roughly once a day.</p>
|
||
|
|
</div>
|
||
|
|
</blockquote>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>So regressions are fought all the time by developers, and indeed it is
|
||
|
|
well known that bugs should be fixed as soon as possible, so as soon
|
||
|
|
as they are found. That’s why it is interesting to have good tools for
|
||
|
|
this purpose.</p>
|
||
|
|
</div>
|
||
|
|
</div>
|
||
|
|
<div class="sect2">
|
||
|
|
<h3 id="_other_tools_to_fight_regressions">Other tools to fight regressions</h3>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>So what are the tools used to fight regressions? They are nearly the
|
||
|
|
same as those used to fight regular bugs. The only specific tools are
|
||
|
|
test suites and tools similar as "git bisect".</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>Test suites are very nice. But when they are used alone, they are
|
||
|
|
supposed to be used so that all the tests are checked after each
|
||
|
|
commit. This means that they are not very efficient, because many
|
||
|
|
tests are run for no interesting result, and they suffer from
|
||
|
|
combinational explosion.</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>In fact the problem is that big software often has many different
|
||
|
|
configuration options and that each test case should pass for each
|
||
|
|
configuration after each commit. So if you have for each release: N
|
||
|
|
configurations, M commits and T test cases, you should perform:</p>
|
||
|
|
</div>
|
||
|
|
<div class="listingblock">
|
||
|
|
<div class="content">
|
||
|
|
<pre>N * M * T tests</pre>
|
||
|
|
</div>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>where N, M and T are all growing with the size your software.</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>So very soon it will not be possible to completely test everything.</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>And if some bugs slip through your test suite, then you can add a test
|
||
|
|
to your test suite. But if you want to use your new improved test
|
||
|
|
suite to find where the bug slipped in, then you will either have to
|
||
|
|
emulate a bisection process or you will perhaps bluntly test each
|
||
|
|
commit backward starting from the "bad" commit you have which may be
|
||
|
|
very wasteful.</p>
|
||
|
|
</div>
|
||
|
|
</div>
|
||
|
|
</div>
|
||
|
|
</div>
|
||
|
|
<div class="sect1">
|
||
|
|
<h2 id="_git_bisect_overview">"git bisect" overview</h2>
|
||
|
|
<div class="sectionbody">
|
||
|
|
<div class="sect2">
|
||
|
|
<h3 id="_starting_a_bisection">Starting a bisection</h3>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>The first "git bisect" subcommand to use is "git bisect start" to
|
||
|
|
start the search. Then bounds must be set to limit the commit
|
||
|
|
space. This is done usually by giving one "bad" and at least one
|
||
|
|
"good" commit. They can be passed in the initial call to "git bisect
|
||
|
|
start" like this:</p>
|
||
|
|
</div>
|
||
|
|
<div class="listingblock">
|
||
|
|
<div class="content">
|
||
|
|
<pre>$ git bisect start [BAD [GOOD...]]</pre>
|
||
|
|
</div>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>or they can be set using:</p>
|
||
|
|
</div>
|
||
|
|
<div class="listingblock">
|
||
|
|
<div class="content">
|
||
|
|
<pre>$ git bisect bad [COMMIT]</pre>
|
||
|
|
</div>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>and:</p>
|
||
|
|
</div>
|
||
|
|
<div class="listingblock">
|
||
|
|
<div class="content">
|
||
|
|
<pre>$ git bisect good [COMMIT...]</pre>
|
||
|
|
</div>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>where BAD, GOOD and COMMIT are all names that can be resolved to a
|
||
|
|
commit.</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>Then "git bisect" will checkout a commit of its choosing and ask the
|
||
|
|
user to test it, like this:</p>
|
||
|
|
</div>
|
||
|
|
<div class="listingblock">
|
||
|
|
<div class="content">
|
||
|
|
<pre>$ git bisect start v2.6.27 v2.6.25
|
||
|
|
Bisecting: 10928 revisions left to test after this (roughly 14 steps)
|
||
|
|
[2ec65f8b89ea003c27ff7723525a2ee335a2b393] x86: clean up using max_low_pfn on 32-bit</pre>
|
||
|
|
</div>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>Note that the example that we will use is really a toy example, we
|
||
|
|
will be looking for the first commit that has a version like
|
||
|
|
"2.6.26-something", that is the commit that has a "SUBLEVEL = 26" line
|
||
|
|
in the top level Makefile. This is a toy example because there are
|
||
|
|
better ways to find this commit with Git than using "git bisect" (for
|
||
|
|
example "git blame" or "git log -S<string>").</p>
|
||
|
|
</div>
|
||
|
|
</div>
|
||
|
|
<div class="sect2">
|
||
|
|
<h3 id="_driving_a_bisection_manually">Driving a bisection manually</h3>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>At this point there are basically 2 ways to drive the search. It can
|
||
|
|
be driven manually by the user or it can be driven automatically by a
|
||
|
|
script or a command.</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>If the user is driving it, then at each step of the search, the user
|
||
|
|
will have to test the current commit and say if it is "good" or "bad"
|
||
|
|
using the "git bisect good" or "git bisect bad" commands respectively
|
||
|
|
that have been described above. For example:</p>
|
||
|
|
</div>
|
||
|
|
<div class="listingblock">
|
||
|
|
<div class="content">
|
||
|
|
<pre>$ git bisect bad
|
||
|
|
Bisecting: 5480 revisions left to test after this (roughly 13 steps)
|
||
|
|
[66c0b394f08fd89236515c1c84485ea712a157be] KVM: kill file->f_count abuse in kvm</pre>
|
||
|
|
</div>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>And after a few more steps like that, "git bisect" will eventually
|
||
|
|
find a first bad commit:</p>
|
||
|
|
</div>
|
||
|
|
<div class="listingblock">
|
||
|
|
<div class="content">
|
||
|
|
<pre>$ git bisect bad
|
||
|
|
2ddcca36c8bcfa251724fe342c8327451988be0d is the first bad commit
|
||
|
|
commit 2ddcca36c8bcfa251724fe342c8327451988be0d
|
||
|
|
Author: Linus Torvalds <torvalds@linux-foundation.org>
|
||
|
|
Date: Sat May 3 11:59:44 2008 -0700
|
||
|
|
|
||
|
|
Linux 2.6.26-rc1
|
||
|
|
|
||
|
|
:100644 100644 5cf82581... 4492984e... M Makefile</pre>
|
||
|
|
</div>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>At this point we can see what the commit does, check it out (if it’s
|
||
|
|
not already checked out) or tinker with it, for example:</p>
|
||
|
|
</div>
|
||
|
|
<div class="listingblock">
|
||
|
|
<div class="content">
|
||
|
|
<pre>$ git show HEAD
|
||
|
|
commit 2ddcca36c8bcfa251724fe342c8327451988be0d
|
||
|
|
Author: Linus Torvalds <torvalds@linux-foundation.org>
|
||
|
|
Date: Sat May 3 11:59:44 2008 -0700
|
||
|
|
|
||
|
|
Linux 2.6.26-rc1
|
||
|
|
|
||
|
|
diff --git a/Makefile b/Makefile
|
||
|
|
index 5cf8258..4492984 100644
|
||
|
|
--- a/Makefile
|
||
|
|
+++ b/Makefile
|
||
|
|
@@ -1,7 +1,7 @@
|
||
|
|
VERSION = 2
|
||
|
|
PATCHLEVEL = 6
|
||
|
|
-SUBLEVEL = 25
|
||
|
|
-EXTRAVERSION =
|
||
|
|
+SUBLEVEL = 26
|
||
|
|
+EXTRAVERSION = -rc1
|
||
|
|
NAME = Funky Weasel is Jiggy wit it
|
||
|
|
|
||
|
|
# *DOCUMENTATION*</pre>
|
||
|
|
</div>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>And when we are finished we can use "git bisect reset" to go back to
|
||
|
|
the branch we were in before we started bisecting:</p>
|
||
|
|
</div>
|
||
|
|
<div class="listingblock">
|
||
|
|
<div class="content">
|
||
|
|
<pre>$ git bisect reset
|
||
|
|
Checking out files: 100% (21549/21549), done.
