Example using git bisect to narrow in on a commit

I learned about git bisect from this Stack Overflow poll: What are your favorite git features or tricks? I thought it was so cool that I wanted to share an example.

After upgrading from Django 1.2.3 to Django 1.3, something broke on the website I was working on. To figure out what was wrong, I used git bisect to find the Django revision that introduced the relevant change. I cloned the Django github repo and pip install -e'd it into my virtualenv. Then I used git bisect as follows:

• Start git bisect

  $ git bisect start 
  

  $ git bisect bad 1.3 
  

  $ git bisect good 1.2.3 
  Bisecting: a merge base must be tested
    [618153bd3b047529f6cfb50917c88270f30e8ea8] Fixed #10843: the textile tests now pass against the latest textile library.
  

  Note: 1.3 and 1.2.3 are git tags in the Django git repo. Commit SHA1 hashes can be used instead.

• Start the development server in another terminal. Load the webpage in the browser. The page loads without error.

  [04/Aug/2011 19:13:20] "GET /my/webpage/ HTTP/1.1" 200 16279

• Mark the revision good

  $ git bisect good 
  Bisecting: 774 revisions left to test after this (roughly 10 steps)
  [39591cddccffdf3b66cfaaa60b95257daa4ef8c5] Fixed #14781 - Setting "CACHE_PREFIX" should be "CACHE_KEY_PREFIX". Thanks to adamv for report and patch.
  

• Reload the page. This time it throws a 500 error.

  [04/Aug/2011 19:13:59] "GET /my/webpage/ HTTP/1.1" 500 118861

• Mark the revision bad

  $ git bisect bad 
  Bisecting: 387 revisions left to test after this (roughly 9 steps)
  [6630bd24f45c7967c49b90adf82c63dd7d93e6f7] Fixed #13863 -- Corrected decimal separator in Swedish format module. Thanks, Ulf Urdén.
  

• Reload

  [04/Aug/2011 19:15:45] "GET /my/webpage/ HTTP/1.1" 500 117536

• Mark the revision bad

  $ git bisect bad 
  Bisecting: 193 revisions left to test after this (roughly 8 steps)
  [4d19282beb19b95e73aec119b40ed40456065a21] Migrated custom_methods doctests. Thanks to Alex Gaynor.
  

• Etc...

  [04/Aug/2011 19:16:17] "GET /my/webpage/ HTTP/1.1" 500 117536

  $ git bisect bad 
  Bisecting: 96 revisions left to test after this (roughly 7 steps)
  [ed63689b9e3be80c3af51bfc5d6805bcedbae9f0] Added file missing from r13590.
  

  [04/Aug/2011 19:16:46] "GET /my/webpage/ HTTP/1.1" 200 16279

  $ git bisect good 
  Bisecting: 48 revisions left to test after this (roughly 6 steps)
  [2eca0cd7f5fe6efd1f177724a948cdde7475f54c] Fixed #13754 - Add a note about a test client session property gotcha
  

  [04/Aug/2011 19:17:12] "GET /my/webpage/ HTTP/1.1" 200 16279

  $ git bisect good 
  Bisecting: 24 revisions left to test after this (roughly 5 steps)
  [d7e3c7bad40e5f66169b8b951d74c87103ece07d] Fixed #13095 -- `formfield_callback` keyword argument is now more sane and works with widgets defined in `ModelForm.Meta.widgets`.  Thanks, hvdklauw for bug report, vung for initial patch, and carljm for review.
  

  [04/Aug/2011 19:17:31] "GET /my/webpage/ HTTP/1.1" 200 16279

  $ git bisect good 
  [I forgot to copy/paste the console output for this one]
  

  [04/Aug/2011 19:17:54] "GET /my/webpage/ HTTP/1.1" 500 117536

  $ git bisect bad 
  Bisecting: 5 revisions left to test after this (roughly 3 steps)
  [622bf3b2199e5e7015edccb00f1ab694291ca121] Fixed #11905: Raise an error on model form creation if a non-existent field was listed in fields. Thanks ben and copelco.
  

  [04/Aug/2011 19:18:17] "GET /my/webpage/ HTTP/1.1" 500 117536

  $ git bisect bad 
  Bisecting: 2 revisions left to test after this (roughly 2 steps)
  [57427e9d758eb37b43f6b7804b24c6f47c2fa456] Fixed a test so that it actually tests what it's supposed to test.
  

  [04/Aug/2011 19:18:37] "GET /my/webpage/ HTTP/1.1" 200 16279

  $ git bisect good 
  Bisecting: 0 revisions left to test after this (roughly 1 step)
  [dcb12158881cdcc619de0ae6d3f6cf674a0d4abb] Better error message for calling get_next_by_* on unsaved models. Patch from Marc Fargas. Fixed #7435.
  

  [04/Aug/2011 19:19:07] "GET /my/webpage/ HTTP/1.1" 200 16279

• After marking the last commit, git bisect shows the first bad commit as 622bf3b2199e5e7015edccb00f1ab694291ca121

  $ git bisect good
  622bf3b2199e5e7015edccb00f1ab694291ca121 is the first bad commit
  commit 622bf3b2199e5e7015edccb00f1ab694291ca121
  Author: kmtracey <kmtracey>
  Date:   Sat Sep 11 01:39:16 2010 +0000
  
      Fixed #11905: Raise an error on model form creation if a non-existent field was listed in fields. Thanks ben and copelco.
      
      
      git-svn-id: http://code.djangoproject.com/svn/django/trunk@13739 bcc190cf-cafb-0310-a4f2-bffc1f526a37
  
  :040000 040000 87ec1601b970938b55382a5aa66db74716552c3d 2d418dfe8dd9f67fea408e022aa13838086ce33b M      django
  :040000 040000 054e6f42b3b467e47f295f8fda6cdc8a7f2c054c 52ba5265ba9d6458f23c8cb4a8a9bdcdbfc1f590 M      tests</kmtracey>
  

Note: checking for failed unit tests instead of checking for a 500 error in the browser is another (probably better) way to test with git bisect.

Note 2: The problem with my code was that I was adding extra fields to a model formset in the wrong way. I was trying to use add_fields on a base model formset instead of creating a custom Form with the extra field and specifying it in the factory. This Stack Overflow answer solved the problem for me.

Extra details

Here's how I installed Django from github

• Clone the Django repo and pip install -e it in my virtualenv

  $ git clone https://github.com/django/django.git django-github 
  $ cd django-github 
  $ workon myenv 
  $ pip install -e ./ 
  

• In another terminal run the Django development server

  $ cd /myproject 
  $ workon myenv 
  $ python manage.py runserver