git bisect: A practical example with yum

update: This post is now on my new blog.

I used git bisect to track down a bug in yum last night. It was so easy and practical that I figured I should record it here, so that others might want to give git a try.

I was attempting to install mutt, and yum failed (printing a traceback) after the rpms had been downloaded, but before the test transaction finished. So I started git bisect, and marked the current point as bad:

$> git bisect start
$> git bisect bad


The yum 3.1.0 release didn’t have this bug (it was the version I had installed at the time), so I marked it as good:

$> git bisect good yum-3-1-0
Bisecting: 15 revisions left to test after this
[1d0454af41ef6361604cafa8c7a13d80bc183c63] make it so that we see that the local rpm is present and then don't download


Git automatically checks out the next revision for you to test. This one happened to be good, so I marked it as such. I continued to test and mark revisions as either good or bad, until:

$> git bisect bad
832814e6b037621c4f26ee6a47e4b7b6dc7eb073 is first bad commit
commit 832814e6b037621c4f26ee6a47e4b7b6dc7eb073
Author: XXX
Date: XXX
XXXXXXXXXXXXXXXX
:100644 100644 8ea07cda8441687da2f0e3dd794c3a1c50a0f161 567ef25557eacbd932bc5f8c20cd34e49c169f57 M cli.py
:100644 100644 50fb320c9c31a0f394985e244dc35b9766fb28ce 3875b70c4f8a7b6a9cf7d06de6df47e8a0ae5777 M yum-updatesd.py
:040000 040000 28296caad31015e1573b19dd84d12c2e3db2b90b 98048391465ca3da06c210d6f45c3f234dc12e0a M yum


At this point, with the traceback and the diff from the commit, it was easy enough figure out what the problem was, and commit a fix.