|
||
|
|
Previous HEAD position was 2ddcca3... Linux 2.6.26-rc1
|
||
|
|
Switched to branch 'master'</pre>
|
||
|
|
</div>
|
||
|
|
</div>
|
||
|
|
</div>
|
||
|
|
<div class="sect2">
|
||
|
|
<h3 id="_driving_a_bisection_automatically">Driving a bisection automatically</h3>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>The other way to drive the bisection process is to tell "git bisect"
|
||
|
|
to launch a script or command at each bisection step to know if the
|
||
|
|
current commit is "good" or "bad". To do that, we use the "git bisect
|
||
|
|
run" command. For example:</p>
|
||
|
|
</div>
|
||
|
|
<div class="listingblock">
|
||
|
|
<div class="content">
|
||
|
|
<pre>$ git bisect start v2.6.27 v2.6.25
|
||
|
|
Bisecting: 10928 revisions left to test after this (roughly 14 steps)
|
||
|
|
[2ec65f8b89ea003c27ff7723525a2ee335a2b393] x86: clean up using max_low_pfn on 32-bit
|
||
|
|
$
|
||
|
|
$ git bisect run grep '^SUBLEVEL = 25' Makefile
|
||
|
|
running grep ^SUBLEVEL = 25 Makefile
|
||
|
|
Bisecting: 5480 revisions left to test after this (roughly 13 steps)
|
||
|
|
[66c0b394f08fd89236515c1c84485ea712a157be] KVM: kill file->f_count abuse in kvm
|
||
|
|
running grep ^SUBLEVEL = 25 Makefile
|
||
|
|
SUBLEVEL = 25
|
||
|
|
Bisecting: 2740 revisions left to test after this (roughly 12 steps)
|
||
|
|
[671294719628f1671faefd4882764886f8ad08cb] V4L/DVB(7879): Adding cx18 Support for mxl5005s
|
||
|
|
...
|
||
|
|
...
|
||
|
|
running grep ^SUBLEVEL = 25 Makefile
|
||
|
|
Bisecting: 0 revisions left to test after this (roughly 0 steps)
|
||
|
|
[2ddcca36c8bcfa251724fe342c8327451988be0d] Linux 2.6.26-rc1
|
||
|
|
running grep ^SUBLEVEL = 25 Makefile
|
||
|
|
2ddcca36c8bcfa251724fe342c8327451988be0d is the first bad commit
|
||
|
|
commit 2ddcca36c8bcfa251724fe342c8327451988be0d
|
||
|
|
Author: Linus Torvalds <torvalds@linux-foundation.org>
|
||
|
|
Date: Sat May 3 11:59:44 2008 -0700
|
||
|
|
|
||
|
|
Linux 2.6.26-rc1
|
||
|
|
|
||
|
|
:100644 100644 5cf82581... 4492984e... M Makefile
|
||
|
|
bisect run success</pre>
|
||
|
|
</div>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>In this example, we passed "grep <em>^SUBLEVEL = 25</em> Makefile" as
|
||
|
|
parameter to "git bisect run". This means that at each step, the grep
|
||
|
|
command we passed will be launched. And if it exits with code 0 (that
|
||
|
|
means success) then git bisect will mark the current state as
|
||
|
|
"good". If it exits with code 1 (or any code between 1 and 127
|
||
|
|
included, except the special code 125), then the current state will be
|
||
|
|
marked as "bad".</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>Exit code between 128 and 255 are special to "git bisect run". They
|
||
|
|
make it stop immediately the bisection process. This is useful for
|
||
|
|
example if the command passed takes too long to complete, because you
|
||
|
|
can kill it with a signal and it will stop the bisection process.</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>It can also be useful in scripts passed to "git bisect run" to "exit
|
||
|
|
255" if some very abnormal situation is detected.</p>
|
||
|
|
</div>
|
||
|
|
</div>
|
||
|
|
<div class="sect2">
|
||
|
|
<h3 id="_avoiding_untestable_commits">Avoiding untestable commits</h3>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>Sometimes it happens that the current state cannot be tested, for
|
||
|
|
example if it does not compile because there was a bug preventing it
|
||
|
|
at that time. This is what the special exit code 125 is for. It tells
|
||
|
|
"git bisect run" that the current commit should be marked as
|
||
|
|
untestable and that another one should be chosen and checked out.</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>If the bisection process is driven manually, you can use "git bisect
|
||
|
|
skip" to do the same thing. (In fact the special exit code 125 makes
|
||
|
|
"git bisect run" use "git bisect skip" in the background.)</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>Or if you want more control, you can inspect the current state using
|
||
|
|
for example "git bisect visualize". It will launch gitk (or "git log"
|
||
|
|
if the <code>DISPLAY</code> environment variable is not set) to help you find a
|
||
|
|
better bisection point.</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>Either way, if you have a string of untestable commits, it might
|
||
|
|
happen that the regression you are looking for has been introduced by
|
||
|
|
one of these untestable commits. In this case it’s not possible to
|
||
|
|
tell for sure which commit introduced the regression.</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>So if you used "git bisect skip" (or the run script exited with
|
||
|
|
special code 125) you could get a result like this:</p>
|
||
|
|
</div>
|
||
|
|
<div class="listingblock">
|
||
|
|
<div class="content">
|
||
|
|
<pre>There are only 'skip'ped commits left to test.
|
||
|
|
The first bad commit could be any of:
|
||
|
|
15722f2fa328eaba97022898a305ffc8172db6b1
|
||
|
|
78e86cf3e850bd755bb71831f42e200626fbd1e0
|
||
|
|
e15b73ad3db9b48d7d1ade32f8cd23a751fe0ace
|
||
|
|
070eab2303024706f2924822bfec8b9847e4ac1b
|
||
|
|
We cannot bisect more!</pre>
|
||
|
|
</div>
|
||
|
|
</div>
|
||
|
|
</div>
|
||
|
|
<div class="sect2">
|
||
|
|
<h3 id="_saving_a_log_and_replaying_it">Saving a log and replaying it</h3>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>If you want to show other people your bisection process, you can get a
|
||
|
|
log using for example:</p>
|
||
|
|
</div>
|
||
|
|
<div class="listingblock">
|
||
|
|
<div class="content">
|
||
|
|
<pre>$ git bisect log > bisect_log.txt</pre>
|
||
|
|
</div>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>And it is possible to replay it using:</p>
|
||
|
|
</div>
|
||
|
|
<div class="listingblock">
|
||
|
|
<div class="content">
|
||
|
|
<pre>$ git bisect replay bisect_log.txt</pre>
|
||
|
|
</div>
|
||
|
|
</div>
|
||
|
|
</div>
|
||
|
|
</div>
|
||
|
|
</div>
|
||
|
|
<div class="sect1">
|
||
|
|
<h2 id="_git_bisect_details">"git bisect" details</h2>
|
||
|
|
<div class="sectionbody">
|
||
|
|
<div class="sect2">
|
||
|
|
<h3 id="_bisection_algorithm">Bisection algorithm</h3>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>As the Git commits form a directed acyclic graph (DAG), finding the
|
||
|
|
best bisection commit to test at each step is not so simple. Anyway
|
||
|
|
Linus found and implemented a "truly stupid" algorithm, later improved
|
||
|
|
by Junio Hamano, that works quite well.</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>So the algorithm used by "git bisect" to find the best bisection
|
||
|
|
commit when there are no skipped commits is the following:</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>1) keep only the commits that:</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>a) are ancestor of the "bad" commit (including the "bad" commit itself),
|
||
|
|
b) are not ancestor of a "good" commit (excluding the "good" commits).</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>This means that we get rid of the uninteresting commits in the DAG.</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>For example if we start with a graph like this:</p>
|
||
|
|
</div>
|
||
|
|
<div class="listingblock">
|
||
|
|
<div class="content">
|
||
|
|
<pre>G-Y-G-W-W-W-X-X-X-X
|
||
|
|
\ /
|
||
|
|
W-W-B
|
||
|
|
/
|
||
|
|
Y---G-W---W
|
||
|
|
\ / \
|
||
|
|
Y-Y X-X-X-X
|
||
|
|
|
||
|
|
-> time goes this way -></pre>
|
||
|
|
</div>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>where B is the "bad" commit, "G" are "good" commits and W, X, and Y
|
||
|
|
are other commits, we will get the following graph after this first
|
||
|
|
step:</p>
|
||
|
|
</div>
|
||
|
|
<div class="listingblock">
