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1359 lines
48 KiB
1359 lines
48 KiB
15 years ago
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Fighting regressions with git bisect
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====================================
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:Author: Christian Couder
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:Email: chriscool@tuxfamily.org
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:Date: 2009/11/08
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Abstract
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--------
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"git bisect" enables software users and developers to easily find the
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commit that introduced a regression. We show why it is important to
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have good tools to fight regressions. We describe how "git bisect"
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works from the outside and the algorithms it uses inside. Then we
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explain how to take advantage of "git bisect" to improve current
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practices. And we discuss how "git bisect" could improve in the
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future.
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Introduction to "git bisect"
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----------------------------
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Git is a Distributed Version Control system (DVCS) created by Linus
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Torvalds and maintained by Junio Hamano.
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In Git like in many other Version Control Systems (VCS), the different
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states of the data that is managed by the system are called
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commits. And, as VCS are mostly used to manage software source code,
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sometimes "interesting" changes of behavior in the software are
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introduced in some commits.
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In fact people are specially interested in commits that introduce a
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"bad" behavior, called a bug or a regression. They are interested in
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these commits because a commit (hopefully) contains a very small set
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of source code changes. And it's much easier to understand and
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properly fix a problem when you only need to check a very small set of
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changes, than when you don't know where look in the first place.
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So to help people find commits that introduce a "bad" behavior, the
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"git bisect" set of commands was invented. And it follows of course
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that in "git bisect" parlance, commits where the "interesting
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behavior" is present are called "bad" commits, while other commits are
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called "good" commits. And a commit that introduce the behavior we are
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interested in is called a "first bad commit". Note that there could be
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more than one "first bad commit" in the commit space we are searching.
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So "git bisect" is designed to help find a "first bad commit". And to
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be as efficient as possible, it tries to perform a binary search.
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Fighting regressions overview
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-----------------------------
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Regressions: a big problem
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~~~~~~~~~~~~~~~~~~~~~~~~~~
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Regressions are a big problem in the software industry. But it's
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difficult to put some real numbers behind that claim.
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There are some numbers about bugs in general, like a NIST study in
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2002 <<1>> that said:
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_____________
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Software bugs, or errors, are so prevalent and so detrimental that
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they cost the U.S. economy an estimated $59.5 billion annually, or
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about 0.6 percent of the gross domestic product, according to a newly
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released study commissioned by the Department of Commerce's National
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Institute of Standards and Technology (NIST). At the national level,
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over half of the costs are borne by software users and the remainder
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by software developers/vendors. The study also found that, although
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all errors cannot be removed, more than a third of these costs, or an
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estimated $22.2 billion, could be eliminated by an improved testing
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infrastructure that enables earlier and more effective identification
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and removal of software defects. These are the savings associated with
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finding an increased percentage (but not 100 percent) of errors closer
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to the development stages in which they are introduced. Currently,
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over half of all errors are not found until "downstream" in the
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development process or during post-sale software use.
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_____________
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And then:
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_____________
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Software developers already spend approximately 80 percent of
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development costs on identifying and correcting defects, and yet few
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products of any type other than software are shipped with such high
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levels of errors.
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_____________
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Eventually the conclusion started with:
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_____________
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The path to higher software quality is significantly improved software
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testing.
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_____________
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There are other estimates saying that 80% of the cost related to
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software is about maintenance <<2>>.
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Though, according to Wikipedia <<3>>:
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_____________
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A common perception of maintenance is that it is merely fixing
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bugs. However, studies and surveys over the years have indicated that
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the majority, over 80%, of the maintenance effort is used for
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non-corrective actions (Pigosky 1997). This perception is perpetuated
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by users submitting problem reports that in reality are functionality
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enhancements to the system.
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_____________
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But we can guess that improving on existing software is very costly
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because you have to watch out for regressions. At least this would
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make the above studies consistent among themselves.
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Of course some kind of software is developed, then used during some
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time without being improved on much, and then finally thrown away. In
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this case, of course, regressions may not be a big problem. But on the
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other hand, there is a lot of big software that is continually
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developed and maintained during years or even tens of years by a lot
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of people. And as there are often many people who depend (sometimes
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critically) on such software, regressions are a really big problem.
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One such software is the linux kernel. And if we look at the linux
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kernel, we can see that a lot of time and effort is spent to fight
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regressions. The release cycle start with a 2 weeks long merge
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window. Then the first release candidate (rc) version is tagged. And
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after that about 7 or 8 more rc versions will appear with around one
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week between each of them, before the final release.
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The time between the first rc release and the final release is
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supposed to be used to test rc versions and fight bugs and especially
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regressions. And this time is more than 80% of the release cycle
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time. But this is not the end of the fight yet, as of course it
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continues after the release.
