Thread: PostmasterIsAlive() in recovery (non-USE_POST_MASTER_DEATH_SIGNAL builds)

Hello,

In commits 9f095299 and f98b8476 we improved recovery performance on
Linux and FreeBSD but we didn't help other operating systems.  David
just confirmed for me that commenting out the PostmasterIsAlive() call
in the main recovery loop speeds up crash recovery considerably on his
Windows system: 93s -> 70s or 1.32x faster.  So I think we should do
something like what Heikki originally proposed to lower the frequency
of checks, on systems where we don't have the ability to skip the
check completely.  Please see attached.

On 17/09/2020 12:48, Thomas Munro wrote:
> Hello,
> 
> In commits 9f095299 and f98b8476 we improved recovery performance on
> Linux and FreeBSD but we didn't help other operating systems.  David
> just confirmed for me that commenting out the PostmasterIsAlive() call
> in the main recovery loop speeds up crash recovery considerably on his
> Windows system: 93s -> 70s or 1.32x faster.

Nice speedup!

> So I think we should do
> something like what Heikki originally proposed to lower the frequency
> of checks, on systems where we don't have the ability to skip the
> check completely.  Please see attached.

If you put the counter in HandleStartupProcInterrupts(), it could be a 
long wait if the startup process is e.g. waiting for WAL to arrive in 
the loop in WaitForWALToBecomeAvailable(), or in recoveryPausesHere(). 
My original patch only reduced the frequency in the WAL redo loop, when 
you're actively replaying records.

We could probably do better on Windows. Maybe the signal handler thread 
could wait on the PostmasterHandle at the same time that it waits on the 
signal pipe, and set postmaster_possibly_dead. But I'm not going to work 
on that, and it would only help on Windows, so I'm OK with just adding 
the counter.

- Heikki

On Thu, Sep 17, 2020 at 10:19 PM Heikki Linnakangas <hlinnaka@iki.fi> wrote:
> On 17/09/2020 12:48, Thomas Munro wrote:
> > So I think we should do
> > something like what Heikki originally proposed to lower the frequency
> > of checks, on systems where we don't have the ability to skip the
> > check completely.  Please see attached.
>
> If you put the counter in HandleStartupProcInterrupts(), it could be a
> long wait if the startup process is e.g. waiting for WAL to arrive in
> the loop in WaitForWALToBecomeAvailable(), or in recoveryPausesHere().
> My original patch only reduced the frequency in the WAL redo loop, when
> you're actively replaying records.

Oh, I checked that WaitForWALToBecomeAvailable() already handled
postmaster death via events rather than polling, with
WL_EXIT_ON_PM_DEATH, but I hadn't clocked that recoveryPausesHere()
uses pg_usleep() and polling.  Hmm.  Perhaps we should change that
instead?  The reason I did it that way is that I didn't want to make
the new ProcSignalBarrierPending handler less reactive.

> We could probably do better on Windows. Maybe the signal handler thread
> could wait on the PostmasterHandle at the same time that it waits on the
> signal pipe, and set postmaster_possibly_dead. But I'm not going to work
> on that, and it would only help on Windows, so I'm OK with just adding
> the counter.

Yeah, I had the same thought.

On 17/09/2020 13:31, Thomas Munro wrote:
> On Thu, Sep 17, 2020 at 10:19 PM Heikki Linnakangas <hlinnaka@iki.fi> wrote:
>> If you put the counter in HandleStartupProcInterrupts(), it could be a
>> long wait if the startup process is e.g. waiting for WAL to arrive in
>> the loop in WaitForWALToBecomeAvailable(), or in recoveryPausesHere().
>> My original patch only reduced the frequency in the WAL redo loop, when
>> you're actively replaying records.
> 
> Oh, I checked that WaitForWALToBecomeAvailable() already handled
> postmaster death via events rather than polling, with
> WL_EXIT_ON_PM_DEATH, but I hadn't clocked that recoveryPausesHere()
> uses pg_usleep() and polling.  Hmm.  Perhaps we should change that
> instead?  The reason I did it that way is that I didn't want to make
> the new ProcSignalBarrierPending handler less reactive.

