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Align to 4-byte boundaries for 32-bit environments #315

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1 change: 1 addition & 0 deletions statsd/buffered_metric_context.go
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Original file line number Diff line number Diff line change
Expand Up @@ -11,6 +11,7 @@ import (
// and Timing. Since those 3 metric types behave the same way and are sampled
// with the same type they're represented by the same class.
type bufferedMetricContexts struct {
_ [4]byte // Aligin to a 4-byte boundary

@atanzu atanzu Jun 3, 2026

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I have a concern that this approach can move the problem, but not eliminate it.

Aggregator is heap-allocated so it starts at an 8-byte aligned base address. On 32-bit the layout of aggregator up to histograms sums to offset 64 (83 + 4 + 123), so histograms.nbContext is fine. The problem is distributions and timings, which follow histograms at offset 64 + sizeof(bufferedMetricContexts). If that size is not a multiple of 8, the nbContext field in distributions/timings falls at a 4-byte but not 8-byte aligned address.

If we add 4 bytes at the start of the struct, it moves nbContext to offset 4 within bufferedMetricContexts, but then histograms starts at base+64, histograms.nbContext is now at base+68 so not aligned.

@unionsep unionsep Jun 5, 2026

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Thank you for your detailed comment.

You are right. I wanted to fix this panic on a 32-bit binary with Go 1.22.4:

panic: unaligned 64-bit atomic operation

The panic happened around atomic.AddUint64(&bc.nbContext, ...) .

But I now understand that my fix is not enough.
Adding 4 bytes before nbContext may only move the alignment problem to another field, as you said.

Also, some things have changed on my side since I opened this PR.
So I will debug this again in my current environment.

I will check the struct layout with GOARCH=386.
If my understanding is correct, a better fix may be to keep nbContext as the first field, and add [4]byte padding after randomLock .

Thank you again for explaining this carefully.

nbContext uint64
mutex sync.RWMutex
values bufferedMetricMap
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