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And then this is what Ingo Molnar (a well known linux kernel
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developer) says about his use of git bisect:
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_____________
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I most actively use it during the merge window (when a lot of trees
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get merged upstream and when the influx of bugs is the highest) - and
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yes, there have been cases that i used it multiple times a day. My
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average is roughly once a day.
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_____________
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So regressions are fought all the time by developers, and indeed it is
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well known that bugs should be fixed as soon as possible, so as soon
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as they are found. That's why it is interesting to have good tools for
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this purpose.
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Other tools to fight regressions
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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So what are the tools used to fight regressions? They are nearly the
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same as those used to fight regular bugs. The only specific tools are
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test suites and tools similar as "git bisect".
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Test suites are very nice. But when they are used alone, they are
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supposed to be used so that all the tests are checked after each
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commit. This means that they are not very efficient, because many
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tests are run for no interesting result, and they suffer from
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combinational explosion.
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In fact the problem is that big software often has many different
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configuration options and that each test case should pass for each
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configuration after each commit. So if you have for each release: N
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configurations, M commits and T test cases, you should perform:
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-------------
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N * M * T tests
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-------------
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where N, M and T are all growing with the size your software.
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So very soon it will not be possible to completely test everything.
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And if some bugs slip through your test suite, then you can add a test
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to your test suite. But if you want to use your new improved test
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suite to find where the bug slipped in, then you will either have to
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emulate a bisection process or you will perhaps bluntly test each
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commit backward starting from the "bad" commit you have which may be
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very wasteful.
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"git bisect" overview
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---------------------
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Starting a bisection
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~~~~~~~~~~~~~~~~~~~~
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The first "git bisect" subcommand to use is "git bisect start" to
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start the search. Then bounds must be set to limit the commit
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space. This is done usually by giving one "bad" and at least one
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"good" commit. They can be passed in the initial call to "git bisect
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start" like this:
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-------------
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$ git bisect start [BAD [GOOD...]]
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-------------
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or they can be set using:
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-------------
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$ git bisect bad [COMMIT]
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-------------
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and:
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-------------
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$ git bisect good [COMMIT...]
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-------------
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where BAD, GOOD and COMMIT are all names that can be resolved to a
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commit.
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Then "git bisect" will checkout a commit of its choosing and ask the
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user to test it, like this:
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-------------
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$ git bisect start v2.6.27 v2.6.25
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Bisecting: 10928 revisions left to test after this (roughly 14 steps)
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[2ec65f8b89ea003c27ff7723525a2ee335a2b393] x86: clean up using max_low_pfn on 32-bit
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-------------
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Note that the example that we will use is really a toy example, we
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will be looking for the first commit that has a version like
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"2.6.26-something", that is the commit that has a "SUBLEVEL = 26" line
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in the top level Makefile. This is a toy example because there are
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better ways to find this commit with git than using "git bisect" (for
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example "git blame" or "git log -S<string>").
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Driving a bisection manually
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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At this point there are basically 2 ways to drive the search. It can
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be driven manually by the user or it can be driven automatically by a
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script or a command.
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If the user is driving it, then at each step of the search, the user
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will have to test the current commit and say if it is "good" or "bad"
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using the "git bisect good" or "git bisect bad" commands respectively
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that have been described above. For example:
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-------------
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$ git bisect bad
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Bisecting: 5480 revisions left to test after this (roughly 13 steps)
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[66c0b394f08fd89236515c1c84485ea712a157be] KVM: kill file->f_count abuse in kvm
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-------------
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And after a few more steps like that, "git bisect" will eventually
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find a first bad commit:
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-------------
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$ git bisect bad
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2ddcca36c8bcfa251724fe342c8327451988be0d is the first bad commit
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commit 2ddcca36c8bcfa251724fe342c8327451988be0d
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Author: Linus Torvalds <torvalds@linux-foundation.org>
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Date: Sat May 3 11:59:44 2008 -0700
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Linux 2.6.26-rc1
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:100644 100644 5cf8258195331a4dbdddff08b8d68642638eea57 4492984efc09ab72ff6219a7bc21fb6a957c4cd5 M Makefile
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-------------
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At this point we can see what the commit does, check it out (if it's
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not already checked out) or tinker with it, for example:
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-------------
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$ git show HEAD
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commit 2ddcca36c8bcfa251724fe342c8327451988be0d
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Author: Linus Torvalds <torvalds@linux-foundation.org>
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Date: Sat May 3 11:59:44 2008 -0700
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Linux 2.6.26-rc1
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diff --git a/Makefile b/Makefile
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index 5cf8258..4492984 100644
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--- a/Makefile
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+++ b/Makefile
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@@ -1,7 +1,7 @@
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VERSION = 2
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PATCHLEVEL = 6
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-SUBLEVEL = 25
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-EXTRAVERSION =
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+SUBLEVEL = 26
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+EXTRAVERSION = -rc1
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NAME = Funky Weasel is Jiggy wit it
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# *DOCUMENTATION*
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-------------
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And when we are finished we can use "git bisect reset" to go back to
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the branch we were in before we started bisecting:
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-------------
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$ git bisect reset
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Checking out files: 100% (21549/21549), done.
