An environment variable indicating the tox worker number in --parallel

[nedbat]

What's the problem this feature will solve?

When using tox --parallel, coverage writes a number of data files and tries to combine them. But multiple processes are working in the same directory, creating a race condition. If we had an environment variable that distinguished among the parallel workers, we could name the data files differently and avoid collisions.

Here are reports of the problem:

• Race conditions when using 'tox --parallel' nedbat/coveragepy#1514
• Data files missing or empty when using tox --parallel auto nedbat/coveragepy#1642

Describe the solution you'd like

An environment variable called TOX_WORKER, a number for each worker, similar to pytest xdist's PYTEST_XDIST_WORKER.

[gaborbernat]

We already inject the talks environment name as an environment variable, any reason why that will not suffice?

[nedbat]

In this case, it looks like that will work. I think it could be useful to have the environment variable anyway.

[gaborbernat]

tox environments are unordered so assigning numbers doesn't make sense, so for now I'll say no to such request. The name is a better identifier.