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romanett
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## Problem `WebSocketByteTransportTests.CloseFailsQueuedSendInsteadOfOrphaningItAsync` fails intermittently. One example is test-linux-net10.0 (3/16) in run 39114639155 on #4831, an unrelated MachineTool change. The queued send failed with a raw `WebSocketException` ("…transitioned into an invalid state") instead of `ServiceResultException` with `BadConnectionClosed`. ## Cause This is a product race in `WebSocketByteTransportBase`. - `Close()` cancels the shared send token. That wakes both the in-flight send and the queued `SemaphoreSlim.WaitAsync`. - `SemaphoreSlim` removes a cancelled waiter on a later continuation. If the in-flight sender's `finally` releases the lock first, the release *grants* the queued waiter despite its cancelled token. - `EnterSendAsync` then returned normally, and the queued sender called `SendAsync` on the already aborted socket. - That socket error is not an `OperationCanceledException`, so it reached the caller untranslated. ## Fix After acquiring the lock, `EnterSendAsync` checks whether the transport closed. If it did, it gives the lock back and throws `BadConnectionClosed`. `Close()` sets the closed flag before it cancels, so every waiter woken by `Close()` sees it. ## Tests - The existing test body moved into a helper. New `CloseFailsQueuedSendEvenWhenTheLockIsGrantedAsync` repeats the interleaving 2000 times (about 140 ms). - Without the fix it fails 3/3 runs within about 100 ms, with the CI exception. With the fix it passes 3/3. - `Opc.Ua.Core.Tests` `~WebSocket` (net10.0): 39/39 passed. 🤖 Generated with [Claude Code](https://claude.com/claude-code) Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>