bpo-41544: Add missing parameters to dummy.DummyProcess and dummy.Pool

[moi90]

Add missing parameters to multiprocessing.dummy.DummyProcess in order to match the API of multiprocessing.Process.

https://bugs.python.org/issue41544

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[ZackerySpytz]

This needs a unit test.

[moi90]

It seems to me that there are not tests at all for DummyProcess. Is that right?
Any directions where to put them and how to get startet?

I'd appreciate any help with this.

[moi90]

I added test_multiprocessing_dummy and it runs successfully. It shouldn't, however, because install_tests_in_module_dict calls multiprocessing.set_start_method("dummy") (in setUpModule) which should fail with a ValueError that is converted to a unittest.SkipTest. But the test is not skipped. Can someone explain this behavior?

EDIT: It seems that if setUpModule raises SkipTest, the file is displayed as "successful" by the test runner. Isn't that inaccurate?

In my eyes, to make use of the current test machinery, we need to make "dummy" a supported (but maybe undocumented) start method.

EDIT: I see why this doesn't work: The threading module requires a properly initialized multiprocessing context (which of cause can not be a threading-based dummy).

[moi90]

I moved on to checking only the signatures. Also, I added the missing maxtasksperchild for dummy.Pool. Technically, this would be another pull request but I think it can be included here.

[moi90]

@ZackerySpytz What do you think?

@applio, @pitrou Could you have a look?