[issue11978] Report correct coverage.py data for tests that invoke subprocesses

[Brett Cannon]

Brett Cannon added the comment:

I learned a few thing trying to make this work.

One is that the COVERAGE_PROCESS_START does need to point to an actual .ini file (which includes an empty file).

Two, https://bitbucket.org/ned/coveragepy/src/63dfe482a849e9cb0b82214242315a806a6a3d53/coverage/control.py?at=default&fileviewer=file-view-default#control.py-1265 shows that stdlib coverage is left off by this, so even if you do turn on tracing in a subprocess, it will still ignore the stdlib. That means the coverage config file must turn on stdlib coverage in order or it to be picked up in the subprocesses.

Three, you need to run `coverage combine` to create a unified .coverage file for `coverage report` to pick up (I don't know if codecov explicitly needs the merge step.

When I have time I'll make a PR to test if this change actually helps speed things up.

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue11978>
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