|
||
|
|
<div class="content">
|
||
|
|
<pre>W-W-W
|
||
|
|
\
|
||
|
|
W-W-B
|
||
|
|
/
|
||
|
|
W---W</pre>
|
||
|
|
</div>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>So only the W and B commits will be kept. Because commits X and Y will
|
||
|
|
have been removed by rules a) and b) respectively, and because commits
|
||
|
|
G are removed by rule b) too.</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>Note for Git users, that it is equivalent as keeping only the commit
|
||
|
|
given by:</p>
|
||
|
|
</div>
|
||
|
|
<div class="listingblock">
|
||
|
|
<div class="content">
|
||
|
|
<pre>git rev-list BAD --not GOOD1 GOOD2...</pre>
|
||
|
|
</div>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>Also note that we don’t require the commits that are kept to be
|
||
|
|
descendants of a "good" commit. So in the following example, commits W
|
||
|
|
and Z will be kept:</p>
|
||
|
|
</div>
|
||
|
|
<div class="listingblock">
|
||
|
|
<div class="content">
|
||
|
|
<pre>G-W-W-W-B
|
||
|
|
/
|
||
|
|
Z-Z</pre>
|
||
|
|
</div>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>2) starting from the "good" ends of the graph, associate to each
|
||
|
|
commit the number of ancestors it has plus one</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>For example with the following graph where H is the "bad" commit and A
|
||
|
|
and D are some parents of some "good" commits:</p>
|
||
|
|
</div>
|
||
|
|
<div class="listingblock">
|
||
|
|
<div class="content">
|
||
|
|
<pre>A-B-C
|
||
|
|
\
|
||
|
|
F-G-H
|
||
|
|
/
|
||
|
|
D---E</pre>
|
||
|
|
</div>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>this will give:</p>
|
||
|
|
</div>
|
||
|
|
<div class="listingblock">
|
||
|
|
<div class="content">
|
||
|
|
<pre>1 2 3
|
||
|
|
A-B-C
|
||
|
|
\6 7 8
|
||
|
|
F-G-H
|
||
|
|
1 2/
|
||
|
|
D---E</pre>
|
||
|
|
</div>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>3) associate to each commit: min(X, N - X)</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>where X is the value associated to the commit in step 2) and N is the
|
||
|
|
total number of commits in the graph.</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>In the above example we have N = 8, so this will give:</p>
|
||
|
|
</div>
|
||
|
|
<div class="listingblock">
|
||
|
|
<div class="content">
|
||
|
|
<pre>1 2 3
|
||
|
|
A-B-C
|
||
|
|
\2 1 0
|
||
|
|
F-G-H
|
||
|
|
1 2/
|
||
|
|
D---E</pre>
|
||
|
|
</div>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>4) the best bisection point is the commit with the highest associated
|
||
|
|
number</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>So in the above example the best bisection point is commit C.</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>5) note that some shortcuts are implemented to speed up the algorithm</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>As we know N from the beginning, we know that min(X, N - X) can’t be
|
||
|
|
greater than N/2. So during steps 2) and 3), if we would associate N/2
|
||
|
|
to a commit, then we know this is the best bisection point. So in this
|
||
|
|
case we can just stop processing any other commit and return the
|
||
|
|
current commit.</p>
|
||
|
|
</div>
|
||
|
|
</div>
|
||
|
|
<div class="sect2">
|
||
|
|
<h3 id="_bisection_algorithm_debugging">Bisection algorithm debugging</h3>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>For any commit graph, you can see the number associated with each
|
||
|
|
commit using "git rev-list --bisect-all".</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>For example, for the above graph, a command like:</p>
|
||
|
|
</div>
|
||
|
|
<div class="listingblock">
|
||
|
|
<div class="content">
|
||
|
|
<pre>$ git rev-list --bisect-all BAD --not GOOD1 GOOD2</pre>
|
||
|
|
</div>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>would output something like:</p>
|
||
|
|
</div>
|
||
|
|
<div class="listingblock">
|
||
|
|
<div class="content">
|
||
|
|
<pre>e15b73ad3db9b48d7d1ade32f8cd23a751fe0ace (dist=3)
|
||
|
|
15722f2fa328eaba97022898a305ffc8172db6b1 (dist=2)
|
||
|
|
78e86cf3e850bd755bb71831f42e200626fbd1e0 (dist=2)
|
||
|
|
a1939d9a142de972094af4dde9a544e577ddef0e (dist=2)
|
||
|
|
070eab2303024706f2924822bfec8b9847e4ac1b (dist=1)
|
||
|
|
a3864d4f32a3bf5ed177ddef598490a08760b70d (dist=1)
|
||
|
|
a41baa717dd74f1180abf55e9341bc7a0bb9d556 (dist=1)
|
||
|
|
9e622a6dad403b71c40979743bb9d5be17b16bd6 (dist=0)</pre>
|
||
|
|
</div>
|
||
|
|
</div>
|
||
|
|
</div>
|
||
|
|
<div class="sect2">
|
||
|
|
<h3 id="_bisection_algorithm_discussed">Bisection algorithm discussed</h3>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>First let’s define "best bisection point". We will say that a commit X
|
||
|
|
is a best bisection point or a best bisection commit if knowing its
|
||
|
|
state ("good" or "bad") gives as much information as possible whether
|
||
|
|
the state of the commit happens to be "good" or "bad".</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>This means that the best bisection commits are the commits where the
|
||
|
|
following function is maximum:</p>
|
||
|
|
</div>
|
||
|
|
<div class="listingblock">
|
||
|
|
<div class="content">
|
||
|
|
<pre>f(X) = min(information_if_good(X), information_if_bad(X))</pre>
|
||
|
|
</div>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>where information_if_good(X) is the information we get if X is good
|
||
|
|
and information_if_bad(X) is the information we get if X is bad.</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>Now we will suppose that there is only one "first bad commit". This
|
||
|
|
means that all its descendants are "bad" and all the other commits are
|
||
|
|
"good". And we will suppose that all commits have an equal probability
|
||
|
|
of being good or bad, or of being the first bad commit, so knowing the
|
||
|
|
state of c commits gives always the same amount of information
|
||
|
|
wherever these c commits are on the graph and whatever c is. (So we
|
||
|
|
suppose that these commits being for example on a branch or near a
|
||
|
|
good or a bad commit does not give more or less information).</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>Let’s also suppose that we have a cleaned up graph like one after step
|
||
|
|
1) in the bisection algorithm above. This means that we can measure
|
||
|
|
the information we get in terms of number of commit we can remove from
|
||
|
|
the graph..</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>And let’s take a commit X in the graph.</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>If X is found to be "good", then we know that its ancestors are all
|
||
|
|
"good", so we want to say that:</p>
|
||
|
|
</div>
|
||
|
|
<div class="listingblock">
|
||
|
|
<div class="content">
|
||
|
|
<pre>information_if_good(X) = number_of_ancestors(X) (TRUE)</pre>
|
||
|
|
</div>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>And this is true because at step 1) b) we remove the ancestors of the
|
||
|
|
"good" commits.</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>If X is found to be "bad", then we know that its descendants are all
|
||
|
|
"bad", so we want to say that:</p>
|
||
|
|
</div>
|
||
|
|
<div class="listingblock">
|
||
|
|
<div class="content">
|
||
|
|
<pre>information_if_bad(X) = number_of_descendants(X) (WRONG)</pre>
|
||
|
|
</div>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>But this is wrong because at step 1) a) we keep only the ancestors of
|
||
|
|
the bad commit. So we get more information when a commit is marked as
|
||
|
|
"bad", because we also know that the ancestors of the previous "bad"
|
||
|
|
commit that are not ancestors of the new "bad" commit are not the
|
||
|
|
first bad commit. We don’t know if they are good or bad, but we know
|
||
|
|