Hmm, so for speedy response to postmaster death, you're relying on the 
loops to have other postmaster-death checks besides 
HandleStartupProcInterrupts(), in the form of WL_EXIT_ON_PM_DEATH. That 
seems a bit fragile, at the very least it needs a comment in 
HandleStartupProcInterrupts() to call it out.

Note that there's one more loop in ShutdownWalRcv() that uses pg_usleep().

- Heikki

On Thu, Sep 17, 2020 at 10:47 PM Heikki Linnakangas <hlinnaka@iki.fi> wrote:
> Hmm, so for speedy response to postmaster death, you're relying on the
> loops to have other postmaster-death checks besides
> HandleStartupProcInterrupts(), in the form of WL_EXIT_ON_PM_DEATH. That
> seems a bit fragile, at the very least it needs a comment in
> HandleStartupProcInterrupts() to call it out.

Surely that's the direction we want to go in, though, no?  Commit
cfdf4dc4 was intended to standardise the way we react to postmaster
death where waiting is involved.  I updated the comment in
HandleStartupProcInterrupts() to highlight that the
PostmasterIsAlive() check in there is only for the benefit of
CPU-bound loops.

> Note that there's one more loop in ShutdownWalRcv() that uses pg_usleep().

Updating that one required me to invent a new wait_event for
pg_stat_activity, which seems like progress.

Unfortunately, while I was doing that I realised that WaitLatch()
without WL_SET_LATCH was broken by commit 3347c982bab (in master
only), in a way not previously reached by the tests.  So 0001 is a
patch to fix that.

On 2020/09/18 9:30, Thomas Munro wrote:
> On Thu, Sep 17, 2020 at 10:47 PM Heikki Linnakangas <hlinnaka@iki.fi> wrote:
>> Hmm, so for speedy response to postmaster death, you're relying on the
>> loops to have other postmaster-death checks besides
>> HandleStartupProcInterrupts(), in the form of WL_EXIT_ON_PM_DEATH. That
>> seems a bit fragile, at the very least it needs a comment in
>> HandleStartupProcInterrupts() to call it out.
> 
> Surely that's the direction we want to go in, though, no?  Commit
> cfdf4dc4 was intended to standardise the way we react to postmaster
> death where waiting is involved.  I updated the comment in
> HandleStartupProcInterrupts() to highlight that the
> PostmasterIsAlive() check in there is only for the benefit of
> CPU-bound loops.
> 
>> Note that there's one more loop in ShutdownWalRcv() that uses pg_usleep().
> 
> Updating that one required me to invent a new wait_event for
> pg_stat_activity, which seems like progress.
> 
> Unfortunately, while I was doing that I realised that WaitLatch()
> without WL_SET_LATCH was broken by commit 3347c982bab (in master
> only), in a way not previously reached by the tests.  So 0001 is a
> patch to fix that.

-        pgstat_report_wait_start(WAIT_EVENT_RECOVERY_PAUSE);
-        pg_usleep(1000000L);    /* 1000 ms */
-        pgstat_report_wait_end();
+        WaitLatch(NULL, WL_EXIT_ON_PM_DEATH | WL_TIMEOUT, 1000,
+                  WAIT_EVENT_RECOVERY_PAUSE);

This change may cause at most one second delay against the standby
promotion request during WAL replay pause? It's only one second,
but I'd like to avoid this (unnecessary) wait to shorten the failover time
as much as possible basically. So what about using WL_SET_LATCH here?

But when using WL_SET_LATCH, one concern is that walreceiver can
wake up the startup process too frequently even during WAL replay pause.
This is also problematic? I'm ok with this, but if not, using pg_usleep()
might be better as the original code does.

Regards,

-- 
Fujii Masao
Advanced Computing Technology Center
Research and Development Headquarters
NTT DATA CORPORATION

On Sat, Sep 19, 2020 at 6:07 AM Fujii Masao <masao.fujii@oss.nttdata.com> wrote:
> -               pgstat_report_wait_start(WAIT_EVENT_RECOVERY_PAUSE);
> -               pg_usleep(1000000L);    /* 1000 ms */
> -               pgstat_report_wait_end();
> +               WaitLatch(NULL, WL_EXIT_ON_PM_DEATH | WL_TIMEOUT, 1000,
> +                                 WAIT_EVENT_RECOVERY_PAUSE);
>
> This change may cause at most one second delay against the standby
> promotion request during WAL replay pause? It's only one second,
> but I'd like to avoid this (unnecessary) wait to shorten the failover time
> as much as possible basically. So what about using WL_SET_LATCH here?

