ReadFromBuffer and HasData() take the read lock separately, so a write
that lands between them can make a subscriber which just read a
momentarily empty buffer return ResumeFromDiskError even though the data
is now servable from memory. Re-read under a fresh lock and only bail
when the position is genuinely behind the in-memory window (flushed to
disk); otherwise loop back and read it.
Chris Lu
f711868fb6