hg precommit hooks and the Clang Static Analyzer

2009-04-03 09:24:09 UTC

Fraser Speirs has a post about configuring your Git repository to vet commits with the Clang Static Analyzer.

The idea of pre-commit hooks is that you get to run a script before the commit happens. Depending on the result code of the script, the commit will either proceed or be aborted.

I wrote a wrapper around the scan-build tool, so that I could run the analyzer by hand with my preferred options at any time: …

The –status-bugs flag is the trick here: it makes scan-build return a non-zero status code if it detects bugs. That’s what we want with Git pre-commit hooks: a non-zero status indicates a possible bug, and that causes the Git commit to be aborted.

Mercurial, of course, has the same feature. The hg book has instructions; I’ll show you how I set up my repository to do this.

First, I created a shell script named RunClang.zsh:

#!/bin/zsh -f
~/bin/checker-latest/scan-build \
    -checker-cfref -warn-objc-methodsigs -warn-objc-missing-dealloc -warn-objc-unused-ivars \
    --status-bugs -o checker.out \
    xcodebuild -configuration $1

Next, I added my precommit hook to the repository’s .hg/hgrc file:

[hooks]
precommit.RunClang = ~/bin/RunClang.zsh Development

Here’s what an example session looks like:

% echo 'Testing precommit testing hook' >> test.txt
% hg ci -m 'Testing precommit testing hook'
[churn churn churn]
** BUILD SUCCEEDED **
scan-build: 17 bugs found.
scan-build: Run 'scan-view [snip]/growl-boredzo-precommit-test/checker.out/2009-04-03-2' to examine bug reports.
abort: precommit.RunClang hook exited with status 1

(255)% hg log --limit=1
changeset:   4188:b208862a586d
tag:         tip
user:        Peter Hosey
date:        Fri Mar 13 05:40:09 2009 -0700
summary:     Fix encoding of the Norwegian Growl-WithInstaller strings file.

17 bugs—mostly leaks. Glad I didn’t commit this test file!

So now you know how to have Mercurial block you from committing if the clang checker can find bugs. This should also work for your unit tests. And if you have 100% test coverage (lucky!), you can combine them: have scan-build build your test-bundle target. Then, the hook will prevent the commit if the checker can find bugs or any tests fail.

I don’t think I’ll actually use this set-up, though.

First, in order to find bugs, you need to build your entire main product. Any significantly large program is going to take a long time to build and analyze—Growl, for example, takes about one-and-a-quarter minutes for a clean build. Even committing to a Subversion repository over dial-up was quicker.

More significantly, precommit hooks like this interfere with patch queues. The mq extension implements patches as mutable commits, so any qnew or qrefresh will run the hook. It would be useful on qfinish, but it’s just annoying on qnew and especially qrefresh, as the all-too-frequent builds thwart rapid iteration.

So, if you use patch queues, this won’t work for you. But, if you don’t, then this should work as well in Mercurial as it does in Git.

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