Re: Interrupts vs signals

On Wed, Oct 30, 2024 at 01:23:54PM -0400, Robert Haas wrote:
> On Wed, Oct 30, 2024 at 12:03 PM Heikki Linnakangas <hlinnaka(at)iki(dot)fi> wrote:
>> We can provide backwards compatibility macros and a new facility to
>> allocate custom interrupt bits. But how big of a problem is this anyway?
>> In an in-person chat last week, Andres said something like "this will
>> break every extension".
>
> Seems hyperbolic.

Most extensions that rely on bgworkers, at least.

> However, that was also pretty easy to fix. This seems like it might be
> a little more complicated.

The advantage of the breakage is also to force extension maintainers
to look at the changes because these have benefits. I'd like to think
that 6f3bd98ebfc0 was a good move overall even if it broke come
compilations and these require more PG_VERSION_NUM magic. For the
extension maintainer hat on, it is always annoying, but not really
that bad in the long-term.

>> We have a few options for how to deal with backwards-compatibility for
>> extensions:
>>
>> A) If we rip off the bandaid in one go and don't provide any
>> backwards-compatibility macros, we will break 96 extensions. Most of
>> them can be fixed by replacing WaitLatch, SetLatch, ResetLatch with
>> corresponding WaitInterrupt, RaiseInterrupt, ResetInterrupt calls. (With
>> #ifdef for compatibility with older postgres versions)
>>
>> B) If we provide backwards-compatibility macros so that simple Latch
>> calls on MyLatch continue working, we will break about 14 extensions.
>> They will need some tweaking like in option A). A bit more than the
>> simple cases in option A), but nothing too difficult.
>>
>> C) We could provide "forward-compatibility" macros in a separate header
>> file, to make the new "SetInterrupt" etc calls work in old PostgreSQL
>> versions. Many of the extensions already have a header file like this,
>> see e.g. citusdata/citus/src/include/pg_version_compat.h,
>> pipelinedb/pipelinedb/include/compat.h. It might actually be a good idea
>> to provide a semi-official header file like this on the Postgres wiki,
>> to help extension authors. It would encourage extensions to use the
>> latest idioms, while still being able to compile for older versions.
>>
>> I'm leaning towards option C). Let's rip off the band-aid, but provide
>> documentation for how to adapt your extension code. And provide a
>> forwards-compatibility header on the wiki, that extension authors can
>> use to make the new Interrupt calls work against old server versions.
>
> I don't know which of these options is best, but I don't find any of
> them categorically unacceptable.

Looking at the compatibility macros of 0008 for the latches with
INTERRUPT_GENERAL_WAKEUP under latch.h, the changes are not that bad
to adapt to, IMO. It reminds of f25968c49697: hard breakage, no
complaints I've heard of because I guess that most folks have been
using an in-house compatibility headers.

A big disadvantage of B is that someone may decide to add new code in
core that depends on the past routines, and we'd better avoid that for
this new layer of APIs for interrupt handling. A is a subset of C: do
a hard switch in the core code, with C mentioning a compatibility
layer in the wiki that does not exist in the core code. Any of A or C
is OK, I would not choose B for the core backend.
--
Michael