No problem at all, and thanks for clarifying.

> 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:

> 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.

That seems reasonable to me and would make the most sense at a glance. If this is confirmed to be the source of the issue, we should be able to just reduce the size of the socket's send buffer via SO_SNDBUF before sending to it so that it fills up much more quickly.