that they are not the first bad commit because they are not ancestor
|
||
|
|
of the new "bad" commit.</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>So when a commit is marked as "bad" we know we can remove all the
|
||
|
|
commits in the graph except those that are ancestors of the new "bad"
|
||
|
|
commit. This means that:</p>
|
||
|
|
</div>
|
||
|
|
<div class="listingblock">
|
||
|
|
<div class="content">
|
||
|
|
<pre>information_if_bad(X) = N - number_of_ancestors(X) (TRUE)</pre>
|
||
|
|
</div>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>where N is the number of commits in the (cleaned up) graph.</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>So in the end this means that to find the best bisection commits we
|
||
|
|
should maximize the function:</p>
|
||
|
|
</div>
|
||
|
|
<div class="listingblock">
|
||
|
|
<div class="content">
|
||
|
|
<pre>f(X) = min(number_of_ancestors(X), N - number_of_ancestors(X))</pre>
|
||
|
|
</div>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>And this is nice because at step 2) we compute number_of_ancestors(X)
|
||
|
|
and so at step 3) we compute f(X).</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>Let’s take the following graph as an example:</p>
|
||
|
|
</div>
|
||
|
|
<div class="listingblock">
|
||
|
|
<div class="content">
|
||
|
|
<pre> G-H-I-J
|
||
|
|
/ \
|
||
|
|
A-B-C-D-E-F O
|
||
|
|
\ /
|
||
|
|
K-L-M-N</pre>
|
||
|
|
</div>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>If we compute the following non optimal function on it:</p>
|
||
|
|
</div>
|
||
|
|
<div class="listingblock">
|
||
|
|
<div class="content">
|
||
|
|
<pre>g(X) = min(number_of_ancestors(X), number_of_descendants(X))</pre>
|
||
|
|
</div>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>we get:</p>
|
||
|
|
</div>
|
||
|
|
<div class="listingblock">
|
||
|
|
<div class="content">
|
||
|
|
<pre> 4 3 2 1
|
||
|
|
G-H-I-J
|
||
|
|
1 2 3 4 5 6/ \0
|
||
|
|
A-B-C-D-E-F O
|
||
|
|
\ /
|
||
|
|
K-L-M-N
|
||
|
|
4 3 2 1</pre>
|
||
|
|
</div>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>but with the algorithm used by git bisect we get:</p>
|
||
|
|
</div>
|
||
|
|
<div class="listingblock">
|
||
|
|
<div class="content">
|
||
|
|
<pre> 7 7 6 5
|
||
|
|
G-H-I-J
|
||
|
|
1 2 3 4 5 6/ \0
|
||
|
|
A-B-C-D-E-F O
|
||
|
|
\ /
|
||
|
|
K-L-M-N
|
||
|
|
7 7 6 5</pre>
|
||
|
|
</div>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>So we chose G, H, K or L as the best bisection point, which is better
|
||
|
|
than F. Because if for example L is bad, then we will know not only
|
||
|
|
that L, M and N are bad but also that G, H, I and J are not the first
|
||
|
|
bad commit (since we suppose that there is only one first bad commit
|
||
|
|
and it must be an ancestor of L).</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>So the current algorithm seems to be the best possible given what we
|
||
|
|
initially supposed.</p>
|
||
|
|
</div>
|
||
|
|
</div>
|
||
|
|
<div class="sect2">
|
||
|
|
<h3 id="_skip_algorithm">Skip algorithm</h3>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>When some commits have been skipped (using "git bisect skip"), then
|
||
|
|
the bisection algorithm is the same for step 1) to 3). But then we use
|
||
|
|
roughly the following steps:</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>6) sort the commit by decreasing associated value</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>7) if the first commit has not been skipped, we can return it and stop
|
||
|
|
here</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>8) otherwise filter out all the skipped commits in the sorted list</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>9) use a pseudo random number generator (PRNG) to generate a random
|
||
|
|
number between 0 and 1</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>10) multiply this random number with its square root to bias it toward
|
||
|
|
0</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>11) multiply the result by the number of commits in the filtered list
|
||
|
|
to get an index into this list</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>12) return the commit at the computed index</p>
|
||
|
|
</div>
|
||
|
|
</div>
|
||
|
|
<div class="sect2">
|
||
|
|
<h3 id="_skip_algorithm_discussed">Skip algorithm discussed</h3>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>After step 7) (in the skip algorithm), we could check if the second
|
||
|
|
commit has been skipped and return it if it is not the case. And in
|
||
|
|
fact that was the algorithm we used from when "git bisect skip" was
|
||
|
|
developed in Git version 1.5.4 (released on February 1st 2008) until
|
||
|
|
Git version 1.6.4 (released July 29th 2009).</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>But Ingo Molnar and H. Peter Anvin (another well known linux kernel
|
||
|
|
developer) both complained that sometimes the best bisection points
|
||
|
|
all happened to be in an area where all the commits are
|
||
|
|
untestable. And in this case the user was asked to test many
|
||
|
|
untestable commits, which could be very inefficient.</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>Indeed untestable commits are often untestable because a breakage was
|
||
|
|
introduced at one time, and that breakage was fixed only after many
|
||
|
|
other commits were introduced.</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>This breakage is of course most of the time unrelated to the breakage
|
||
|
|
we are trying to locate in the commit graph. But it prevents us to
|
||
|
|
know if the interesting "bad behavior" is present or not.</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>So it is a fact that commits near an untestable commit have a high
|
||
|
|
probability of being untestable themselves. And the best bisection
|
||
|
|
commits are often found together too (due to the bisection algorithm).</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>This is why it is a bad idea to just chose the next best unskipped
|
||
|
|
bisection commit when the first one has been skipped.</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>We found that most commits on the graph may give quite a lot of
|
||
|
|
information when they are tested. And the commits that will not on
|
||
|
|
average give a lot of information are the one near the good and bad
|
||
|
|
commits.</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>So using a PRNG with a bias to favor commits away from the good and
|
||
|
|
bad commits looked like a good choice.</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>One obvious improvement to this algorithm would be to look for a
|
||
|
|
commit that has an associated value near the one of the best bisection
|
||
|
|
commit, and that is on another branch, before using the PRNG. Because
|
||
|
|
if such a commit exists, then it is not very likely to be untestable
|
||
|
|
too, so it will probably give more information than a nearly randomly
|
||
|
|
chosen one.</p>
|
||
|
|
</div>
|
||
|
|
</div>
|
||
|
|
<div class="sect2">
|
||
|
|
<h3 id="_checking_merge_bases">Checking merge bases</h3>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>There is another tweak in the bisection algorithm that has not been
|
||
|
|
described in the "bisection algorithm" above.</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>We supposed in the previous examples that the "good" commits were
|
||
|
|
ancestors of the "bad" commit. But this is not a requirement of "git
|
||
|
|
bisect".</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>Of course the "bad" commit cannot be an ancestor of a "good" commit,
|
||
|
|
because the ancestors of the good commits are supposed to be
|
||
|
|
"good". And all the "good" commits must be related to the bad commit.
