> Did your PR fix the issue?

The bug was that *sometimes* on Travis CI, and only on Travis CI (!?), writing 1 MiB into the multiprocessing pipe didn't block. The bug is really strange because it is only reproduced on the clang Linux job of Travis CI which runs tests in parallel. Not on the Linux gcc which runs tests sequentially in coverage. Moreover, the failure only occurs for a specific order of tests.

You can easily reproduce the issue if you reduce the size of the data written into the pipe at the end of _test_ignore(). If the write (send_bytes) doesn't block, you get the same error.

I'm confident that writing 4 MiB instead of 1 MiB will fix the issue. I saw the test passing with 4 MiB whereas it failed with 1 MiB, when I fixed the test order.