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Previous HEAD position was 2ddcca3... Linux 2.6.26-rc1
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Switched to branch 'master'
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-------------
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Driving a bisection automatically
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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The other way to drive the bisection process is to tell "git bisect"
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to launch a script or command at each bisection step to know if the
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current commit is "good" or "bad". To do that, we use the "git bisect
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run" command. For example:
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-------------
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$ git bisect start v2.6.27 v2.6.25
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Bisecting: 10928 revisions left to test after this (roughly 14 steps)
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[2ec65f8b89ea003c27ff7723525a2ee335a2b393] x86: clean up using max_low_pfn on 32-bit
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$
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$ git bisect run grep '^SUBLEVEL = 25' Makefile
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running grep ^SUBLEVEL = 25 Makefile
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Bisecting: 5480 revisions left to test after this (roughly 13 steps)
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[66c0b394f08fd89236515c1c84485ea712a157be] KVM: kill file->f_count abuse in kvm
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running grep ^SUBLEVEL = 25 Makefile
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SUBLEVEL = 25
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Bisecting: 2740 revisions left to test after this (roughly 12 steps)
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[671294719628f1671faefd4882764886f8ad08cb] V4L/DVB(7879): Adding cx18 Support for mxl5005s
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...
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...
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running grep ^SUBLEVEL = 25 Makefile
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Bisecting: 0 revisions left to test after this (roughly 0 steps)
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[2ddcca36c8bcfa251724fe342c8327451988be0d] Linux 2.6.26-rc1
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running grep ^SUBLEVEL = 25 Makefile
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2ddcca36c8bcfa251724fe342c8327451988be0d is the first bad commit
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commit 2ddcca36c8bcfa251724fe342c8327451988be0d
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Author: Linus Torvalds <torvalds@linux-foundation.org>
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Date: Sat May 3 11:59:44 2008 -0700
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Linux 2.6.26-rc1
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:100644 100644 5cf8258195331a4dbdddff08b8d68642638eea57 4492984efc09ab72ff6219a7bc21fb6a957c4cd5 M Makefile
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bisect run success
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-------------
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In this example, we passed "grep '^SUBLEVEL = 25' Makefile" as
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parameter to "git bisect run". This means that at each step, the grep
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command we passed will be launched. And if it exits with code 0 (that
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means success) then git bisect will mark the current state as
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"good". If it exits with code 1 (or any code between 1 and 127
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included, except the special code 125), then the current state will be
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marked as "bad".
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Exit code between 128 and 255 are special to "git bisect run". They
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make it stop immediately the bisection process. This is useful for
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example if the command passed takes too long to complete, because you
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can kill it with a signal and it will stop the bisection process.
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It can also be useful in scripts passed to "git bisect run" to "exit
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255" if some very abnormal situation is detected.
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Avoiding untestable commits
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~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Sometimes it happens that the current state cannot be tested, for
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example if it does not compile because there was a bug preventing it
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at that time. This is what the special exit code 125 is for. It tells
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"git bisect run" that the current commit should be marked as
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untestable and that another one should be chosen and checked out.
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If the bisection process is driven manually, you can use "git bisect
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skip" to do the same thing. (In fact the special exit code 125 makes
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"git bisect run" use "git bisect skip" in the background.)
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Or if you want more control, you can inspect the current state using
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for example "git bisect visualize". It will launch gitk (or "git log"
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if the DISPLAY environment variable is not set) to help you find a
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better bisection point.
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Either way, if you have a string of untestable commits, it might
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happen that the regression you are looking for has been introduced by
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one of these untestable commits. In this case it's not possible to
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tell for sure which commit introduced the regression.
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So if you used "git bisect skip" (or the run script exited with
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special code 125) you could get a result like this:
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-------------
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There are only 'skip'ped commits left to test.
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The first bad commit could be any of:
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15722f2fa328eaba97022898a305ffc8172db6b1
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78e86cf3e850bd755bb71831f42e200626fbd1e0
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e15b73ad3db9b48d7d1ade32f8cd23a751fe0ace
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070eab2303024706f2924822bfec8b9847e4ac1b
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We cannot bisect more!