|
||
|
|
They cannot be on a branch that has no link with the branch of the
|
||
|
|
"bad" commit. But it is possible for a good commit to be related to a
|
||
|
|
bad commit and yet not be neither one of its ancestor nor one of its
|
||
|
|
descendants.</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>For example, there can be a "main" branch, and a "dev" branch that was
|
||
|
|
forked of the main branch at a commit named "D" like this:</p>
|
||
|
|
</div>
|
||
|
|
<div class="listingblock">
|
||
|
|
<div class="content">
|
||
|
|
<pre>A-B-C-D-E-F-G <--main
|
||
|
|
\
|
||
|
|
H-I-J <--dev</pre>
|
||
|
|
</div>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>The commit "D" is called a "merge base" for branch "main" and "dev"
|
||
|
|
because it’s the best common ancestor for these branches for a merge.</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>Now let’s suppose that commit J is bad and commit G is good and that
|
||
|
|
we apply the bisection algorithm like it has been previously
|
||
|
|
described.</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>As described in step 1) b) of the bisection algorithm, we remove all
|
||
|
|
the ancestors of the good commits because they are supposed to be good
|
||
|
|
too.</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>So we would be left with only:</p>
|
||
|
|
</div>
|
||
|
|
<div class="listingblock">
|
||
|
|
<div class="content">
|
||
|
|
<pre>H-I-J</pre>
|
||
|
|
</div>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>But what happens if the first bad commit is "B" and if it has been
|
||
|
|
fixed in the "main" branch by commit "F"?</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>The result of such a bisection would be that we would find that H is
|
||
|
|
the first bad commit, when in fact it’s B. So that would be wrong!</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>And yes it can happen in practice that people working on one branch
|
||
|
|
are not aware that people working on another branch fixed a bug! It
|
||
|
|
could also happen that F fixed more than one bug or that it is a
|
||
|
|
revert of some big development effort that was not ready to be
|
||
|
|
released.</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>In fact development teams often maintain both a development branch and
|
||
|
|
a maintenance branch, and it would be quite easy for them if "git
|
||
|
|
bisect" just worked when they want to bisect a regression on the
|
||
|
|
development branch that is not on the maintenance branch. They should
|
||
|
|
be able to start bisecting using:</p>
|
||
|
|
</div>
|
||
|
|
<div class="listingblock">
|
||
|
|
<div class="content">
|
||
|
|
<pre>$ git bisect start dev main</pre>
|
||
|
|
</div>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>To enable that additional nice feature, when a bisection is started
|
||
|
|
and when some good commits are not ancestors of the bad commit, we
|
||
|
|
first compute the merge bases between the bad and the good commits and
|
||
|
|
we chose these merge bases as the first commits that will be checked
|
||
|
|
out and tested.</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>If it happens that one merge base is bad, then the bisection process
|
||
|
|
is stopped with a message like:</p>
|
||
|
|
</div>
|
||
|
|
<div class="listingblock">
|
||
|
|
<div class="content">
|
||
|
|
<pre>The merge base BBBBBB is bad.
|
||
|
|
This means the bug has been fixed between BBBBBB and [GGGGGG,...].</pre>
|
||
|
|
</div>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>where BBBBBB is the sha1 hash of the bad merge base and [GGGGGG,…​]
|
||
|
|
is a comma separated list of the sha1 of the good commits.</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>If some of the merge bases are skipped, then the bisection process
|
||
|
|
continues, but the following message is printed for each skipped merge
|
||
|
|
base:</p>
|
||
|
|
</div>
|
||
|
|
<div class="listingblock">
|
||
|
|
<div class="content">
|
||
|
|
<pre>Warning: the merge base between BBBBBB and [GGGGGG,...] must be skipped.
|
||
|
|
So we cannot be sure the first bad commit is between MMMMMM and BBBBBB.
|
||
|
|
We continue anyway.</pre>
|
||
|
|
</div>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>where BBBBBB is the sha1 hash of the bad commit, MMMMMM is the sha1
|
||
|
|
hash of the merge base that is skipped and [GGGGGG,…​] is a comma
|
||
|
|
separated list of the sha1 of the good commits.</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>So if there is no bad merge base, the bisection process continues as
|
||
|
|
usual after this step.</p>
|
||
|
|
</div>
|
||
|
|
</div>
|
||
|
|
</div>
|
||
|
|
</div>
|
||
|
|
<div class="sect1">
|
||
|
|
<h2 id="_best_bisecting_practices">Best bisecting practices</h2>
|
||
|
|
<div class="sectionbody">
|
||
|
|
<div class="sect2">
|
||
|
|
<h3 id="_using_test_suites_and_git_bisect_together">Using test suites and git bisect together</h3>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>If you both have a test suite and use git bisect, then it becomes less
|
||
|
|
important to check that all tests pass after each commit. Though of
|
||
|
|
course it is probably a good idea to have some checks to avoid
|
||
|
|
breaking too many things because it could make bisecting other bugs
|
||
|
|
more difficult.</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>You can focus your efforts to check at a few points (for example rc
|
||
|
|
and beta releases) that all the T test cases pass for all the N
|
||
|
|
configurations. And when some tests don’t pass you can use "git
|
||
|
|
bisect" (or better "git bisect run"). So you should perform roughly:</p>
|
||
|
|
</div>
|
||
|
|
<div class="listingblock">
|
||
|
|
<div class="content">
|
||
|
|
<pre>c * N * T + b * M * log2(M) tests</pre>
|
||
|
|
</div>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>where c is the number of rounds of test (so a small constant) and b is
|
||
|
|
the ratio of bug per commit (hopefully a small constant too).</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>So of course it’s much better as it’s O(N * T) vs O(N * T * M) if
|
||
|
|
you would test everything after each commit.</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>This means that test suites are good to prevent some bugs from being
|
||
|
|
committed and they are also quite good to tell you that you have some
|
||
|
|
bugs. But they are not so good to tell you where some bugs have been
|
||
|
|
introduced. To tell you that efficiently, git bisect is needed.</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>The other nice thing with test suites, is that when you have one, you
|
||
|
|
already know how to test for bad behavior. So you can use this
|
||
|
|
knowledge to create a new test case for "git bisect" when it appears
|
||
|
|
that there is a regression. So it will be easier to bisect the bug and
|
||
|
|
fix it. And then you can add the test case you just created to your
|
||
|
|
test suite.</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>So if you know how to create test cases and how to bisect, you will be
|
||
|
|
subject to a virtuous circle:</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>more tests ⇒ easier to create tests ⇒ easier to bisect ⇒ more tests</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>So test suites and "git bisect" are complementary tools that are very
|
||
|
|
powerful and efficient when used together.</p>
|
||
|
|
</div>
|
||
|
|
</div>
|
||
|
|
<div class="sect2">
|
||
|
|
<h3 id="_bisecting_build_failures">Bisecting build failures</h3>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>You can very easily automatically bisect broken builds using something
|
||
|
|
like:</p>
|
||
|
|
</div>
|
||
|
|
<div class="listingblock">
|
||
|
|
<div class="content">
|
||
|
|
<pre>$ git bisect start BAD GOOD
|
||
|
|
$ git bisect run make</pre>
|
||
|
|
</div>
|
||
|
|
</div>
|
||
|
|
</div>
|
||
|
|
<div class="sect2">
|
||
|
|
<h3 id="_passing_sh_c_some_commands_to_git_bisect_run">Passing sh -c "some commands" to "git bisect run"</h3>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>For example:</p>
|
||
|
|
</div>
|
||
|
|
<div class="listingblock">
|
||
|
|
<div class="content">
|
||
|
|
<pre>$ git bisect run sh -c "make || exit 125; ./my_app | grep 'good output'"</pre>
|
||
|
|
</div>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>On the other hand if you do this often, then it can be worth having
|
||
|
|
scripts to avoid too much typing.</p>
|
||
|
|
</div>
|
||
|
|
</div>
|
||
|
|
<div class="sect2">
|
||
|
|
<h3 id="_finding_performance_regressions">Finding performance regressions</h3>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>Here is an example script that comes slightly modified from a real
|
||
|
|
world script used by Junio Hamano <a href="#4">[4]</a>.</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>This script can be passed to "git bisect run" to find the commit that
|
||
|
|
introduced a performance regression:</p>
|
||
|
|
</div>
|
||
|
|
<div class="listingblock">
|
||
|
|
<div class="content">
|
||
|
|
<pre>#!/bin/sh
|
||
|
|
|
||
|
|
# Build errors are not what I am interested in.
|
||
|
|
make my_app || exit 255
|
||
|
|
|
||
|
|
# We are checking if it stops in a reasonable amount of time, so
|
||
|
|
# let it run in the background...
|
||
|
|
|
||
|
|
./my_app >log 2>&1 &
|
||
|
|
|
||
|
|
# ... and grab its process ID.
|
||
|
|
pid=$!
