Fix cloned handlers to use same mutex#97
Merged
lmittmann merged 5 commits intolmittmann:mainfrom Jun 7, 2025
Merged
Conversation
b39b74b to
901d028
Compare
All handlers cloned from the handler attached to a logger should synchronize writer writes. This allows the logger to be used from multiple go routines. Here we add a test writing to a `bytes.Buffer` which itself is in not synchronized to expose the problem.
Drawing on the information in https://github.com/golang/example/blob/8b405629c4a5215871be932097e099c05ec5cb2e/slog-handler-guide/README.md#concurrency-safety, the handler is adapted to store a pointer to a mutex so that it and all of its clones synchronize with each other.
901d028 to
925ec80
Compare
This file contains hidden or bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment
2 participants
Add this suggestion to a batch that can be applied as a single commit.This suggestion is invalid because no changes were made to the code.Suggestions cannot be applied while the pull request is closed.Suggestions cannot be applied while viewing a subset of changes.Only one suggestion per line can be applied in a batch.Add this suggestion to a batch that can be applied as a single commit.Applying suggestions on deleted lines is not supported.You must change the existing code in this line in order to create a valid suggestion.Outdated suggestions cannot be applied.This suggestion has been applied or marked resolved.Suggestions cannot be applied from pending reviews.Suggestions cannot be applied on multi-line comments.Suggestions cannot be applied while the pull request is queued to merge.Suggestion cannot be applied right now. Please check back later.
Why this PR?
We experienced a race condition being detected in a test logging to a
bytes.Buffer. Although the problem doesn't occur when logging toos.Stderr, presumably because this is synchronized, it is reproducible when a logger uses a cloned handler.What has changed?
Based on https://github.com/golang/example/blob/8b405629c4a5215871be932097e099c05ec5cb2e/slog-handler-guide/README.md#concurrency-safety, synchronization in
handleris changed to share a mutex among all clones, similar toslog.commonHandlerhttps://cs.opensource.google/go/go/+/refs/tags/go1.24.3:src/log/slog/handler.go;l=197.How is it tested?
An explicit test triggering a race condition is added:
TestClonedHandlersSynchronizeWriter.How does it affect users?
There should be no effect on users.