bpo-39104: Fix hanging ProcessPoolExecutor on shutdown nowait with pickling failure (GH-17670)

As reported initially by @rad-pat in #6084, the following script causes a deadlock.

```
from concurrent.futures import ProcessPoolExecutor


class ObjectWithPickleError():
    """Triggers a RuntimeError when sending job to the workers"""

    def __reduce__(self):
        raise RuntimeError()


if __name__ == "__main__":
    e = ProcessPoolExecutor()
    f = e.submit(id, ObjectWithPickleError())
    e.shutdown(wait=False)
    f.result()  # Deadlock on get
```

This is caused by the fact that the main process is closing communication channels that might be necessary to the `queue_management_thread` later. To avoid this, this PR let the `queue_management_thread` manage all the closing.



https://bugs.python.org/issue39104



Automerge-Triggered-By: @pitrou

1 files changed, 2 insertions, 0 deletions

diff --git a/Misc/NEWS.d/next/Library/2020-02-16-18-49-16.bpo-39104.cI5MJY.rst b/Misc/NEWS.d/next/Library/2020-02-16-18-49-16.bpo-39104.cI5MJY.rst
new file mode 100644
index 0000000..52779bf
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2020-02-16-18-49-16.bpo-39104.cI5MJY.rst
@@ -0,0 +1,2 @@
+Fix hanging ProcessPoolExcutor on ``shutdown(wait=False)`` when a task has
+failed pickling.