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-------------
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Saving a log and replaying it
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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If you want to show other people your bisection process, you can get a
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log using for example:
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|
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-------------
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$ git bisect log > bisect_log.txt
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-------------
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And it is possible to replay it using:
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-------------
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$ git bisect replay bisect_log.txt
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-------------
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"git bisect" details
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--------------------
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Bisection algorithm
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||
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~~~~~~~~~~~~~~~~~~~
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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.
|
||
|
|
||
|
So the algorithm used by "git bisect" to find the best bisection
|
||
|
commit when there are no skipped commits is the following:
|
||
|
|
||
|
1) keep only the commits that:
|
||
|
|
||
|
a) are ancestor of the "bad" commit (including the "bad" commit itself),
|
||
|
b) are not ancestor of a "good" commit (excluding the "good" commits).
|
||
|
|
||
|
This means that we get rid of the uninteresting commits in the DAG.
|
||
|
|
||
|
For example if we start with a graph like this:
|
||
|
|
||
|
-------------
|
||
|
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 ->
|
||
|
-------------
|
||
|
|
||
|
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:
|
||
|
|
||
|
-------------
|
||
|
W-W-W
|
||
|
\
|
||
|
W-W-B
|
||
|
/
|
||
|
W---W
|
||
|
-------------
|
||
|
|
||
|
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.
|
||
|
|
||
|
Note for git users, that it is equivalent as keeping only the commit
|
||
|
given by:
|
||
|
|
||
|
-------------
|
||
|
git rev-list BAD --not GOOD1 GOOD2...
|
||
|
-------------
|
||
|
|
||
|
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:
|
||
|
|
||
|
-------------
|
||
|
G-W-W-W-B
|
||
|
/
|
||
|
Z-Z
|
||
|
-------------
|
||
|
|
||
|
2) starting from the "good" ends of the graph, associate to each
|
||
|
commit the number of ancestors it has plus one
|
||
|
|
||
|
For example with the following graph where H is the "bad" commit and A
|
||
|
and D are some parents of some "good" commits:
|
||
|
|
||
|
-------------
|
||
|
A-B-C
|
||
|
\
|
||
|
F-G-H
|
||
|
/
|
||
|
D---E
|
||
|
-------------
|
||
|
|
||
|
this will give:
|
||
|
|
||
|
-------------
|
||
|
1 2 3
|
||
|
A-B-C
|
||
|
\6 7 8
|
||
|
F-G-H
|
||
|
1 2/
|
||
|
D---E
|
||
|
-------------
|
||
|
|
||
|
3) associate to each commit: min(X, N - X)
|
||
|
|
||
|
where X is the value associated to the commit in step 2) and N is the
|
||
|
total number of commits in the graph.
|
||
|
|
||
|
In the above example we have N = 8, so this will give:
|
||
|
|
||
|
-------------
|
||
|
1 2 3
|
||
|
A-B-C
|
||
|
\2 1 0
|
||
|
F-G-H
|
||
|
1 2/
|
||
|
D---E
|
||
|
-------------
|
||
|
|
||
|
4) the best bisection point is the commit with the highest associated
|
||
|
number
|
||
|
|
||
|
So in the above example the best bisection point is commit C.
|
||
|
|
||
|
5) note that some shortcuts are implemented to speed up the algorithm
|
||
|
|
||
|
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.
|
||
|
|
||
|
Bisection algorithm debugging
|
||
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
||
|
|
||
|
For any commit graph, you can see the number associated with each
|
||
|
commit using "git rev-list --bisect-all".
|
||
|
|
||
|
For example, for the above graph, a command like:
|
||
|
|
||
|
-------------
|
||
|
$ git rev-list --bisect-all BAD --not GOOD1 GOOD2
|
||
|
-------------
|
||
|
|
||
|
would output something like:
|
||
|
|
||
|
-------------
|
||
|
e15b73ad3db9b48d7d1ade32f8cd23a751fe0ace (dist=3)
|
||
|
15722f2fa328eaba97022898a305ffc8172db6b1 (dist=2)
|
||
|
78e86cf3e850bd755bb71831f42e200626fbd1e0 (dist=2)
|
||
|
a1939d9a142de972094af4dde9a544e577ddef0e (dist=2)
|
||
|
070eab2303024706f2924822bfec8b9847e4ac1b (dist=1)
|
||
|
a3864d4f32a3bf5ed177ddef598490a08760b70d (dist=1)
|
||
|
a41baa717dd74f1180abf55e9341bc7a0bb9d556 (dist=1)
|
||
|
9e622a6dad403b71c40979743bb9d5be17b16bd6 (dist=0)
|
||
|
-------------
|
||
|
|
||
|
Bisection algorithm discussed
|
||
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
||
|
|
||
|
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".