Right, there is a comment saying that we could do that:

 * XXX Could also be done with shared latch, avoiding the pg_usleep loop.
 * Probably not worth the trouble though.  This state shouldn't be one that
 * anyone cares about server power consumption in.

> But when using WL_SET_LATCH, one concern is that walreceiver can
> wake up the startup process too frequently even during WAL replay pause.
> This is also problematic? I'm ok with this, but if not, using pg_usleep()
> might be better as the original code does.

You're right, at least if we used recoveryWakeupLatch.  Although we'd
react to pg_wal_replay_resume() faster, which would be nice, we
wouldn't be saving energy, we'd be using more energy due to all the
other latch wakeups that we'd be ignoring.  I believe the correct
solution to this problem is to add a ConditionVariable
"recoveryPauseChanged" into XLogCtlData, and then broadcast on it in
SetRecoveryPause().  This would be a trivial change, except for one
small problem: ConditionVariableTimedSleep() contains
CHECK_FOR_INTERRUPTS(), but startup.c has its own special interrupt
handling rather than using ProcessInterrupts() from postgres.c.  Maybe
that's OK, I'm not sure, but it requires more thought, and I propose
to keep the existing sloppy polling for now and leave precise wakeup
improvements for a separate patch.  The primary goal of this patch is
to switch to the standard treatment of postmaster death in wait loops,
so that we're free to reduce the sampling frequency in
HandleStartupProcInterrupts(), to fix a horrible performance problem.
I have at least tweaked that comment about pg_usleep(), though,
because that was out of date; I also used (void) WaitLatch(...) to
make it look like other places where we ignore the return value
(perhaps some static analyser out there somewhere cares?)

By the way, a CV could probably be used for walreceiver state changes
too, to improve ShutdownWalRcv().

Although I know from CI that this builds and passes "make check" on
Windows, I'm hoping to attract some review of the 0001 patch from a
Windows person, and confirmation that it passes "check-world" (or at
least src/test/recovery check) there, because I don't have CI scripts
for that yet.

On Sun, 20 Sep 2020 at 09:29, Thomas Munro <thomas.munro@gmail.com> wrote:
>
> Although I know from CI that this builds and passes "make check" on
> Windows, I'm hoping to attract some review of the 0001 patch from a
> Windows person, and confirmation that it passes "check-world" (or at
> least src/test/recovery check) there, because I don't have CI scripts
> for that yet.

I've gone as far as running the recovery tests on the v3-0001 patch
using a Windows machine. They pass:

L:\Projects\Postgres\d\src\tools\msvc>vcregress taptest src/test/recovery
...
t/001_stream_rep.pl .................. ok
t/002_archiving.pl ................... ok
t/003_recovery_targets.pl ............ ok
t/004_timeline_switch.pl ............. ok
t/005_replay_delay.pl ................ ok
t/006_logical_decoding.pl ............ ok
t/007_sync_rep.pl .................... ok
t/008_fsm_truncation.pl .............. ok
t/009_twophase.pl .................... ok
t/010_logical_decoding_timelines.pl .. ok
t/011_crash_recovery.pl .............. skipped: Test fails on Windows perl
t/012_subtransactions.pl ............. ok
t/013_crash_restart.pl ............... ok
t/014_unlogged_reinit.pl ............. ok
t/015_promotion_pages.pl ............. ok
t/016_min_consistency.pl ............. ok
t/017_shm.pl ......................... skipped: SysV shared memory not
supported by this platform
t/018_wal_optimize.pl ................ ok
t/019_replslot_limit.pl .............. ok
t/020_archive_status.pl .............. ok
All tests successful.
Files=20, Tests=222, 397 wallclock secs ( 0.08 usr +  0.00 sys =  0.08 CPU)
Result: PASS

L:\Projects\Postgres\d\src\tools\msvc>git diff --stat
 src/backend/storage/ipc/latch.c | 23 +++++++++++++++++++----
 1 file changed, 19 insertions(+), 4 deletions(-)

David

On Wed, Sep 23, 2020 at 2:27 PM David Rowley <dgrowleyml@gmail.com> wrote:
> I've gone as far as running the recovery tests on the v3-0001 patch
> using a Windows machine. They pass:

Thanks!  I pushed that one, because it was effectively a bug fix
(WaitLatch() without a latch was supposed to work).

