Re: PoC: Add condition variable support to WaitEventSetWait()

> On Mar 31, 2026, at 15:28, Chao Li <li(dot)evan(dot)chao(at)gmail(dot)com> wrote:
>
> Hi,
>
> There is an XXX comment in WalSndWait():
> ```
> * XXX: A desirable future improvement would be to add support for CVs
> * into WaitEventSetWait().
> ```
>
> I have been exploring a possible approach for that. This patch is a PoC that adds ConditionVariable support to WaitEventSet. This v1 is mainly intended to gather feedback on the design, so I have only done some basic testing so far, such as a normal logical replication workflow.
>
> I’d like to highlight a few key points about the design:
>
> 1. In the current WalSndWait(), although it prepares to sleep on a ConditionVariable, it does not actually check whether the CV has been signaled. In this PoC, I kept that same behavior. However, I tried to make the WaitEventSet support for CVs generic, so that if we want to add actual signal checking in the future, that would be possible.
>
> 2. To keep the design generic, this patch introduces a new wait event type, WL_CONDITION_VARIABLE. A WL_CONDITION_VARIABLE event occupies a position in the event array, similar to latch and socket events. When a CV is signaled, the corresponding WL_CONDITION_VARIABLE event is returned in occurred_events.
>
> 3. The WaitEventSet APIs AddWaitEventToSet() and ModifyWaitEvent() are extended to support CVs by adding one more parameter “cv" to both APIs. The downside of this approach is that all call sites of these two APIs need to be updated. I also considered adding separate APIs for CVs, such as AddWaitEventToSetForCV() and ModifyWaitEventForCV(), since CVs do not rely on the kernel and it might therefore make sense to decouple them from socket and latch handling. But for v1, I chose the more generic approach. I’d be interested in hearing comments on this part of the design.
>
> 4. One important point is that this patch extends the WaitEventSet abstraction, not the underlying kernel wait primitives. A ConditionVariable is still a userspace/shared-memory concept, but with this design it can participate in the same waiting framework as sockets and latches. I think that is useful because it allows mixed waits to be handled through one interface.
>
> Here is the v1 patch.
>
> Best regards,
> --
> Chao Li (Evan)
> HighGo Software Co., Ltd.
> https://www.highgo.com/
>
>
>
>
> <v1-0001-Add-condition-variable-support-to-WaitEventSetWai.patch>

I just noticed that I missed checking in my last edit when switching to the other branch, so attaching an updated v1.

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