[rzikm]

HTTP/2 stream-limit and keepalive tests can issue concurrent writes on one loopback connection, causing SslStream to throw NotSupportedException: This method may not be called when another write operation is pending. A pending ReadFrameAsync can send a transparent PING ACK while the test sends response HEADERS; the keepalive reader can also observe client EOF and send disposal GOAWAY while final response DATA is still being written.

Enable the existing connection-scoped EnsureThreadSafeIO option for the stream-limit test and keepalive fixture. Move keepalive connection disposal to TerminateLoopbackConnectionAsync, after joining the incoming-frame task, so the reader does not independently initiate shutdown on EOF. Protocol assertions are unchanged; there are no product changes or changes to the shared helper implementation.

Validation

Windows x64, using a short subst path:

• .\build.cmd clr+libs -rc release: passed. Existing MsQuic package advisory warnings remain.
• .\dotnet.cmd build src\libraries\System.Net.Http\tests\FunctionalTests\System.Net.Http.Functional.Tests.csproj /t:Test /p:Outerloop=true "/p:XUnitOptions=-method System.Net.Http.Functional.Tests.SocketsHttpHandlerTest_Http2.Http2_MaxConcurrentStreams_LimitEnforced -method System.Net.Http.Functional.Tests.SocketsHttpHandler_Http2KeepAlivePing_Test.KeepAlivePingDelay_Infinite_NoKeepAlivePingIsSent -method System.Net.Http.Functional.Tests.SocketsHttpHandler_Http2KeepAlivePing_Test.KeepAliveConfigured_KeepAlivePingsAreSentEvenWith0ConnectionLifetime -method System.Net.Http.Functional.Tests.SocketsHttpHandler_Http2KeepAlivePing_Test.KeepAliveConfigured_ConnectionAtMaxConcurrentStreams_KeepAlivePingsAreStillSent -method System.Net.Http.Functional.Tests.SocketsHttpHandler_Http2KeepAlivePing_Test.KeepAliveConfigured_HandlerDisposedWithActiveRequest_KeepAlivePingsAreStillSent": passed; repeated twice with /p:TestNoBuild=true, totaling 15 passed, 0 failed, 0 skipped after removing instrumentation.
• .\dotnet.cmd build src\libraries\System.Net.Http\tests\FunctionalTests\System.Net.Http.Functional.Tests.csproj /t:Test /p:Outerloop=true /p:TestNoBuild=true "/p:XUnitOptions=-class System.Net.Http.Functional.Tests.SocketsHttpHandler_Http2KeepAlivePing_Test": passed, 10 tests, 0 skipped.

Temporary connection-scoped tracing and a 50-ms delay of transport write completion below SslStream reproduced competing writers in four of the five methods. Across three repetitions of all five tests, the original code had 12 failures; serialization alone passed 15/15, joining alone passed 9/15, and this fix passed 15/15 under the same delay. Traces confirmed queued ACK writes reached TLS only after the preceding response write completed. All instrumentation and delay controls were removed from this patch.

No spontaneous failure occurred in the unamplified baseline. The max-concurrent-streams keepalive variant did not reproduce locally, although it passed all fixed runs. Linux/ARM, NativeAOT, WinHTTP, and the full System.Net.Http suite were not run.

Note

This pull request description and code changes were generated with GitHub Copilot.

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[rzikm]

/ba-g mutex tests are unrelated