I'll wait longer for feedback on the main patch; perhaps someone has a
better idea, or wants to take issue with the magic number 1024 (ie
limit on how many records we'll replay before we notice the postmaster
has exited), or my plan to harmonise those wait loops?  It has a CF
entry for the next CF.

On 2020/09/23 12:47, Thomas Munro wrote:
> On Wed, Sep 23, 2020 at 2:27 PM David Rowley <dgrowleyml@gmail.com> wrote:
>> I've gone as far as running the recovery tests on the v3-0001 patch
>> using a Windows machine. They pass:
> 
> Thanks!  I pushed that one, because it was effectively a bug fix
> (WaitLatch() without a latch was supposed to work).

Great!


> 
> I'll wait longer for feedback on the main patch; perhaps someone has a
> better idea, or wants to take issue with the magic number 1024 (ie
> limit on how many records we'll replay before we notice the postmaster
> has exited), or my plan to harmonise those wait loops?  It has a CF
> entry for the next CF.

Does this patch work fine with warm-standby case using pg_standby?
IIUC the startup process doesn't call WaitLatch() in that case, so ISTM that,
with the patch, it cannot detect the postmaster death immediately.

Regards,

-- 
Fujii Masao
Advanced Computing Technology Center
Research and Development Headquarters
NTT DATA CORPORATION

On Thu, Sep 24, 2020 at 2:39 AM Fujii Masao <masao.fujii@oss.nttdata.com> wrote:
> Does this patch work fine with warm-standby case using pg_standby?
> IIUC the startup process doesn't call WaitLatch() in that case, so ISTM that,
> with the patch, it cannot detect the postmaster death immediately.

Right, RestoreArchivedFile() uses system(), so I guess it can hang
around for a long time after unexpected postmaster exit on every OS if
the command waits.  To respond to various kinds of important
interrupts, I suppose that'd ideally use something like
OpenPipeStream() and a typical WaitLatch() loop with CFI().  I'm not
sure what our policy is or should be for exiting while we have running
subprocesses.  I guess that is a separate issue.

Here's a rebase, no code change.

On Thu, Sep 24, 2020 at 05:55:17PM +1200, Thomas Munro wrote:
> Right, RestoreArchivedFile() uses system(), so I guess it can hang
> around for a long time after unexpected postmaster exit on every OS if
> the command waits.  To respond to various kinds of important
> interrupts, I suppose that'd ideally use something like
> OpenPipeStream() and a typical WaitLatch() loop with CFI().  I'm not
> sure what our policy is or should be for exiting while we have running
> subprocesses.  I guess that is a separate issue.

-   if (IsUnderPostmaster && !PostmasterIsAlive())
+   if (IsUnderPostmaster &&
+#ifndef USE_POSTMASTER_DEATH_SIGNAL
+       count++ % 1024 == 0 &&
+#endif
+       !PostmasterIsAlive())
That's pretty hack-ish, still efficient.  Could we consider a
different approach like something relying on
PostmasterIsAliveInternal() with repetitive call handling?  This may
not be the only place where we care about that, particularly for
non-core code.

No objections with the two changes from pg_usleep() to WaitLatch() so
they could be applied separately first.
--
Michael

On 2021/03/02 10:10, Thomas Munro wrote:
> On Tue, Mar 2, 2021 at 12:00 AM Thomas Munro <thomas.munro@gmail.com> wrote:
>> On Mon, Nov 16, 2020 at 8:56 PM Michael Paquier <michael@paquier.xyz> wrote:
>>> No objections with the two changes from pg_usleep() to WaitLatch() so
>>> they could be applied separately first.
>>
>> I thought about committing that first part, and got as far as
>> splitting the patch into two (see attached), but then I re-read
>> Fujii-san's message about the speed of promotion and realised that we
>> really should have something like a condition variable for walRcvState
>> changes.  I'll look into that.
> 
> Here's an experimental attempt at that, though I'm not sure if it's
> the right approach.  Of course it's not necessary to use condition
> variables here: we could use recoveryWakeupLatch, because we're not in
> any doubt about who needs to be woken up.  But then you could get
> constant wakeups while recovery is paused, unless you also suppressed
> that somehow.  You could use the startup process's procLatch,
> advertised in shmem, but that's almost a condition variable.  With a
> condition variable, you get to name it something like
> walRcvStateChanged, and then the programming rule is very clear: if
> you change walRcvState, you need to broadcast that fact (and you don't
> have to worry about who might be interested).  One question I haven't
> got to the bottom of: is it a problem for the startup process that CVs
> use CHECK_FOR_INTERRUPTS()?

