On 17/06/2021 09:26, Mark Thomas wrote:
I think I might have found one contributing factor to this bug. I need
to run a series of tests to determine whether I am seeing random
variation in test results or a genuine effect.
It was random effects but I believe I have now found the bug.
Consider two threads, T1 and T2 writing HTTP/2 response bodies
concurrently in the same HTTP/2 Connection.
You'll need to have the code in front of you to follow what is going on
The write:
https://github.com/apache/tomcat/blob/main/java/org/apache/tomcat/util/net/SocketWrapperBase.java#L1364
and the associated completion handler
https://github.com/apache/tomcat/blob/main/java/org/apache/tomcat/util/net/SocketWrapperBase.java#L1044
The detail of the code is fairly complex but all you really need to keep
in mind is the following:
- the writePending semaphore ensures only one thread can write at a time
- the state of the write is maintained in a OperationState instance that
is stored in SocketWrapperBase.writeOperation (L1390)
- the completion handler clears this state (L1050) and releases the
semaphore (L1046)
The sequence of events for a failure is as follows:
- T1 obtains the write semaphore (L1366)
- T1 creates an OperationState and sets writeOperation (L1390)
- the async write for T1 completes and the completion handler is called
- T1's completion handler releases the semaphore (L1046)
- T2 obtains the write semaphore (L1366)
- T2 creates an OperationState and sets writeOperation (L1390)
- T1's completion handler clears writeOperation (L1050)
- the async write for T2 does not complete and the socket is added to
the Poller
- The Poller signals the socket is ready for write
- The Poller finds writeOperation is null so performs a normal dispatch
for write
- The async write times out as it never receives the notification from
the Poller
The fix is to swap the order of clearing writeOperation and releasing
the semaphore.
Concurrent reads will have the same problem and will be fixed by the
same solution.
Fix will be applied shortly.
Mark
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