Revert "[r3.4] logger: avoid mutex contention"#20652
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This reverts commit 26be98b.
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LGTM. The revert makes sense on release/3.4. The earlier change optimized away synchronization in StreamHandler/rotatingWriter, but the stuck stacks here point to blocking writes on stderr/os.File.Write and fdMutex serialization in the runtime. Restoring SyncHandler at the StreamHandler boundary and reinstating rotatingWriter locking is the safer behavior for production logging, even if it gives up the no-contention benchmark path.
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Reverts #20487
reason: gnosis has regression
The rotatingWriter.mu does guard the file — it wraps both Write and Close. But a mutex doesn't make the write non-blocking. It just serializes access. If
r.file.Write() blocks in the kernel (pipe full), r.mu stays locked and other goroutines pile up at r.mu.Lock().
But this isn't the current problem. Look at the stuck goroutine stacks — they go through streamHandler directly to os.File.Write(). None of them go through
rotatingWriter. They're writing to stdout or stderr (plain os.File, no rotating writer, no mutex).
The node is set up with log.Root().SetHandler(... log.StderrHandler) — just a streamHandler wrapping os.Stderr. No mutex, no rotation. Multiple goroutines
call os.Stderr.Write() concurrently, serialize at the internal fdMutex, and one of them is stuck in the kernel write() syscall.
So: rotatingWriter.mu is correct but irrelevant to this bug. The bug is the stderr StreamHandler has no Go-level protection and when the stderr pipe
consumer backs up, the kernel write stalls the execution goroutine.