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fix(server): do not cache an ECS query's SERVFAIL under the global failure key (F55) - #86
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…ilure key A SERVFAIL learned while a client-subnet option was in flight is not portable to other clients (resolver.go already skips failure caching under ECS), but Handle wrote it to the global key, so unscoped clients got the cached SERVFAIL for the failure TTL.
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An upstream SERVFAIL for a query carrying ECS was stored in the global RFC 9520 failure cache, so clients without ECS received that SERVFAIL for the cache period. The resolver already skips this when ECS is outbound; the handler did not.
Fix: skip
StoreFailurewhenoutboundECS != nilon both the error and SERVFAIL result paths.Regression test:
TestHandle_ECS_ServfailNotCachedGlobally. Proof failed on unmodified code; verifier passed 3x with-race.