Stop asking the filesystem the same question three times per append

[bjmeetsfo]

An append stats the log to check nothing else has touched it, stats it again to learn the offset it is about to write at, writes, then stats it a third time for the new length.

The append lock is held across all three — taken before the first, released after the last — so nothing can change the file in between, and the second answer is one the caller already has.

• the second stat is gone: the check hands its answer to the append
• the third is now an fstat on the handle just written — cheaper, and a better question: it reports the length of the file this record went into, rather than of whatever the name refers to afterwards

The numbers, and what I am not claiming

stat by path	2.47 us
fstat on an open handle	1.67 us
open + close	5.93 us
a 64 B asynchronous append	~77 us

The saving is those component figures — one stat removed, one downgraded to an fstat. I am not quoting an end-to-end delta. One run suggested 77 us → 49 us, which is far more than removing ~3 us of syscalls can explain; the machine had a different load between the two, and a few microseconds is not something to claim from runs taken under a load average that swung between 2 and 20 during this work.

What I chose not to do

Holding the file handle open between appends. It would save the open and the close, 5.93 us, of which about 4.3 us survives once an identity check replaces them — an fstat, since another process can reclaim the log and reclaim replaces it by rename. A handle held across that would be writing into a file nobody will ever read again.

About 5% of an asynchronous append, and nothing at all on a durable one where the fsync dominates, is not worth a failure mode that loses data silently. Recorded here so the next person does not have to re-measure it to reach the same conclusion.

Testing

Library suite: 1020 passed, 9 failed. Four names differ from the main comparison run and all four were isolated clean earlier today — manifest_fold_reload_reconstructs_catalog_with_band_manifest_deleted, topology_check_stays_off_the_request_path, http_raft_transport_sends_append_vote_and_snapshot_over_tcp, distributed_raft_chunks_large_sequence_add_under_default_entry_limit.