* 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.
Chris Lu