Avoid spurious wakeups when stream capacity is not available#661
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seanmonstar merged 2 commits intohyperium:masterfrom Feb 20, 2023
Merged
Avoid spurious wakeups when stream capacity is not available#661seanmonstar merged 2 commits intohyperium:masterfrom
seanmonstar merged 2 commits intohyperium:masterfrom
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Fixes hyperium#628 Sometimes `poll_capacity` returns `Ready(Some(0))` - in which case caller will have no way to wait for the stream capacity to become available. The previous attempt on the fix has addressed only a part of the problem. The root cause - in a nutshell - is the race condition between the application tasks that performs stream I/O and the task that serves the underlying HTTP/2 connection. The application thread that is about to send data calls `reserve_capacity/poll_capacity`, is provided with some send capacity and proceeds to `send_data`. Meanwhile the service thread may send some buffered data and/or receive some window updates - either way the stream's effective allocated send capacity may not change, but, since the capacity still available, `send_capacity_inc` flag may be set. The sending task calls `send_data` and uses the entire allocated capacity, leaving the flag set. Next time `poll_capacity` returns `Ready(Some(0))`. This change sets the flag and dispatches the wakeup event only in cases when the effective capacity reported by `poll_capacity` actually increases.
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Excellent work, thank you!
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Fixes #628
Sometimes
poll_capacityreturnsReady(Some(0))- in which casecaller will have no way to wait for the stream capacity to become available.
The previous attempt on the fix has addressed only a part of the problem.
The root cause - in a nutshell - is the race condition between the
application tasks that performs stream I/O and the task that serves
the underlying HTTP/2 connection. The application thread that is about
to send data calls
reserve_capacity/poll_capacity, is providedwith some send capacity and proceeds to
send_data.Meanwhile the service thread may send some buffered data and/or
receive some window updates - either way the stream's effective
allocated send capacity may not change, but, since the capacity still
available,
send_capacity_incflag may be set.The sending task calls
send_dataand uses the entire allocatedcapacity, leaving the flag set. Next time
poll_capacityreturnsReady(Some(0)).This change sets the flag and dispatches the wakeup event only in
cases when the effective capacity reported by
poll_capacityactuallyincreases.