commit_txn issued an unconditional fsync per COMMIT, so N concurrent
writers serialized behind N disk syncs (p99 hit the 3s socket timeout
under 8 writers). Add Wal::sync_upto: committers queue on an fsync_gate,
the first one flushes the whole accumulated buffer, and waiters that find
committed_seq past their target return with zero I/O. N commits now cost
~1 fsync with the same durability guarantee.
Also fix a durability over-report: flush_pending stamped committed_seq
from the LIVE seq counter, so sequences taken by appenders that had not
yet buffered their bytes were reported durable. Track max_seq alongside
the pending buffer and advance committed_seq only to what was written.
Wire --wal-fsync-ms / --checkpoint-ms in cube-server (previously
hardcoded to defaults, so the documented knob did nothing).
Both regressions are mutation-verified: each test fails when its bug is
reintroduced.
CUBELinux-2
ec84fbc725