Re: [PATCH] fsmonitor: flush pending FSEvents before cookie wait

[Junio C Hamano <gitster@pobox.com>]

Koji Nakamaru <koji.nakamaru@gree.net> writes:

> On Wed, Jul 22, 2026 at 6:05 AM Tamir Duberstein <tamird@gmail.com> wrote:
>>
>> 56cef9cb1a (fsmonitor: use pthread_cond_timedwait for cookie wait,
>> 2026-04-15) limits the cookie wait to one second so that a filesystem
>> which never delivers events cannot hang fsmonitor clients. A client that
>> times out receives a trivial response and scans the entire index.
>>
>> FSEvents can defer delivery while it batches notifications and does not
>> guarantee that its queue is drained in one latency interval. A loaded
>> macOS system can therefore time out even though the event stream is
>> working.
>>
>> On an Apple M4 Max (16 cores, 128 GiB RAM) running macOS 26.5.2, two
>> worktrees with a 1,001,178-entry index timed out 484 of 545 and 297 of
>> 365 fsmonitor requests. One status call performed 934,519 lstat() calls
>> during a 47-second preload and took 52 seconds overall.
>>
>> Ask FSEvents to flush pending notifications after creating the cookie
>> and before starting the timed wait. Use the asynchronous form because
>> the client handler holds main_lock, which the listener callback also
>> acquires. Keep the timeout and the behavior of the other backends
>> unchanged.
>>
>> Signed-off-by: Tamir Duberstein <tamird@gmail.com>
>> ---
>>...
> This patch is carefully designed to minimize any risks. To drain events,
> we could also call FSEventStreamFlushSync before acquiring main_lock in
> do_handle_client(), but this patch should be sufficient if it mitigates
> the issue. The commit message would be much more convincing if you also
> included benchmark results showing how many timeouts were reduced.

Thanks for a review.