coverage.py can't trace child processes of a fork()
I wanted to get code coverage for tests of a project that makes heavy use of python 2.6 multiprocessing, and discovered I wasn't getting any coverage data for the child processes, though they were obviously running. This is even with the -p option.
I distilled it down to a simple test case that called os.fork() and ran a few lines of code in the child. (will attach).
I then discovered that the problem is that in parallel mode, the filename (including pid) is decided when the CollectorData object is *created*, rather than saved.
When one postpones creating the filename suffix until save() time, as the to-be-attached patch does, it works beautifully for fork() as far as I can tell in initial testing.
The attached patch should only change behavior when the suffix is due to parallel=True, not when an explicit suffix is given.
Of course, the fork()'d processes will have duplicate coverage data from their parents. But as long as coverage.py only tracks whether or not a line of code was run or branch taken, not how many times, this duplication is a non-issue, as the results from combine will be what is desired.
I haven't done extensive testing yet, and couldn't find a test suite for coverage.py? But thus far it's working for me.