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Chris LuandGitHub
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* filer: tolerance-window read pattern detection for concurrent readahead ReaderPattern detected sequential access via strict contiguity (lastReadStopOffset == offset). Under concurrent/reordered ReadAt (parallel readahead, which ReadAt already supports via RLock + atomics), that exact match frequently misses even for a genuinely sequential stream, drifting toward random mode and disabling prefetch. Track a read frontier (max offset+size, advanced with a lock-free CAS loop) and treat a read as sequential when its start is within SeqTolerance (8 MiB) of the frontier. The window absorbs reordered/concurrent readahead while still rejecting far random jumps; the existing ModeChangeLimit hysteresis is unchanged. API (NewReaderPattern/MonitorReadAt/IsRandomMode) is unchanged, so reader_at is unaffected. Adds reader_pattern_test.go. * filer: read the read frontier inside the CAS loop Capture the frontier from the CAS loop's own load rather than a separate up-front snapshot, so the sequentiality diff is judged against the freshest pre-image even if a concurrent readahead advances the frontier while we loop. Also drops the now-redundant load. Mirrors the same fix on the write-side ReaderPattern twin (review feedback on the companion PR). * filer: add frontier/boundary/recovery guard tests for ReaderPattern Mirror the regression guards added to the write-side WriterPattern: assert the max-frontier never regresses on a backward read (the CAS invariant), pin both sides of the inclusive SeqTolerance boundary, and verify sustained near reads recover sequential mode out of the negative floor. Each guards an invariant the original four tests left uncovered.