fix(server/sse): potential goroutine leak in Heartbeat sender

[cryo-zd]

  Currently, there's one potential goroutine leak scenario caused by blocking sends to the session.eventQueue channel in the SSEServer heartbeat logic. The problematic case occurs under the following conditions:

1. The session.eventQueue channel is full(buffered channel with a fixed size of 100)
2. The heartbeat goroutine attempts to send a ping message but is blocked because the channel is full:

session.eventQueue <- pingMsg

3. Meanwhile, the main SSE handler goroutine exists(e.g., due to r.Context.Done() or session.done being closed), and no one is left consuming from session.eventQueue
4. As a result, the heartbeat goroutine becomes permanently blocked and never exits, which leads to:
   4.1 A goroutine leak
   4.2 The session and its associated channels(eventQueue, etc.) staying alive longer than needed
   4.3 Potential memory pressure and scalability issues under high-concurrency or long-running conditions

 This issue can be reproduced in edge cases where the client disconnects or becomes slow, and the server continues trying to send heartbeat messages at a fixed interval.

[Que1 ]Why not add case <- r.Context.Done() in the new added select statement ?
  Note that the main SSE handler will close(session.done) when r.Context is cancelled, so I think there's no need to monitor this case in the added select statement
[Que2] Is it possible that the main SSE handler will exit when session.eventQueue is full?
 Due to Go's select semantics, when multiple cases are ready (e.g., a message is available in eventQueue and r.Context().Done() is also ready), the runtime picks one at random. This can cause the main goroutine to exit before draining eventQueue, leaving producer goroutines blocked indefinitely.

Summary by CodeRabbit

• Bug Fixes
  • Improved the reliability of server-sent events by ensuring keep-alive ping messages do not cause blocking or deadlocks when a session is closed.

[coderabbitai]

Walkthrough

This change updates the keep-alive ping mechanism in the handleSSE method of the SSEServer. Instead of sending ping messages directly to the session's event queue, the operation is now wrapped in a select statement. This allows the code to either send the ping message or exit gracefully if the session is closed, preventing potential blocking or deadlocks. No changes were made to the signatures or declarations of exported or public entities.

Changes

File(s)	Change Summary
server/sse.go	Modified keep-alive ping logic in handleSSE to use a select statement for non-blocking sends and graceful exit on session closure.

Possibly related PRs

• feat: add ping for sse server #80: Refines the keep-alive ping logic in handleSSE, directly building upon the ping feature implementation.
• fix sse shutdown #128: Modifies handleSSE to improve session closure handling by adding select cases for graceful exit.

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• server/sse.go (1 hunks)

🔇 Additional comments (1)

server/sse.go (1)

370-375: Good fix for the goroutine leak issue!

This change effectively prevents the heartbeat goroutine from leaking when the session.eventQueue is full and the main SSE handler goroutine exits. By using a select statement with a case for session.done, the heartbeat goroutine can now exit gracefully when the session is terminated, even if the event queue is full.

The implementation is consistent with similar patterns used elsewhere in the codebase (like in the notification handler at line 339 and the SendEventToSession method at line 521).

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[rwjblue-glean]

Nice catch!