fix(client): potential risk of sending on closed channel

[cryo-zd]

  Note that the local context variable ctx in func handleSSEResponse() may be cancelled at any time because its parent context is canceled, and then func handleSSEResponse() will exit and close channel responsChan. But it is possible that we are going to write to channel responsChan in func readSSE and panic: send on closed channel although the possibility of this scenario is rather low.
[Que1] Is it really has the potential of panic ? Proof is as follows:

  In the picture above, we utilized sync.WaitGroup for mocking the possible scenario and controlling the execution sequence of cancelling context and sending message to responseChan, and we will find that the test will fail because of panic
  Now let's see what will happen after change( only test fails, no longer panic):

[Que 2] Why not check ctx is cancelled or not before we send message to responseChan:

    select {
    case responseChan <- &message:
    case <-ctx.Done():
    }

  In my opinion, this plan is not particularly rigorous in concurrent scenario. One boundary case is that when both two cases in select are ready, GoRuntime chooses the case at random and may choose to send message to closed channel responseChan, for which we encounter panic again
[Que 3] Why not directly close(responseChan) directly after sending message to responseChan:

    responseChan <- &message
    close(responseChan)

  I think, this plan is also not rigorous because func readSSE may call its argument handler for zero, one or more than one times, and we can't make sure that it will call handler for exactly once:

• If func readSSE call its argument handler for more than one times, we will fail because of panic: close of closed channel
• If func readSSE does not call handler, we will miss the chance of closing responseChan that may bury the risk of resource leakage in the near future?

Summary by CodeRabbit

• Bug Fixes
  • Improved the reliability of server-sent event (SSE) response handling, preventing interruptions or missed messages during streaming responses.

[coderabbitai]

Walkthrough

The update revises the logic for handling Server-Sent Events (SSE) response streams in the client transport layer. The main change is to defer the closure of the response channel until after all SSE events have been processed, rather than closing it immediately after channel creation. This is achieved by launching a goroutine to process the SSE stream and closing the channel only after stream processing is complete. The event unmarshaling and notification handling logic remains unchanged.

Changes

File(s)	Change Summary
client/transport/streamable_http.go	Restructured SSE response handling by moving channel closure into a goroutine, ensuring the channel closes only after all SSE events are processed. No changes to public API or exported entities.

Possibly related PRs

• fix: SSE client hangs after 30 seconds #88: Modifies SSE stream reading logic, specifically removing the SSE read timeout in readSSE, which is closely related to the restructuring of SSE processing and channel management in this PR.

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Reviewing files that changed from the base of the PR and between 781b732 and 570b52a.

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• client/transport/streamable_http.go (1 hunks)

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🧬 Code Graph Analysis (1)

client/transport/streamable_http.go (1)

mcp/types.go (1)

• JSONRPCNotification (206-209)

🔇 Additional comments (1)

client/transport/streamable_http.go (1)

244-275: Great solution to avoid the "send on closed channel" panic!

This change properly addresses the concurrency issue described in the PR. By moving the closure of responseChan into a deferred call inside the goroutine that processes the SSE stream, you ensure that:

1. The channel remains open throughout the entire SSE processing lifecycle
2. The channel is only closed after readSSE has completed, preventing any late sends to a closed channel
3. There's no risk of goroutine leaks since the goroutine will always complete, either due to normal processing or context cancellation

The approach is clean and effectively handles the race condition. The select statement in the main function body still properly waits for either a response or context cancellation.

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