|
||
|
|
||
|
This means that the best bisection commits are the commits where the
|
||
|
following function is maximum:
|
||
|
|
||
|
-------------
|
||
|
f(X) = min(information_if_good(X), information_if_bad(X))
|
||
|
-------------
|
||
|
|
||
|
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.
|
||
|
|
||
|
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).
|
||
|
|
||
|
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..
|
||
|
|
||
|
And let's take a commit X in the graph.
|
||
|
|
||
|
If X is found to be "good", then we know that its ancestors are all
|
||
|
"good", so we want to say that:
|
||
|
|
||
|
-------------
|
||
|
information_if_good(X) = number_of_ancestors(X) (TRUE)
|
||
|
-------------
|
||
|
|
||
|
And this is true because at step 1) b) we remove the ancestors of the
|
||
|
"good" commits.
|
||
|
|
||
|
If X is found to be "bad", then we know that its descendants are all
|
||
|
"bad", so we want to say that:
|
||
|
|
||
|
-------------
|
||
|
information_if_bad(X) = number_of_descendants(X) (WRONG)
|
||
|
-------------
|
||
|
|
||
|
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.
|
||
|
|
||
|
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:
|
||
|
|
||
|
-------------
|
||
|
information_if_bad(X) = N - number_of_ancestors(X) (TRUE)
|
||
|
-------------
|
||
|
|
||
|
where N is the number of commits in the (cleaned up) graph.
|
||
|
|
||
|
So in the end this means that to find the best bisection commits we
|
||
|
should maximize the function:
|
||
|
|
||
|
-------------
|
||
|
f(X) = min(number_of_ancestors(X), N - number_of_ancestors(X))
|
||
|
-------------
|
||
|
|
||
|
And this is nice because at step 2) we compute number_of_ancestors(X)
|
||
|
and so at step 3) we compute f(X).
|
||
|
|
||
|
Let's take the following graph as an example:
|
||
|
|
||
|
-------------
|
||
|
G-H-I-J
|
||
|
/ \
|
||
|
A-B-C-D-E-F O
|
||
|
\ /
|
||
|
K-L-M-N
|
||
|
-------------
|
||
|
|
||
|
If we compute the following non optimal function on it:
|
||
|
|
||
|
-------------
|
||
|
g(X) = min(number_of_ancestors(X), number_of_descendants(X))
|
||
|
-------------
|
||
|
|
||
|
we get:
|
||
|
|
||
|
-------------
|
||
|
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
|
||
|
-------------
|
||
|
|
||
|
but with the algorithm used by git bisect we get:
|
||
|
|
||
|
-------------
|
||
|
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
|
||
|
-------------
|
||
|
|
||
|
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).
|
||
|
|
||
|
So the current algorithm seems to be the best possible given what we
|
||
|
initially supposed.
|
||
|
|
||
|
Skip algorithm
|
||
|
~~~~~~~~~~~~~~
|
||
|
|
||
|
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:
|
||
|
|
||
|
6) sort the commit by decreasing associated value
|
||
|
|
||
|
7) if the first commit has not been skipped, we can return it and stop
|
||
|
here
|
||
|
|
||
|
8) otherwise filter out all the skipped commits in the sorted list
|
||
|
|
||
|
9) use a pseudo random number generator (PRNG) to generate a random
|
||
|
number between 0 and 1
|
||
|
|
||
|
10) multiply this random number with its square root to bias it toward
|
||
|
0
|
||
|
|
||
|
11) multiply the result by the number of commits in the filtered list
|
||
|
to get an index into this list
|
||
|
|
||
|
12) return the commit at the computed index
|
||
|
|
||
|
Skip algorithm discussed
|
||
|
~~~~~~~~~~~~~~~~~~~~~~~~
|
||
|
|
||
|
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).
|
||
|
|
||
|
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.
|
||
|
|
||
|
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.
|
||
|
|
||
|
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.
|
||
|
|
||
|
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).
|
||
|
|
||
|
This is why it is a bad idea to just chose the next best unskipped
|
||
|
bisection commit when the first one has been skipped.
|
||
|
|
||
|
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.
|
||
|
|
||
|
So using a PRNG with a bias to favor commits away from the good and
|
||
|
bad commits looked like a good choice.
|
||
|
|
||
|
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.
|
||
|
|
||
|
Checking merge bases
|
||
|
~~~~~~~~~~~~~~~~~~~~
|
||
|
|
||
|
There is another tweak in the bisection algorithm that has not been
|
||
|
described in the "bisection algorithm" above.