|
||
|
|
|
||
|
|
# ... and then wait for sufficiently long.
|
||
|
|
sleep $NORMAL_TIME
|
||
|
|
|
||
|
|
# ... and then see if the process is still there.
|
||
|
|
if kill -0 $pid
|
||
|
|
then
|
||
|
|
# It is still running -- that is bad.
|
||
|
|
kill $pid; sleep 1; kill $pid;
|
||
|
|
exit 1
|
||
|
|
else
|
||
|
|
# It has already finished (the $pid process was no more),
|
||
|
|
# and we are happy.
|
||
|
|
exit 0
|
||
|
|
fi</pre>
|
||
|
|
</div>
|
||
|
|
</div>
|
||
|
|
</div>
|
||
|
|
<div class="sect2">
|
||
|
|
<h3 id="_following_general_best_practices">Following general best practices</h3>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>It is obviously a good idea not to have commits with changes that
|
||
|
|
knowingly break things, even if some other commits later fix the
|
||
|
|
breakage.</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>It is also a good idea when using any VCS to have only one small
|
||
|
|
logical change in each commit.</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>The smaller the changes in your commit, the most effective "git
|
||
|
|
bisect" will be. And you will probably need "git bisect" less in the
|
||
|
|
first place, as small changes are easier to review even if they are
|
||
|
|
only reviewed by the committer.</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>Another good idea is to have good commit messages. They can be very
|
||
|
|
helpful to understand why some changes were made.</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>These general best practices are very helpful if you bisect often.</p>
|
||
|
|
</div>
|
||
|
|
</div>
|
||
|
|
<div class="sect2">
|
||
|
|
<h3 id="_avoiding_bug_prone_merges">Avoiding bug prone merges</h3>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>First merges by themselves can introduce some regressions even when
|
||
|
|
the merge needs no source code conflict resolution. This is because a
|
||
|
|
semantic change can happen in one branch while the other branch is not
|
||
|
|
aware of it.</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>For example one branch can change the semantic of a function while the
|
||
|
|
other branch add more calls to the same function.</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>This is made much worse if many files have to be fixed to resolve
|
||
|
|
conflicts. That’s why such merges are called "evil merges". They can
|
||
|
|
make regressions very difficult to track down. It can even be
|
||
|
|
misleading to know the first bad commit if it happens to be such a
|
||
|
|
merge, because people might think that the bug comes from bad conflict
|
||
|
|
resolution when it comes from a semantic change in one branch.</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>Anyway "git rebase" can be used to linearize history. This can be used
|
||
|
|
either to avoid merging in the first place. Or it can be used to
|
||
|
|
bisect on a linear history instead of the non linear one, as this
|
||
|
|
should give more information in case of a semantic change in one
|
||
|
|
branch.</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>Merges can be also made simpler by using smaller branches or by using
|
||
|
|
many topic branches instead of only long version related branches.</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>And testing can be done more often in special integration branches
|
||
|
|
like linux-next for the linux kernel.</p>
|
||
|
|
</div>
|
||
|
|
</div>
|
||
|
|
<div class="sect2">
|
||
|
|
<h3 id="_adapting_your_work_flow">Adapting your work-flow</h3>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>A special work-flow to process regressions can give great results.</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>Here is an example of a work-flow used by Andreas Ericsson:</p>
|
||
|
|
</div>
|
||
|
|
<div class="ulist">
|
||
|
|
<ul>
|
||
|
|
<li>
|
||
|
|
<p>write, in the test suite, a test script that exposes the regression</p>
|
||
|
|
</li>
|
||
|
|
<li>
|
||
|
|
<p>use "git bisect run" to find the commit that introduced it</p>
|
||
|
|
</li>
|
||
|
|
<li>
|
||
|
|
<p>fix the bug that is often made obvious by the previous step</p>
|
||
|
|
</li>
|
||
|
|
<li>
|
||
|
|
<p>commit both the fix and the test script (and if needed more tests)</p>
|
||
|
|
</li>
|
||
|
|
</ul>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>And here is what Andreas said about this work-flow <a href="#5">[5]</a>:</p>
|
||
|
|
</div>
|
||
|
|
<div class="quoteblock">
|
||
|
|
<blockquote>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>To give some hard figures, we used to have an average report-to-fix
|
||
|
|
cycle of 142.6 hours (according to our somewhat weird bug-tracker
|
||
|
|
which just measures wall-clock time). Since we moved to Git, we’ve
|
||
|
|
lowered that to 16.2 hours. Primarily because we can stay on top of
|
||
|
|
the bug fixing now, and because everyone’s jockeying to get to fix
|
||
|
|
bugs (we’re quite proud of how lazy we are to let Git find the bugs
|
||
|
|
for us). Each new release results in ~40% fewer bugs (almost certainly
|
||
|
|
due to how we now feel about writing tests).</p>
|
||
|
|
</div>
|
||
|
|
</blockquote>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>Clearly this work-flow uses the virtuous circle between test suites
|
||
|
|
and "git bisect". In fact it makes it the standard procedure to deal
|
||
|
|
with regression.</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>In other messages Andreas says that they also use the "best practices"
|
||
|
|
described above: small logical commits, topic branches, no evil
|
||
|
|
merge,…​ These practices all improve the bisectability of the commit
|
||
|
|
graph, by making it easier and more useful to bisect.</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>So a good work-flow should be designed around the above points. That
|
||
|
|
is making bisecting easier, more useful and standard.</p>
|
||
|
|
</div>
|
||
|
|
</div>
|
||
|
|
<div class="sect2">
|
||
|
|
<h3 id="_involving_qa_people_and_if_possible_end_users">Involving QA people and if possible end users</h3>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>One nice about "git bisect" is that it is not only a developer
|
||
|
|
tool. It can effectively be used by QA people or even end users (if
|
||
|
|
they have access to the source code or if they can get access to all
|
||
|
|
the builds).</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>There was a discussion at one point on the linux kernel mailing list
|
||
|
|
of whether it was ok to always ask end user to bisect, and very good
|
||
|
|
points were made to support the point of view that it is ok.</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>For example David Miller wrote <a href="#6">[6]</a>:</p>
|
||
|
|
</div>
|
||
|
|
<div class="quoteblock">
|
||
|
|
<blockquote>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>What people don’t get is that this is a situation where the "end node
|
||
|
|
principle" applies. When you have limited resources (here: developers)
|
||
|
|
you don’t push the bulk of the burden upon them. Instead you push
|
||
|
|
things out to the resource you have a lot of, the end nodes (here:
|
||
|
|
users), so that the situation actually scales.</p>
|
||
|
|
</div>
|
||
|
|
</blockquote>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>This means that it is often "cheaper" if QA people or end users can do
|
||
|
|
it.</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>What is interesting too is that end users that are reporting bugs (or
|
||
|
|
QA people that reproduced a bug) have access to the environment where
|
||
|
|
the bug happens. So they can often more easily reproduce a
|
||
|
|
regression. And if they can bisect, then more information will be
|
||
|
|
