On 07/07/2026 00:11, Heikki Linnakangas wrote:
> On 05/07/2026 08:03, Thomas Munro wrote:
>> On Sat, Jul 4, 2026 at 11:15 PM Jelte Fennema-Nio <postgres@jeltef.nl>
>> wrote:
>>> On Sat, 4 Jul 2026 at 02:51, Thomas Munro <thomas.munro@gmail.com>
>>> wrote:
>>>> We don't actually care about the threads
>>>> themselves, and it doesn't seem that great if we have to introduce an
>>>> IPC ping-pong of some kind with each thread.
>>>
>>> Agreed. But I do agree with Heikki that swapping out stderr seems pretty
>>> hacky. At the very least because now the main thread cannot write to
>>> stderr either anymore (which is why you removed the "terminated by user"
>>> write I guess).
>>>
>>> How about instead we do something like the attached?
>>
>> That's definitely nicer, if we know that all potential error logging
>> caused by cancellation happens in a context that can check the flag.
>
> +1, much nicer!
>
>> I didn't even look into that, because I was deliberately trying to
>> avoid needing atomics from here, because I need this to work on Unix
>> too, and I didn't want to open too many cans of worms at the same
>> time. Hence the appeal of a simple async-signal-safe system call that
>> has the right concurrency properties already and works also on Windows
>> without a separate code path. But... reaching for the can opener...
>>
>> 1. If we're ready to drop VS < 2022 and GCC < 4.9, we could just use
>> <stdatomic.h> directly in frontend code (independently of the project
>> to use it in the backend).
>> 2. If we're not ready yet we could make "port/atomics.h" or selected
>> parts of it frontend-allowed.
>> 3. Maybe all we really need for this case is memory barriers, and we
>> could move those out to a frontend-allowed header.
>
> To be honest, I didn't realize we didn't allow "port/atomics.h" in
> frontend code. I think spinlock-simulated 64-bit atomics is the only
> thing that wouldn't just work.
In this case, though, I think all we need is a "volatile sigatomic_t"
flag. Sending the query cancellation over the network surely acts as a
full compiler and memory barrier in the cancelling thread. And similarly
receiving the error message from the network acts as a full barrier in
the other threads that might receive the cancellation error from the
backend.
- Heikki
