promhttp: add regression test for concurrent map writes (#1274) (#2000)

promhttp: add regression test for concurrent map writes (#1274)

Without the fix in #1318, instrumenting a metric vec partitioned only by labels supplied via WithLabelFromCtx (i.e. with no “code” or “method” label) would cause concurrent requests to share and mutate a single ‘emptyLabels’ map, producing ‘fatal error: concurrent map writes’ under load. The fix made labels() allocate a fresh map per call but no regression test was added at the time.

This adds a test that exercises both InstrumentHandlerCounter and InstrumentHandlerDuration from many goroutines so the bug would be caught either by the race detector or the runtime’s concurrent-map- writes detection.

Verified: the test fails (DATA RACE / concurrent map writes panic) when the pre-#1318 labels() implementation is reintroduced, and passes on current main with ‘go test -race’.

Closes #1274

Signed-off-by: Pedram pedram@arpdigital.io

Update prometheus/promhttp/instrument_server_test.go

Co-authored-by: Bartlomiej Plotka bwplotka@gmail.com Signed-off-by: Pedram Pd eragon.pedy@gmail.com

Signed-off-by: Pedram pedram@arpdigital.io Signed-off-by: Pedram Pd eragon.pedy@gmail.com Co-authored-by: Pedram pedram@arpdigital.io Co-authored-by: Bartlomiej Plotka bwplotka@gmail.com

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