coverage.py can't trace child processes of a fork()

jmalicki avatarjmalicki created an issue

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.

Comments?

Comments (2)

  1. jmalicki

    coverage-3.3.1-fork.patch is the patch to fix coverage. testcover.py is the test case. Both with and without patch, do:

    coverage run -p testcover.py coverage combine coverage report -m

    and note the difference, in that with the patch the child shows coverage.

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