extracted from the environment where the bug happens, which means that
|
||
|
|
it will be easier to understand and then fix the bug.</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>For open source projects it can be a good way to get more useful
|
||
|
|
contributions from end users, and to introduce them to QA and
|
||
|
|
development activities.</p>
|
||
|
|
</div>
|
||
|
|
</div>
|
||
|
|
<div class="sect2">
|
||
|
|
<h3 id="_using_complex_scripts">Using complex scripts</h3>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>In some cases like for kernel development it can be worth developing
|
||
|
|
complex scripts to be able to fully automate bisecting.</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>Here is what Ingo Molnar says about that <a href="#7">[7]</a>:</p>
|
||
|
|
</div>
|
||
|
|
<div class="quoteblock">
|
||
|
|
<blockquote>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>i have a fully automated bootup-hang bisection script. It is based on
|
||
|
|
"git-bisect run". I run the script, it builds and boots kernels fully
|
||
|
|
automatically, and when the bootup fails (the script notices that via
|
||
|
|
the serial log, which it continuously watches - or via a timeout, if
|
||
|
|
the system does not come up within 10 minutes it’s a "bad" kernel),
|
||
|
|
the script raises my attention via a beep and i power cycle the test
|
||
|
|
box. (yeah, i should make use of a managed power outlet to 100%
|
||
|
|
automate it)</p>
|
||
|
|
</div>
|
||
|
|
</blockquote>
|
||
|
|
</div>
|
||
|
|
</div>
|
||
|
|
<div class="sect2">
|
||
|
|
<h3 id="_combining_test_suites_git_bisect_and_other_systems_together">Combining test suites, git bisect and other systems together</h3>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>We have seen that test suites and git bisect are very powerful when
|
||
|
|
used together. It can be even more powerful if you can combine them
|
||
|
|
with other systems.</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>For example some test suites could be run automatically at night with
|
||
|
|
some unusual (or even random) configurations. And if a regression is
|
||
|
|
found by a test suite, then "git bisect" can be automatically
|
||
|
|
launched, and its result can be emailed to the author of the first bad
|
||
|
|
commit found by "git bisect", and perhaps other people too. And a new
|
||
|
|
entry in the bug tracking system could be automatically created too.</p>
|
||
|
|
</div>
|
||
|
|
</div>
|
||
|
|
</div>
|
||
|
|
</div>
|
||
|
|
<div class="sect1">
|
||
|
|
<h2 id="_the_future_of_bisecting">The future of bisecting</h2>
|
||
|
|
<div class="sectionbody">
|
||
|
|
<div class="sect2">
|
||
|
|
<h3 id="_git_replace">"git replace"</h3>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>We saw earlier that "git bisect skip" is now using a PRNG to try to
|
||
|
|
avoid areas in the commit graph where commits are untestable. The
|
||
|
|
problem is that sometimes the first bad commit will be in an
|
||
|
|
untestable area.</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>To simplify the discussion we will suppose that the untestable area is
|
||
|
|
a simple string of commits and that it was created by a breakage
|
||
|
|
introduced by one commit (let’s call it BBC for bisect breaking
|
||
|
|
commit) and later fixed by another one (let’s call it BFC for bisect
|
||
|
|
fixing commit).</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>For example:</p>
|
||
|
|
</div>
|
||
|
|
<div class="listingblock">
|
||
|
|
<div class="content">
|
||
|
|
<pre>...-Y-BBC-X1-X2-X3-X4-X5-X6-BFC-Z-...</pre>
|
||
|
|
</div>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>where we know that Y is good and BFC is bad, and where BBC and X1 to
|
||
|
|
X6 are untestable.</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>In this case if you are bisecting manually, what you can do is create
|
||
|
|
a special branch that starts just before the BBC. The first commit in
|
||
|
|
this branch should be the BBC with the BFC squashed into it. And the
|
||
|
|
other commits in the branch should be the commits between BBC and BFC
|
||
|
|
rebased on the first commit of the branch and then the commit after
|
||
|
|
BFC also rebased on.</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>For example:</p>
|
||
|
|
</div>
|
||
|
|
<div class="listingblock">
|
||
|
|
<div class="content">
|
||
|
|
<pre> (BBC+BFC)-X1'-X2'-X3'-X4'-X5'-X6'-Z'
|
||
|
|
/
|
||
|
|
...-Y-BBC-X1-X2-X3-X4-X5-X6-BFC-Z-...</pre>
|
||
|
|
</div>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>where commits quoted with ' have been rebased.</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>You can easily create such a branch with Git using interactive rebase.</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>For example using:</p>
|
||
|
|
</div>
|
||
|
|
<div class="listingblock">
|
||
|
|
<div class="content">
|
||
|
|
<pre>$ git rebase -i Y Z</pre>
|
||
|
|
</div>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>and then moving BFC after BBC and squashing it.</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>After that you can start bisecting as usual in the new branch and you
|
||
|
|
should eventually find the first bad commit.</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>For example:</p>
|
||
|
|
</div>
|
||
|
|
<div class="listingblock">
|
||
|
|
<div class="content">
|
||
|
|
<pre>$ git bisect start Z' Y</pre>
|
||
|
|
</div>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>If you are using "git bisect run", you can use the same manual fix up
|
||
|
|
as above, and then start another "git bisect run" in the special
|
||
|
|
branch. Or as the "git bisect" man page says, the script passed to
|
||
|
|
"git bisect run" can apply a patch before it compiles and test the
|
||
|
|
software <a href="#8">[8]</a>. The patch should turn a current untestable commits
|
||
|
|
into a testable one. So the testing will result in "good" or "bad" and
|
||
|
|
"git bisect" will be able to find the first bad commit. And the script
|
||
|
|
should not forget to remove the patch once the testing is done before
|
||
|
|
exiting from the script.</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>(Note that instead of a patch you can use "git cherry-pick BFC" to
|
||
|
|
apply the fix, and in this case you should use "git reset --hard
|
||
|
|
HEAD^" to revert the cherry-pick after testing and before returning
|
||
|
|
from the script.)</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>But the above ways to work around untestable areas are a little bit
|
||
|
|
clunky. Using special branches is nice because these branches can be
|
||
|
|
shared by developers like usual branches, but the risk is that people
|
||
|
|
will get many such branches. And it disrupts the normal "git bisect"
|
||
|
|
work-flow. So, if you want to use "git bisect run" completely
|
||
|
|
automatically, you have to add special code in your script to restart
|
||
|
|
bisection in the special branches.</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>Anyway one can notice in the above special branch example that the Z'
|
||
|
|
and Z commits should point to the same source code state (the same
|
||
|
|
"tree" in git parlance). That’s because Z' result from applying the
|
||
|
|
same changes as Z just in a slightly different order.</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>So if we could just "replace" Z by Z' when we bisect, then we would
|
||
|
|
not need to add anything to a script. It would just work for anyone in
|
||
|
|
the project sharing the special branches and the replacements.</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>With the example above that would give:</p>
|
||
|
|
</div>
|
||
|
|
<div class="listingblock">
|
||
|
|
<div class="content">
|
||
|
|
<pre> (BBC+BFC)-X1'-X2'-X3'-X4'-X5'-X6'-Z'-...
