Bisecting your favourite bug.

Ran into a new bug? Wasn't there with the last version? Want a developer to fix it? Too easy, just bisect it. And here's how:

First of all, we assume you've been running whatever your distribution ships. Find out that version and find the upstream repository. Now clone that repository and check out the matching version. For the example below, let's assume 1.2 is fine and 1.3 is broken.

$> git clone git://host/path/to/project
$> cd project
$> git tag
# find tag name for the broken version
$> git checkout project-1.3 # the broken version
$> ./configure $CONFIGUREFLAGS
$> make && make install
# test

You can get the configure flags from your distribution's build system. e.g. Fedora's koji. Look up the build of the version, go into the build.log and copy the configure flags from there. After that, the project will be installed in the same location, with the same flags as the rpm/deb/whatever file.
Verify it's still broken with this version, if not it might be distribution-specific patches. If it's still broken, check out the working version and verify that one too.

$> git checkout project-1.2 # the working version
$> make && make install
# test

So, you've just verified that vanilla 1.2 works and 1.3 doesn't? Great, off to bisecting.

$> git bisect start
$> git bisect good project-1.2
$> git bisect bad project-1.3

git now gives you a version in between those two, simply build and install it, test it and depending on whether it works or not, run "git bisect good" or "git bisect bad". After a number of test runs, git will tell you which commit introduced the bug. And you're done, send this commit info to the developers and they'll have a much easier time fixing the bug.

Now, of course nothing is as easy as it seems so you may see versions that you can't build (git bisect skip) or you'll run into dependency issues between versions so you can't bisect further. I've had some reasonable success with the process above and the smaller the range of commits you can provide to the developers the better.

You can use the same process for distribution-specific patches too. After checking out a a working version, just apply all patches from that distro in-order and then bisect between the working version and HEAD. Works like a treat.