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<title>Fighting regressions with git bisect</title>
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<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
combinatorial 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">