|
||
|
|
/
|
||
|
|
...-Y-BBC-X1-X2-X3-X4-X5-X6-BFC-Z</pre>
|
||
|
|
</div>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>That’s why the "git replace" command was created. Technically it
|
||
|
|
stores replacements "refs" in the "refs/replace/" hierarchy. These
|
||
|
|
"refs" are like branches (that are stored in "refs/heads/") or tags
|
||
|
|
(that are stored in "refs/tags"), and that means that they can
|
||
|
|
automatically be shared like branches or tags among developers.</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>"git replace" is a very powerful mechanism. It can be used to fix
|
||
|
|
commits in already released history, for example to change the commit
|
||
|
|
message or the author. And it can also be used instead of git "grafts"
|
||
|
|
to link a repository with another old repository.</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>In fact it’s this last feature that "sold" it to the Git community, so
|
||
|
|
it is now in the "master" branch of Git’s Git repository and it should
|
||
|
|
be released in Git 1.6.5 in October or November 2009.</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>One problem with "git replace" is that currently it stores all the
|
||
|
|
replacements refs in "refs/replace/", but it would be perhaps better
|
||
|
|
if the replacement refs that are useful only for bisecting would be in
|
||
|
|
"refs/replace/bisect/". This way the replacement refs could be used
|
||
|
|
only for bisecting, while other refs directly in "refs/replace/" would
|
||
|
|
be used nearly all the time.</p>
|
||
|
|
</div>
|
||
|
|
</div>
|
||
|
|
<div class="sect2">
|
||
|
|
<h3 id="_bisecting_sporadic_bugs">Bisecting sporadic bugs</h3>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>Another possible improvement to "git bisect" would be to optionally
|
||
|
|
add some redundancy to the tests performed so that it would be more
|
||
|
|
reliable when tracking sporadic bugs.</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>This has been requested by some kernel developers because some bugs
|
||
|
|
called sporadic bugs do not appear in all the kernel builds because
|
||
|
|
they are very dependent on the compiler output.</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>The idea is that every 3 test for example, "git bisect" could ask the
|
||
|
|
user to test a commit that has already been found to be "good" or
|
||
|
|
"bad" (because one of its descendants or one of its ancestors has been
|
||
|
|
found to be "good" or "bad" respectively). If it happens that a commit
|
||
|
|
has been previously incorrectly classified then the bisection can be
|
||
|
|
aborted early, hopefully before too many mistakes have been made. Then
|
||
|
|
the user will have to look at what happened and then restart the
|
||
|
|
bisection using a fixed bisect log.</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>There is already a project called BBChop created by Ealdwulf Wuffinga
|
||
|
|
on Github that does something like that using Bayesian Search Theory
|
||
|
|
<a href="#9">[9]</a>:</p>
|
||
|
|
</div>
|
||
|
|
<div class="quoteblock">
|
||
|
|
<blockquote>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>BBChop is like <em>git bisect</em> (or equivalent), but works when your bug
|
||
|
|
is intermittent. That is, it works in the presence of false negatives
|
||
|
|
(when a version happens to work this time even though it contains the
|
||
|
|
bug). It assumes that there are no false positives (in principle, the
|
||
|
|
same approach would work, but adding it may be non-trivial).</p>
|
||
|
|
</div>
|
||
|
|
</blockquote>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>But BBChop is independent of any VCS and it would be easier for Git
|
||
|
|
users to have something integrated in Git.</p>
|
||
|
|
</div>
|
||
|
|
</div>
|
||
|
|
</div>
|
||
|
|
</div>
|
||
|
|
<div class="sect1">
|
||
|
|
<h2 id="_conclusion">Conclusion</h2>
|
||
|
|
<div class="sectionbody">
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>We have seen that regressions are an important problem, and that "git
|
||
|
|
bisect" has nice features that complement very well practices and
|
||
|
|
other tools, especially test suites, that are generally used to fight
|
||
|
|
regressions. But it might be needed to change some work-flows and
|
||
|
|
(bad) habits to get the most out of it.</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>Some improvements to the algorithms inside "git bisect" are possible
|
||
|
|
and some new features could help in some cases, but overall "git
|
||
|
|
bisect" works already very well, is used a lot, and is already very
|
||
|
|
useful. To back up that last claim, let’s give the final word to Ingo
|
||
|
|
Molnar when he was asked by the author how much time does he think
|
||
|
|
"git bisect" saves him when he uses it:</p>
|
||
|
|
</div>
|
||
|
|
<div class="quoteblock">
|
||
|
|
<blockquote>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>a <em>lot</em>.</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>About ten years ago did i do my first <em>bisection</em> of a Linux patch
|
||
|
|
queue. That was prior the Git (and even prior the BitKeeper) days. I
|
||
|
|
literally days spent sorting out patches, creating what in essence
|
||
|
|
were standalone commits that i guessed to be related to that bug.</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>It was a tool of absolute last resort. I’d rather spend days looking
|
||
|
|
at printk output than do a manual <em>patch bisection</em>.</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>With Git bisect it’s a breeze: in the best case i can get a ~15 step
|
||
|
|
kernel bisection done in 20-30 minutes, in an automated way. Even with
|
||
|
|
manual help or when bisecting multiple, overlapping bugs, it’s rarely
|
||
|
|
more than an hour.</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>In fact it’s invaluable because there are bugs i would never even
|
||
|
|
<em>try</em> to debug if it wasn’t for git bisect. In the past there were bug
|
||
|
|
patterns that were immediately hopeless for me to debug - at best i
|
||
|
|
could send the crash/bug signature to lkml and hope that someone else
|
||
|
|
can think of something.</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>And even if a bisection fails today it tells us something valuable
|
||
|
|
about the bug: that it’s non-deterministic - timing or kernel image
|
||
|
|
layout dependent.</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>So git bisect is unconditional goodness - and feel free to quote that
|
||
|
|
;-)</p>
|
||
|
|
</div>
|
||
|
|
</blockquote>
|
||
|
|
</div>
|
||
|
|
</div>
|
||
|
|
</div>
|
||
|
|
<div class="sect1">
|
||
|
|
<h2 id="_acknowledgments">Acknowledgments</h2>
|
||
|
|
<div class="sectionbody">
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>Many thanks to Junio Hamano for his help in reviewing this paper, for
|
||
|
|
reviewing the patches I sent to the Git mailing list, for discussing
|
||
|
|
some ideas and helping me improve them, for improving "git bisect" a
|
||
|
|
lot and for his awesome work in maintaining and developing Git.</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>Many thanks to Ingo Molnar for giving me very useful information that
|
||
|
|
appears in this paper, for commenting on this paper, for his
|
||
|
|
suggestions to improve "git bisect" and for evangelizing "git bisect"
|
||
|
|
on the linux kernel mailing lists.</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>Many thanks to Linus Torvalds for inventing, developing and
|
||
|
|
evangelizing "git bisect", Git and Linux.</p>
|
||
|
|
</div>
|
||
|
|
<div class="paragraph">
|
||
|
|
<p>Many thanks to the many other great people who helped one way or
|
||
|
|
another when I worked on Git, especially to Andreas Ericsson, Johannes
|
||
|
|
Schindelin, H. Peter Anvin, Daniel Barkalow, Bill Lear, John Hawley,
|
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Shawn O. Pierce, Jeff King, Sam Vilain, Jon Seymour.</p>
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</div>
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<div class="paragraph">
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<p>Many thanks to the Linux-Kongress program committee for choosing the
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author to given a talk and for publishing this paper.</p>
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</div>
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</div>
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</div>
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<div class="sect1">
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<h2 id="_references">References</h2>
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<div class="sectionbody">
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<div class="ulist">
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<ul>
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<li>
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<p>[[[1]]] <a href="https://www.nist.gov/sites/default/files/documents/director/planning/report02-3.pdf"><em>The Economic Impacts of Inadequate Infratructure for Software Testing</em>. Nist Planning Report 02-3</a>, see Executive Summary and Chapter 8.</p>
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</li>
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||
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<li>
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||
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<p>[[[2]]] <a href="http://www.oracle.com/technetwork/java/codeconvtoc-136057.html"><em>Code Conventions for the Java Programming Language</em>. Sun Microsystems.</a></p>
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||
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</li>
|
||
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|
<li>
|
||
|
|
<p>[[[3]]] <a href="https://en.wikipedia.org/wiki/Software_maintenance"><em>Software maintenance</em>. Wikipedia.</a></p>
|
||
|
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</li>
|
||
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<li>
|
||
|
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<p>[[[4]]] <a href="https://public-inbox.org/git/7vps5xsbwp.fsf_-_@assigned-by-dhcp.cox.net/">Junio C Hamano. <em>Automated bisect success story</em>.</a></p>
|
||
|
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</li>
|
||
|
|
<li>
|
||
|
|
<p>[[[5]]] <a href="https://lwn.net/Articles/317154/">Christian Couder. <em>Fully automated bisecting with "git bisect run"</em>. LWN.net.</a></p>
|
||
|
|
</li>
|
||
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<li>
|
||
|
|
<p>[[[6]]] <a href="https://lwn.net/Articles/277872/">Jonathan Corbet. <em>Bisection divides users and developers</em>. LWN.net.</a></p>
|
||
|
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</li>
|
||
|
|
<li>
|
||
|
|
<p>[[[7]]] <a href="http://marc.info/?l=linux-kernel&m=119702753411680&w=2">Ingo Molnar. <em>Re: BUG 2.6.23-rc3 can’t see sd partitions on Alpha</em>. Linux-kernel mailing list.</a></p>
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</li>
|
||
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<li>
|
||
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<p>[[[8]]] <a href="https://www.kernel.org/pub/software/scm/git/docs/git-bisect.html">Junio C Hamano and the git-list. <em>git-bisect(1) Manual Page</em>. Linux Kernel Archives.</a></p>
|
||
|
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</li>
|
||
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<li>
|
||
|
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<p>[[[9]]] <a href="https://github.com/Ealdwulf/bbchop">Ealdwulf. <em>bbchop</em>. GitHub.</a></p>
|
||
|
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</li>
|
||
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</ul>
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||
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</div>
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</div>
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</div>
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</div>
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<div id="footer">
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<div id="footer-text">
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Last updated 2019-02-26 19:31:11 UTC
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