FIX: don't drop requests that arrive before v8_roundtrip is re-entered - #440
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When an attached Ruby callback makes its own request on the same context (perform_microtask_checkpoint, a nested call, etc), the Ruby thread can enqueue its next request in the gap between the nested dispatch finishing and the V8 thread re-entering v8_roundtrip. The buf_reset(&c->req) on entry then destroyed that request, leaving both threads waiting on each other forever. The reset is a leftover from before 478c763, when v8_roundtrip returned a pointer into the live request buffer and had to clear it on the next entry. Requests are now handed off with buf_move, so the buffer is already empty on re-entry unless it holds a new, unread message.
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When an attached Ruby callback makes its own request on the same context (perform_microtask_checkpoint, a nested call, etc), the Ruby thread can enqueue its next request in the gap between the nested dispatch finishing and the V8 thread re-entering v8_roundtrip. The buf_reset(&c->req) on entry then destroyed that request, leaving both threads waiting on each other forever.
The reset is a leftover from before 478c763, when v8_roundtrip returned a pointer into the live request buffer and had to clear it on the next entry. Requests are now handed off with buf_move, so the buffer is already empty on re-entry unless it holds a new, unread message.
Writing a deterministic test here is very difficult. I was only able to reproduce by looping thousands of times.