|
||
|
|
||
|
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".
|
||
|
|
||
|
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.
|
||
|
|
||
|
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:
|
||
|
|
||
|
-------------
|
||
|
A-B-C-D-E-F-G <--main
|
||
|
\
|
||
|
H-I-J <--dev
|
||
|
-------------
|
||
|
|
||
|
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.
|
||
|
|
||
|
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.
|
||
|
|
||
|
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.
|
||
|
|
||
|
So we would be left with only:
|
||
|
|
||
|
-------------
|
||
|
H-I-J
|
||
|
-------------
|
||
|
|
||
|
But what happens if the first bad commit is "B" and if it has been
|
||
|
fixed in the "main" branch by commit "F"?
|
||
|
|
||
|
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!
|
||
|
|
||
|
And yes it's 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.
|
||
|
|
||
|
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:
|
||
|
|
||
|
-------------
|
||
|
$ git bisect start dev main
|
||
|
-------------
|
||
|
|
||
|
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.
|
||
|
|
||
|
If it happens that one merge base is bad, then the bisection process
|
||
|
is stopped with a message like:
|
||
|
|
||
|
-------------
|
||
|
The merge base BBBBBB is bad.
|
||
|
This means the bug has been fixed between BBBBBB and [GGGGGG,...].
|
||
|
-------------
|
||
|
|
||
|
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.
|
||
|
|
||
|
If some of the merge bases are skipped, then the bisection process
|
||
|
continues, but the following message is printed for each skipped merge
|
||
|
base:
|
||
|
|
||
|
-------------
|
||
|
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.
|
||
|
-------------
|
||
|
|
||
|
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.
|
||
|
|
||
|
So if there is no bad merge base, the bisection process continues as
|
||
|
usual after this step.
|
||
|
|
||
|
Best bisecting practices
|
||
|
------------------------
|
||
|
|
||
|
Using test suites and git bisect together
|
||
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
||
|
|
||
|
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.
|
||
|
|
||
|
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:
|
||
|
|
||
|
-------------
|
||
|
c * N * T + b * M * log2(M) tests
|
||
|
-------------
|
||
|
|
||
|
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).
|
||
|
|
||
|
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.
|
||
|
|
||
|
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.
|
||
|
|
||
|
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.
|
||
|
|
||
|
So if you know how to create test cases and how to bisect, you will be
|
||
|
subject to a virtuous circle:
|
||
|
|
||
|
more tests => easier to create tests => easier to bisect => more tests
|
||
|
|
||
|
So test suites and "git bisect" are complementary tools that are very
|
||
|
powerful and efficient when used together.
|
||
|
|
||
|
Bisecting build failures
|
||
|
~~~~~~~~~~~~~~~~~~~~~~~~
|
||
|
|
||
|
You can very easily automatically bisect broken builds using something
|
||
|
like:
|
||
|
|
||
|
-------------
|
||
|
$ git bisect start BAD GOOD
|
||
|
$ git bisect run make
|
||
|
-------------
|
||
|
|
||
|
Passing sh -c "some commands" to "git bisect run"
|
||
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
||
|
|
||
|
For example:
|
||
|
|
||
|
-------------
|
||
|
$ git bisect run sh -c "make || exit 125; ./my_app | grep 'good output'"
|
||
|
-------------
|
||
|
|
||
|
On the other hand if you do this often, then it can be worth having
|
||
|
scripts to avoid too much typing.
|
||
|
|
||
|
Finding performance regressions
|
||
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
||
|
|
||
|
Here is an example script that comes slightly modified from a real
|
||
|
world script used by Junio Hamano <<4>>.
|
||
|
|
||
|
This script can be passed to "git bisect run" to find the commit that
|
||
|
introduced a performance regression:
|
||
|
|
||
|
-------------
|
||
|
#!/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
|
||
|
-------------
|
||
|
|
||
|
Following general best practices
|
||
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
||
|
|
||
|
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.
|
||
|
|
||
|
It is also a good idea when using any VCS to have only one small
|
||
|
logical change in each commit.
|
||
|
|
||
|
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 commiter.
|
||
|
|
||
|
Another good idea is to have good commit messages. They can be very
|
||
|
helpful to understand why some changes were made.
|
||
|
|
||
|
These general best practices are very helpful if you bisect often.
|
||
|
|
||
|
Avoiding bug prone merges
|
||
|
~~~~~~~~~~~~~~~~~~~~~~~~~
|
||
|
|
||
|
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.
|
||
|
|
||
|
For example one branch can change the semantic of a function while the
|
||
|
other branch add more calls to the same function.
