Fix cloned handlers to use same mutex #97
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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.
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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.