The Git bisect command

From the Git documentation:

This command uses a binary search algorithm to find which commit in your project’s history introduced a bug.

But what is a binary search algorithm?

Binary search algorithm

It searches for a value inside an ordered list/array

Easy example:
we are searching for number 4
inside the [1...9] interval

Step 1

Step 2

Step 3

My use case: Paraunit

facile-it/paraunit

• https://github.com/facile-it/paraunit
• Tool for running multiple PHPUnit test in parallel
• Faster test builds!
• Built with Symfony components

The landscape

I use Paraunit in 2 different projects:

1. One (big) project,
   based onto Symfony 2.8
2. One (smaller, newer) project,
   based onto Symfony 3.0

Note: the 2.8 version in the bridge between 2.7 and 3.0,
so they receive the same patches

Here comes the bug!

A few months ago,
I was upgrading to the latest available patch...

2.8.3 2.8.4
3.0.3 3.0.4

• Functional tests failing randomly
• I wasn't able to reproduce the issue outside my projects
• I had to debug it from my vendor folder

Setup

$ rm -rf vendor/symfony/symfony
                            

$ composer install --prefer-source

Loading composer repositories with package information
Installing dependencies (including require-dev) from lock file
  - Installing symfony/symfony (v2.8.9)
    Cloning df02dd5d3f7decb3a05c6d0f31054b4263625dcb

[...]
                            

$ cd vendor/symfony/symfony
$ git status
HEAD detached at v2.8.9
nothing to commit, working directory clean
                            

Let's start!

$ git bisect start
                        

$ git checkout v2.8.3
Previous HEAD position was df02dd5... Merge pull request #19487 from fabpot/release-2.8.9
HEAD is now at 7a9a5fc... Merge pull request #17952 from fabpot/release-2.8.3
                            
$ git bisect good
                        

$ git checkout v2.8.4
Previous HEAD position was 7a9a5fc... Merge pull request #17952 from fabpot/release-2.8.3
HEAD is now at 9e14f9f... Merge branch '2.7' into 2.8
                            
$ git bisect bad
Bisecting: 154 revisions left to test after this (roughly 7 steps)
[ff93c1a0266d766c0643c6fd862b00fe0e4dd54e] bug #18087 [WebProfiler] Sidebar button padding (rvanlaak)
                        

Moving forward

$ git bisect bad
Bisecting: 37 revisions left to test after this (roughly 5 steps)
[4fece2877337b182927331a728a3bed90f9fb394] bug #18175 [Translation] Add support for fuzzy tags in PoFileLoader (nud)
                        

Roughly 5 steps later...

$ git bisect bad
b2293515a491db91e8ee85178558461374161a77 is the first bad commit
commit b2293515a491db91e8ee85178558461374161a77
Author: Nicolas Grekas <nicolas.grekas@gmail.com>
Date:   Wed Mar 16 16:22:42 2016 +0100

    [PhpUnitBridge] Revert 7f62133939b1172e1c9924d211e1d315230b3be8

:040000 040000 4cf2abdee62e5708f91a8025e81a1fad5a91e154 182f7ddb10abb302c077a8a248cf285fd1a43c2e M	src
                        

Using alternate terms

• If you don't like good and bad,
  you can use old and new!
• You shouldn't mix different pairs of terms;
  use git bisect terms to get a reminder
• You can also define your own terms!

  
  $ git bisect start --term-old fast --term-new slow
                                  

Manual checkout

$ git checkout baf35cf3c0e09a2de186e135cfcc001956a923f0
                            
$ git bisect bad
Bisecting: 2 revisions left to test after this (roughly 1 step)
[478c49a8aae66c4701ccd8fcba979cb17c038327] update CONTRIBUTORS for 2.3.38
                        

Skipping

$ git bisect skip
Bisecting: 37 revisions left to test after this (roughly 5 steps)
[2a2aefa4b7c3ee573f0d11b36ce4411787b95180] minor #18119 [Hack Day] Issue for newcomers (2). Replace perfom by perform (JuanLuisGarciaBorrego)
                        

If you skip too much...

$ git bisect skip
                            
There are only 'skip'ped commits left to test.
The first bad commit could be any of:
478c49a8aae66c4701ccd8fcba979cb17c038327
84203be72071a16aa8e552b88effc63d64abab62
78f4e00140b9925eb83df48e19e33642b7a25db9
c51503db3f4e8f22a075cc9a226cfe7c42a7663e
baf35cf3c0e09a2de186e135cfcc001956a923f0
We cannot bisect more!