Fix race in wireguard metrics collection.#12781
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Prometheus can call Gather() and Collect() concurrently. Fix is different for each: - Gather() was incorrect: it returned the current list of metrics, but that was dynamic. For a dynamic collector, the Collector interface specs that we should return no metrics instead. - Collect() was concurrently updating the rate limit fields and, when the rate limit fired, it incorrectly returned no metrics at all. Cache the most recent metrics and re-use those if the rate limit fires instead. Crive-by cleanups: remove unused fields, address missing err check.
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Pull request overview
Fixes a concurrency bug in the wireguard Prometheus metrics collector where Gather() and Collect() could race, and where rate-limited collections returned no metrics at all.
Changes:
- Make
Describe()send nothing (marking the collector as "unchecked") since the collector is dynamic. - Add a mutex around
Collect(), cache the most recent metrics, and return the cache when within the rate-limit interval instead of returning nothing. - Drive-by: remove unused
peerRx/peerTxfields and add a missing error check onwgClient.Close().
- Remove Unregister() test; can't unregister a dynamic collector. - Fix rate limiter test to expect cached results.
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Prometheus can call Gather() and Collect() concurrently. Fix is different for each:
Gather() was incorrect: it returned the current list of metrics, but that was dynamic. For a dynamic collector, the Collector interface specs that we should return no metrics instead.
Collect() was concurrently updating the rate limit fields and, when the rate limit fired, it incorrectly returned no metrics at all. Cache the most recent metrics and re-use those if the rate limit fires instead.
Drive-by cleanups: remove unused fields, address missing err check.
Race detector hit:
Release note: