Automated bisect success story

[Junio C Hamano <junkio@cox.net>]

Learning from example by Steven Grimm, let me share a success
story.

Earlier I noticed that "fsck --full" from 'master' took forever
in linux-2.6 repository, but the one from 'maint' finished in 2
to 3 minutes.

We recently had a few enhancements by Christian Couder to
git-bisect, and this was a perfect opportunity to see how well
they worked:

(1) "git bisect start" now takes one bad and then one or more
    good commits, before suggesting the first revision to try.

Traditionally, immediately after you gave a bad and a good
commit, it did a single bisection and then a checkout.  This
avoids repeated bisect computation and checkout when you know
more than one good revisions before starting to bisect, and also
let you bootstrap with a single command (you could instead give
one good commit at a time and then finally a single bad commit
to avoid the waste).

Not only I know 'maint' is good, I also know that the tips of
"foreign projects" merged to git.git, that do not share any
codepath the fsck takes, are irrelevant to the problem.  So I
want to mark tips of commit ancestry I merged from git-gui
projects as good.  Hence:

	$ git bisect start master maint remotes/git-gui/master

Mnemonic.  Start takes a Bad before Goods, because B comes
before G.

(2) "git bisect run <script>" takes a script to judge the
    goodness of the given revision.  Because I know each round
    of test takes around 3 minutes, I wrote a little script to
    automate the process and gave it to "git bisect run":

	$ git bisect run ./+run-script

This ran for a while (I do not know how long it took -- I was
away from the machine and doing other things) and came back with
the "object decoration" one Linus has fixed yesterday with his
patch.

Here is the "+run-script".  I have git.git repository and
linux-2.6 repository next to each other.

-- >8 --
#!/bin/sh

# Build errors are not what I am interested in.
make git-fsck && cd ../linux-2.6 || exit 255

# We are checking if it stops in a reasonable amount of time, so
# let it run in the background...

../git.git/git-fsck --full >:log 2>&1 &

# ... and grab its process ID.
fsck=$!

# ... and then wait for sufficiently long.
sleep 240

# ... and then see if the process is still there.
if kill -0 $fsck
then
	# It is still running -- that is bad.
        # Three-kill is just a ritual and has no real meaning.
        # It is like "sync;sync;sync;reboot".
	kill $fsck; sleep 1; kill $fsck; sleep 1; kill $fsck;
	exit 1
else
	# It has already finished (the $fsck process was no more),
        # and we are happy.
	exit 0
fi