Fix poll_capacity returning Ready(Some(0))

[vadim-eg]

poll_capacity should never return Some(0) - because the caller has no way to be woken up when send capacity is available.

Check capacity and return Pending if it is zero. The wakeup code is already correctly triggers notification when capacity changes to non-zero.

[seanmonstar]

Thanks for the PR! I'm curious, do you know if we have a unit test that exercises?

[vadim-eg]

Thanks for the PR! I'm curious, do you know if we have a unit test that exercises?

It looks like poll_capacity_after_send_data_and_reserve[_with_max_send_buffer_size] do attempt to test the condition - frankly I haven't figured out yet the exact conditions triggering it. In our case Some(0) happens fairly sporadically usually involving multiple streams doing bidirectional io. If I find some cycles I'll try to take a closer look. Thanks!

[seanmonstar]

It looks like in CI, this causes a test to hang:

test reserved_capacity_assigned_in_multi_window_updates has been running for over 60 seconds

[vadim-eg]

It looks like in CI, this causes a test to hang:

test reserved_capacity_assigned_in_multi_window_updates has been running for over 60 seconds

@seanmonstar Yes, I'll try to see what is going on.

[vadim-eg]

@seanmonstar I think I got an explanation on the original issue - Poll(Ready(Some(0))). It appears to be a concurrency/timing issue - it can happen when wakeup is called from one thread while the outer is sending data on a stream - e.g. -

• thread A - sender called poll_capacity, got 16K and proceeded to send_data
• thread B - connection got a window update and sent out some pending frames. capacity did not increase but wakeup was scheduled and send_capacity_inc set. (may be this can e avoided)
• thread A - sent the data and capacity is now 0, send_capacity_inc set
• thread A - poll_capacity returns 0

[vadim-eg]

I think I have an idea what is going on I'll open an issue and take it there