git bisect run

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This feature of git is too cool not to blog about: git bisect, and more specifically, git bisect run.

“git bisect” is a tool that facilitates a binary search of changes to your git repository to help find where a bug was introduced. You can walk through the process manually using “git bisect {good,bad,skip}“, or if you can write a script that automates checking for the bug, you can use “git bisect run scriptname” to have git do all the work for you.

The script should return 0 if the bug does not exist, and some other number (except 125) if the bug does exist.

In my case, Cappuccino’s “steam” build tool was failing, so I wrote a simple script that would test it by trying to run “steam” on Foundation:

#!/bin/sh

# install a known working copy of the build tools
pushd ../tmp/Tools
sudo sh install-tools
popd

# build whatever version of Cappuccino git bisect has checked out for us
rm -rf $STEAM_BUILD
ant release

# install the freshly built tools
pushd $STEAM_BUILD/Cappuccino/Tools
sudo sh install-tools
popd

# run steam on Foundation to see if the built tools work
rm -rf $STEAM_BUILD
pushd Foundation
steam
RETURN=$?
popd

# return the recorded return value
exit $RETURN

I provided “git bisect start” with a known bad commit and a known good commit, then ran “git bisect run”:

git bisect start c1e882ace1dd29aea98d9247db304fe5d5077df7 d6c0f8802a2fd3a07e14418de7744ae04ae4499e
git bisect run ../test.sh

Sure enough, a few minutes later “git bisect” reported exactly which commit caused the problem:

01177a7e0237b1bd026cf0c4ca923fced8536772 is first bad commit
commit 01177a7e0237b1bd026cf0c4ca923fced8536772
Author: name removed to protect the not-so-innocent

Date:   Tue Sep 30 17:29:56 2008 -0700

   Fix keys conflict in CPDictionary

   [#77 state:resolved]

:040000 040000 bf4ceafbf439aa54790fc57a2a78dec8283abadf b269f85f3f4ed4d08e4c8261ee409e9d88255b1d M    AppKit
:040000 040000 f9c1fdb3c02c7c899285c8cb8123b29e99a17e46 48cc5aefde707119fe956dd882dcd8f8182e7012 M    Foundation
:040000 040000 b780743d17ddb0ebda188703725e9dabf23fd458 c72750dabd99ca42d9144f3074c68c42b727d028 M    Objective-J
bisect run success