|
||
|
|
||
|
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.
|
||
|
|
||
|
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.
|
||
|
|
||
|
Merges can be also made simpler by using smaller branches or by using
|
||
|
many topic branches instead of only long version related branches.
|
||
|
|
||
|
And testing can be done more often in special integration branches
|
||
|
like linux-next for the linux kernel.
|
||
|
|
||
|
Adapting your work-flow
|
||
|
~~~~~~~~~~~~~~~~~~~~~~~
|
||
|
|
||
|
A special work-flow to process regressions can give great results.
|
||
|
|
||
|
Here is an example of a work-flow used by Andreas Ericsson:
|
||
|
|
||
|
* write, in the test suite, a test script that exposes the regression
|
||
|
* use "git bisect run" to find the commit that introduced it
|
||
|
* fix the bug that is often made obvious by the previous step
|
||
|
* commit both the fix and the test script (and if needed more tests)
|
||
|
|
||
|
And here is what Andreas said about this work-flow <<5>>:
|
||
|
|
||
|
_____________
|
||
|
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).
|
||
|
_____________
|
||
|
|
||
|
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.
|
||
|
|
||
|
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.
|
||
|
|
||
|
So a good work-flow should be designed around the above points. That
|
||
|
is making bisecting easier, more useful and standard.
|
||
|
|
||
|
Involving QA people and if possible end users
|
||
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
||
|
|
||
|
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).
|
||
|
|
||
|
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.
|
||
|
|
||
|
For example David Miller wrote <<6>>:
|
||
|
|
||
|
_____________
|
||
|
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.
|
||
|
_____________
|
||
|
|
||
|
This means that it is often "cheaper" if QA people or end users can do
|
||
|
it.
|
||
|
|
||
|
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.
|
||
|
|
||
|
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.
|
||
|
|
||
|
Using complex scripts
|
||
|
~~~~~~~~~~~~~~~~~~~~~
|
||
|
|
||
|
In some cases like for kernel development it can be worth developing
|
||
|
complex scripts to be able to fully automate bisecting.
|
||
|
|
||
|
Here is what Ingo Molnar says about that <<7>>:
|
||
|
|
||
|
_____________
|
||
|
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)
|
||
|
_____________
|
||
|
|
||
|
Combining test suites, git bisect and other systems together
|
||
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
||
|
|
||
|
We have seen that test suites an git bisect are very powerful when
|
||
|
used together. It can be even more powerful if you can combine them
|
||
|
with other systems.
|
||
|
|
||
|
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.
|
||
|
|
||
|
|
||
|
The future of bisecting
|
||
|
-----------------------
|
||
|
|
||
|
"git replace"
|
||
|
~~~~~~~~~~~~~
|
||
|
|
||
|
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.
|
||
|
|
||
|
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).
|
||
|
|
||
|
For example:
|
||
|
|
||
|
-------------
|
||
|
...-Y-BBC-X1-X2-X3-X4-X5-X6-BFC-Z-...
|
||
|
-------------
|
||
|
|
||
|
where we know that Y is good and BFC is bad, and where BBC and X1 to
|
||
|
X6 are untestable.
|
||
|
|
||
|
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.
|
||
|
|
||
|
For example:
|
||
|
|
||
|
-------------
|
||
|
(BBC+BFC)-X1'-X2'-X3'-X4'-X5'-X6'-Z'
|
||
|
/
|
||
|
...-Y-BBC-X1-X2-X3-X4-X5-X6-BFC-Z-...
|
||
|
-------------
|
||
|
|
||
|
where commits quoted with ' have been rebased.
|
||
|
|
||
|
You can easily create such a branch with Git using interactive rebase.
|
||
|
|
||
|
For example using:
|
||
|
|
||
|
-------------
|
||
|
$ git rebase -i Y Z
|
||
|
-------------
|
||
|
|
||
|
and then moving BFC after BBC and squashing it.
|
||
|
|
||
|
After that you can start bisecting as usual in the new branch and you
|
||
|
should eventually find the first bad commit.
|
||
|
|
||
|
For example:
|
||
|
|
||
|
-------------
|
||
|
$ git bisect start Z' Y
|
||
|
-------------
|
||
|
|
||
|
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 <<8>>. 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.
|
||
|
|
||
|
(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.)
|
||
|
|
||
|
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.
|
||
|
|
||
|
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.
|
||
|
|
||
|
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.
|
||
|
|
||
|
With the example above that would give:
|
||
|
|
||
|
-------------
|
||
|
(BBC+BFC)-X1'-X2'-X3'-X4'-X5'-X6'-Z'-...
