Re: Optimize LISTEN/NOTIFY

> On Oct 21, 2025, at 00:43, Arseniy Mukhin <arseniy(dot)mukhin(dot)dev(at)gmail(dot)com> wrote:
>
>
> I managed to reproduce the race with v20-alt3. I tried to write a TAP
> test reproducing the issue, so it was easier to validate changes.
> Please find the attached TAP test. I added it to some random package
> for simplicity.
>

With alt3, as we have acquired the notification lock after reading every message to update the POS, I think we can do a little bit more optimization:

The notifier: in SignalBackend()
* Now we check if a listener’s pos equals to beforeWritePos, then we do “directly advancement”
* We can change to if a listener’s pos is between beforeWritePos and afterWritePos, then we can do the advancement.

The listener: in asyncQueueReadAllNotifications():
* With alt3, we only lock and update pos
* We can do more. If current pos in shared memory is after that local pos, then meaning some notifier has done an advancement, so it can stop reading.

I tried to run your TAP test on my MacBook, but failed:

```
t/008_listen-pos-race.pl .. Dubious, test returned 32 (wstat 8192, 0x2000)
No subtests run

Test Summary Report
-------------------
t/008_listen-pos-race.pl (Wstat: 8192 (exited 32) Tests: 0 Failed: 0)
Non-zero exit status: 32
Parse errors: No plan found in TAP output
Files=1, Tests=0, 3 wallclock secs ( 0.01 usr 0.01 sys + 0.10 cusr 0.29 csys = 0.41 CPU)
Result: FAIL
```

I didn’t spend time debugging the problem. If you can figure the problem, maybe I can run the test from my side.

Best regards,
--
Chao Li (Evan)
HighGo Software Co., Ltd.
https://www.highgo.com/