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fix: avoid painc caused by writing to closed channel #318

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Merged
merged 1 commit into from
May 22, 2025
Merged

fix: avoid painc caused by writing to closed channel #318

merged 1 commit into from
May 22, 2025

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cryo-zd
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@cryo-zd cryo-zd commented May 22, 2025 •
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Description

  The current implementation of handleGet creates a writechan and closes it from the receiver side using defer close(writechan). However, two goroutines (the notification handler and heartbeat sender) may continue writing to this channel. This pattern introduces a potential risk of painc under concurrent scenarios:

  • When the HTTP request is cancelled (via r.Context().Done()), the handleGet function exits, and writeChan is closed.
  • At the same time, the two goroutines mentioned may still be writing to writeChan, which causes a painc (painc: send on closed channel)

 This issue may not always surface in normal conditions but can randomly crash the server under high concurrency or slow network response (delayed done propagation). Proof and mocking:
fix1
Fixes: Remove the defer close(writeChan) and let the channel be implicitly garbage-collected once all goroutines referencing it exit. Instead, the done channel is used to signal all background goroutines to stop.

link text
Note: Only the sender should close a channel, never the receiver. Sending on a closed channel will cause a panic.
Another note: Channels aren't like files; you don't usually need to close them. Closing is only necessary when the receiver must be told there are no more values coming, such as to terminate a range loop.

fix2

Type of Change

  • Bug fix (non-breaking change that fixes an issue)
  • New feature (non-breaking change that adds functionality)
  • MCP spec compatibility implementation
  • Breaking change (fix or feature that would cause existing functionality to not work as expected)
  • Documentation update
  • Code refactoring (no functional changes)
  • Performance improvement
  • Tests only (no functional changes)
  • Other (please describe):

Checklist

  • My code follows the code style of this project
  • I have performed a self-review of my own code
  • I have added tests that prove my fix is effective or that my feature works
  • I have updated the documentation accordingly

Summary by CodeRabbit

  • Bug Fixes
    • Improved reliability of real-time notifications and heartbeat updates, reducing the likelihood of unexpected errors or interruptions during streaming.

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coderabbitai bot commented May 22, 2025 •
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Walkthrough

The changes update the internal goroutine logic in the handleGet method of server/streamable_http.go. Message forwarding to writeChan is now guarded by a select statement that checks the done channel, preventing sends after closure. The explicit closure of writeChan is also removed to avoid unsafe channel operations.

Changes

Files Change Summary
server/streamable_http.go Updated goroutines in handleGet to use select on done when sending to writeChan, and removed channel close logic.

Possibly related PRs

Suggested reviewers

  • robert-jackson-glean

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📒 Files selected for processing (1)
  • server/streamable_http.go (2 hunks)
🔇 Additional comments (3)
server/streamable_http.go (3)

360-364: Well implemented synchronization pattern!

This change effectively prevents the notification handler goroutine from writing to a potentially closed channel. By adding a select statement that checks if the done channel is closed before writing to writeChan, you ensure safe termination of the goroutine without risking "send on closed channel" panics.

385-389: Good consistency in error handling pattern!

Similar to the notification handler, this change properly guards the heartbeat message sending with a select statement to check the done channel. This consistent approach ensures both goroutines safely terminate without attempting to write to writeChan after the parent function exits.

353-355: Correct implementation of Go channel best practices!

The implementation now follows Go concurrency best practices by:

  1. Using a done channel to signal termination to child goroutines
  2. Removing the explicit closure of writeChan (which was previously closed via defer)
  3. Allowing the channel to be garbage-collected naturally when all references are gone

This prevents the "send on closed channel" panic described in the PR objectives, as goroutines now properly check if they should stop before attempting to write to the channel.

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@ezynda3 ezynda3 merged commit 17af676 into mark3labs:main May 22, 2025
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