Checking mochitest test coverage

Firefox Download ButtonOne of the last bugs for Firefox Panorama was bug 625818: “Check Panorama mochitest test suite coverage”. Our automated tests ensure that we do not regress on existing functionality, but it’s only as good as its coverage: how much of the Panorama code base is actually being “hit” through the process of running the test suite.

Panorama went through a pretty rapid development cycle, making it into Firefox 4 which was released today (yay!). Moreover, for a while we were developing outside of mozilla-central, without the regular “patches require tests” requirement. This makes checking its test coverage particularly important.

Check out the final result, the Panorama test coverage report. The good news: our code coverage is 86%! (Some notes on what improvements can be made are in the bug.)

code coverage report

PhiliKON had previously worked on hooking into the JS Debugger service’s interruptHook to test xpcshell tests. I modified this code to run instead in the Mochitest browser chrome tests. This code can be found on the bug.

With this patch applied, I invoked the test suite with the following code: TEST_PATH=browser/base/content/tests/tabview COVERAGE_FILTER="*tabview*" COVERAGE=true make -C obj-ff-dbg mochitest-browser-chrome . That’s a regular mochitest-browser-chrome invocation with the COVERAGE=true flag which turns on code coverage checking, and COVERAGE_FILTER=*tabview* which filters out results from files which don’t have “tabview” in their paths. This creates a file called coverage.json in the working directory of the test suite, meaning, for me, obj-ff-dbg/_tests/testing/mochitest/.

This JSON file is a multidimensional array, with file paths and then line numbers as keys. The file paths here, as best as possible, have been converted into local filesystem paths. PhiliKON built a script which produces beautiful reports based on this output.

A word of warning: running with this JSD interruptHook is ridiculously slow. A number of tests for Panorama are timing-dependent (drag-drop tests, for example), making some of them fail, but that’s okay… as long as it completed not via a timeout, it actually did run through all the code. In order to get this to run through everything with some degree of control, I split up the mochitest tabview suite in to a few chunks. I then took the multiple resulting coverage.json files and passed them into another script, in tools/coverage/, which takes multiple JSON results like this and puts them together into a single JSON file. I then passed this aggregate JSON file to PhiliKON’s wonderful report script and—voila—the Panorama test coverage report! Easy as pie.