Oh thank you for the clue, that's super helpful! Let me try locally.

This fix PR was originally done to solve a hanging call in a race condition, so the test would hang forever if the fix is not working as expected. The 10 seconds wait_for() is to make sure that we don't end up with a test hanging forever.

If enlarging the timeout works in some of your cases, that really inspires me on a potential cause of the issue in the test. To simulate the race condition specifically for loop.sock_sendall(), I needed a connected socket with a full send buffer so that the next sock_sendall() call would block. To achieve this, I wrote something like this:

    with self.assertRaises(BlockingIOError):
        while True:
            sock.send(b' ' * 5)

And another loop in a subtask to consume the data:

    while not data:
        data = await self.loop.sock_recv(server, 1024)
        data = data.strip()

It might be that it cost too much time in these loops (especially the 2nd one because the logs showed that it was blocked on calling sock_sendall()). But that's just a guess yet, let me add some more debug logs and test locally.