|
||
|
/
|
||
|
...-Y-BBC-X1-X2-X3-X4-X5-X6-BFC-Z
|
||
|
-------------
|
||
|
|
||
|
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.
|
||
|
|
||
|
"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.
|
||
|
|
||
|
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.
|
||
|
|
||
|
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.
|
||
|
|
||
|
Bisecting sporadic bugs
|
||
|
~~~~~~~~~~~~~~~~~~~~~~~
|
||
|
|
||
|
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.
|
||
|
|
||
|
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.
|
||
|
|
||
|
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.
|
||
|
|
||
|
There is already a project called BBChop created by Ealdwulf Wuffinga
|
||
|
on Github that does something like that using Bayesian Search Theory
|
||
|
<<9>>:
|
||
|
|
||
|
_____________
|
||
|
BBChop is like 'git bisect' (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).
|
||
|
_____________
|
||
|
|
||
|
But BBChop is independent of any VCS and it would be easier for Git
|
||
|
users to have something integrated in Git.
|
||
|
|
||
|
Conclusion
|
||
|
----------
|
||
|
|
||
|
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.
|
||
|
|
||
|
Some improvements to the algorithms inside "git bisect" are possible
|
||
|
and some new features could help in some cases, but overall "git
|
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bisect" works already very well, is used a lot, and is already very
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|
useful. To back up that last claim, let's give the final word to Ingo
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|
Molnar when he was asked by the author how much time does he think
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|
"git bisect" saves him when he uses it:
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|
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|
_____________
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|
a _lot_.
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|
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|
About ten years ago did i do my first 'bisection' of a Linux patch
|
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|
queue. That was prior the Git (and even prior the BitKeeper) days. I
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|
literally days spent sorting out patches, creating what in essence
|
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|
were standalone commits that i guessed to be related to that bug.
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|
|
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|
It was a tool of absolute last resort. I'd rather spend days looking
|
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|
at printk output than do a manual 'patch bisection'.
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|
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||
|
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
|
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|
manual help or when bisecting multiple, overlapping bugs, it's rarely
|
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|
more than an hour.
|
||
|
|
||
|
In fact it's invaluable because there are bugs i would never even
|
||
|
_try_ 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.
|
||
|
|
||
|
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.
|
||
|
|
||
|
So git bisect is unconditional goodness - and feel free to quote that
|
||
|
;-)
|
||
|
_____________
|
||
|
|
||
|
Acknowledgements
|
||
|
----------------
|
||
|
|
||
|
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.
|
||
|
|
||
|
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.
|
||
|
|
||
|
Many thanks to Linus Torvalds for inventing, developing and
|
||
|
evangelizing "git bisect", Git and Linux.
|
||
|
|
||
|
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,
|
||
|
Shawn O. Pierce, Jeff King, Sam Vilain, Jon Seymour.
|
||
|
|
||
|
Many thanks to the Linux-Kongress program committee for choosing the
|
||
|
author to given a talk and for publishing this paper.
|
||
|
|
||
|
References
|
||
|
----------
|
||
|
|
||
|
- [[[1]]] http://www.nist.gov/public_affairs/releases/n02-10.htm['Software Errors Cost U.S. Economy $59.5 Billion Annually'. Nist News Release.]
|
||
|
- [[[2]]] http://java.sun.com/docs/codeconv/html/CodeConventions.doc.html#16712['Code Conventions for the Java Programming Language'. Sun Microsystems.]
|
||
|
- [[[3]]] http://en.wikipedia.org/wiki/Software_maintenance['Software maintenance'. Wikipedia.]
|
||
|
- [[[4]]] http://article.gmane.org/gmane.comp.version-control.git/45195/[Junio C Hamano. 'Automated bisect success story'. Gmane.]
|
||
|
- [[[5]]] http://lwn.net/Articles/317154/[Christian Couder. 'Fully automated bisecting with "git bisect run"'. LWN.net.]
|
||
|
- [[[6]]] http://lwn.net/Articles/277872/[Jonathan Corbet. 'Bisection divides users and developers'. LWN.net.]
|
||
|
- [[[7]]] http://article.gmane.org/gmane.linux.scsi/36652/[Ingo Molnar. 'Re: BUG 2.6.23-rc3 can't see sd partitions on Alpha'. Gmane.]
|
||
|
- [[[8]]] http://www.kernel.org/pub/software/scm/git/docs/git-bisect.html[Junio C Hamano and the git-list. 'git-bisect(1) Manual Page'. Linux Kernel Archives.]
|
||
|
- [[[9]]] http://github.com/Ealdwulf/bbchop[Ealdwulf. 'bbchop'. GitHub.]
|