Emscripten test worker sometimes fails witth "BlockingIOError: write could not complete without blocking"

tiran

Bug report

Sometimes test worker processes on wasm32-emscripten build bot fails with BlockingIOError: write could not complete without blocking. Example https://buildbot.python.org/all/#/builders/1044/builds/27/steps/10/logs/stdio

0:37:30 load avg: 4.42 [422/436/3] test_pathlib crashed (Exit code 1)
... 
result=Traceback (most recent call last):
  File "<frozen runpy>", line 198, in _run_module_as_main
  File "<frozen runpy>", line 88, in _run_code
  File "/opt/buildbot/bcannon-wasm/3.x.bcannon-wasm.emscripten-node/build/Lib/test/regrtest.py", line 47, in <module>
    _main()
    ^^^^^^^
  File "/opt/buildbot/bcannon-wasm/3.x.bcannon-wasm.emscripten-node/build/Lib/test/regrtest.py", line 43, in _main
    main()
    ^^^^^^
  File "/opt/buildbot/bcannon-wasm/3.x.bcannon-wasm.emscripten-node/build/Lib/test/libregrtest/main.py", line 754, in main
    Regrtest().main(tests=tests, **kwargs)
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/buildbot/bcannon-wasm/3.x.bcannon-wasm.emscripten-node/build/Lib/test/libregrtest/main.py", line 692, in main
    self._main(tests, kwargs)
    ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/buildbot/bcannon-wasm/3.x.bcannon-wasm.emscripten-node/build/Lib/test/libregrtest/main.py", line 713, in _main
    run_tests_worker(self.ns, self.worker_test_name)
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/buildbot/bcannon-wasm/3.x.bcannon-wasm.emscripten-node/build/Lib/test/libregrtest/runtest_mp.py", line 102, in run_tests_worker
    print(json.dumps(result, cls=EncodeTestResult), flush=True)
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
BlockingIOError: [Errno 6] write could not complete without blocking

The error seems to occur at the end of a test job when the worker process returns its status to the control process. The control process runs CPython natively (x86_64-pc-linux-gnu in case of the build bot). The worker processes is wasm32-emscripten CPython with NodeJS as runner. I guess that every now and then, the test worker process produces output faster than the control process can consume from the pipe that connects stdout of the worker and read pipe of the control process. The runtime does not like blocking IO and returns EAGAIN.

>>> import sys, errno
>>> sys.platform
'emscripten'
>>> errno.errorcode[6]
'EAGAIN'