Fix potential deadlock in WebSocket timeout checker

[josesimoes]

Description

• Remove Thread.Suspend/Resume from the timeout checker.
• Guard the shared connection state with a lock that is never held while sending, waiting or running user code.
• Make close, ping and pong state changes atomic, so only one close frame is ever sent.
• Make the ping-timeout close non-blocking.
• Skip a timer check when the previous one is still running.
• Clear the ping flag when the ping can't be queued.
• Stop the timer before closing the receive stream.
• Close() now sends a close frame and ends the connection when the peer replies.
• Close(EndpointUnavailable) and WebSocketServer.Stop() block until the frame is written.
• Data sent before a normal Close() goes out before the close frame.

Motivation and Context

• The timeout checker suspended the receive thread wherever it happened to be, so if that thread was holding a lock the checker needed (e.g. while queueing a Pong, or inside a handler calling BroadCast), the checker blocked forever and the connection hung.
• Stumbled on this potential issue when working in Improve handling of connections closed by the peer #248

How Has This Been Tested?

Screenshots

Types of changes

• Improvement (non-breaking change that improves a feature, code or algorithm)
• Bug fix (non-breaking change which fixes an issue with code or algorithm)
• New feature (non-breaking change which adds functionality to code)
• Breaking change (fix or feature that would cause existing functionality to change)
• Config and build (change in the configuration and build system, has no impact on code or features)
• Dependencies (update dependencies and changes associated, has no impact on code or features)
• Unit Tests (add new Unit Test(s) or improved existing one(s), has no impact on code or features)
• Documentation (changes or updates in the documentation, has no impact on code or features)

Checklist:

• My code follows the code style of this project (only if there are changes in source code).
• My changes require an update to the documentation (there are changes that require the docs website to be updated).
• I have updated the documentation accordingly (the changes require an update on the docs in this repo).
• I have read the CONTRIBUTING document.
• I have tested everything locally and all new and existing tests passed (only if there are changes in source code).
• I have added new tests to cover my changes.

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📝 Summary

Summary by CodeRabbit

• Bug Fixes
  • Improved WebSocket connection reliability by handling keep-alive checks and timeout conditions more consistently.
  • Improved connection closure behavior to avoid duplicate close frames and ensure connections close cleanly.
  • Improved handling of incoming Pong messages and contact updates, helping keep active connections recognized.
  • Reset connection-closing state when reconnecting.

Walkthrough

The change synchronizes WebSocket state and close-frame transitions under a dedicated lock. It centralizes timeout decisions in WebSocket and updates the receive thread to dispatch those decisions, handle close and pong events through WebSocket methods, and stop the timer before closing the receive stream.

Changes

WebSocket State and Liveness

Layer / File(s)	Summary
Synchronize connection and close state WebSockets/WebSocket.cs	Connection state transitions and close-frame queueing use _stateLock. Close-frame tracking and close-received handling are added.
Centralize timeout and ping decisions WebSockets/WebSocket.cs	EvaluateTimeouts determines timeout actions and updates ping state. SendPing uses the resulting state.
Integrate state handling in the receive thread WebSockets/ReceiveAndControllThread.cs	Frame handling and timeout checks call the WebSocket state methods. Timeout ticks use a reentrancy guard, and cleanup stops and disposes the timer before closing the receive stream.

Estimated code review effort: 4 (Complex) | ~45 minutes

Suggested labels: Type: bug

Merge Risk: 🟠 High · up to 4e7f0

A graceful close of a WebSocket through Close() no longer sends a close frame to the peer, and an immediate close no longer hard-closes the socket. Connections stay open until the server timeout closes them, 60 seconds by default. Fix this before merging.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	The title clearly describes the main change: preventing a potential deadlock in the WebSocket timeout checker. It has 51 characters, which is slightly above the preferred 50-character limit, but it re…
Description check	✅ Passed	The description directly explains the deadlock prevention, timeout-checker changes, connection-state synchronization, ping and pong handling, close behavior, and timer cleanup.

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Actionable comments posted: 1

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• 🪄 Fix CodeRabbit comments on this PR

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Inline comments:
In `@WebSockets/WebSocket.cs`:
- Around line 259-263: Update Close() to remove its preemptive State assignment
and ClosingTime update, allowing TryBeginClose() called through RawClose() and
QueueCloseFrame() to atomically validate the state and perform the transition.
Preserve the existing close-request arguments passed to RawClose().

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ℹ️ Review info

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📥 Commits

Reviewing files that changed from the base of the PR and between 6803587 and 4e7f097.

📒 Files selected for processing (2)

• WebSockets/ReceiveAndControllThread.cs
• WebSockets/WebSocket.cs

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