I found 0001 patch was committed in de829ddf23, and which added new
wait event WalrcvExit. This name seems not consistent with other wait
events. I'm thinking it's better to rename it to WalReceiverExit. Thought?
Patch attached.

Regards,

-- 
Fujii Masao
Advanced Computing Technology Center
Research and Development Headquarters
NTT DATA CORPORATION

On Tue, Mar 23, 2021 at 2:44 PM Fujii Masao <masao.fujii@oss.nttdata.com> wrote:
> I found 0001 patch was committed in de829ddf23, and which added new
> wait event WalrcvExit. This name seems not consistent with other wait
> events. I'm thinking it's better to rename it to WalReceiverExit. Thought?
> Patch attached.

Agreed, your names are better.  Thanks.

On 2021/03/23 10:52, Thomas Munro wrote:
> On Tue, Mar 23, 2021 at 2:44 PM Fujii Masao <masao.fujii@oss.nttdata.com> wrote:
>> I found 0001 patch was committed in de829ddf23, and which added new
>> wait event WalrcvExit. This name seems not consistent with other wait
>> events. I'm thinking it's better to rename it to WalReceiverExit. Thought?
>> Patch attached.
> 
> Agreed, your names are better.  Thanks.

Thanks! I will commit the patch.

Regards,

-- 
Fujii Masao
Advanced Computing Technology Center
Research and Development Headquarters
NTT DATA CORPORATION

On 2021/03/23 14:49, Fujii Masao wrote:
> 
> 
> On 2021/03/23 10:52, Thomas Munro wrote:
>> On Tue, Mar 23, 2021 at 2:44 PM Fujii Masao <masao.fujii@oss.nttdata.com> wrote:
>>> I found 0001 patch was committed in de829ddf23, and which added new
>>> wait event WalrcvExit. This name seems not consistent with other wait
>>> events. I'm thinking it's better to rename it to WalReceiverExit. Thought?
>>> Patch attached.
>>
>> Agreed, your names are better.  Thanks.
> 
> Thanks! I will commit the patch.

Pushed. Thanks!

Regards,

-- 
Fujii Masao
Advanced Computing Technology Center
Research and Development Headquarters
NTT DATA CORPORATION

On Fri, Mar 12, 2021 at 7:55 PM Thomas Munro <thomas.munro@gmail.com> wrote:
> On Thu, Mar 11, 2021 at 7:34 PM Michael Paquier <michael@paquier.xyz> wrote:
> > Wow.  This probably means that we would be able to get rid of
> > USE_POSTMASTER_DEATH_SIGNAL?
>
> We'll still need it, because there'd still be systems with no signal:
> NetBSD, OpenBSD, AIX, HPUX, illumos.

Erm, and of course macOS.

There is actually something we could do here: ioctl(I_SETSIG) for
SysV-derived systems and fcntl(O_ASYNC) for BSD-derived systems seems
like a promising way to get a SIGIO signal when the postmaster goes
away and the pipe becomes readable.  Previously I'd discounted this,
because it's not in POSIX and I doubted it would work well on other
systems.  But I was flicking through Stevens' UNIX book while trying
to figure out that POLLHUP stuff from a nearby thread (though it's
useless for that purpose) and I learned from section 12.6 that SIGIO
is fairly ancient, originating in 4.2BSD, adopted by SVR4, so it's
likely present in quite a few systems, maybe even all of our support
platforms if we're prepared to do it two different ways.  Just a
thought.

On Wed, Mar 31, 2021 at 7:02 PM Thomas Munro <thomas.munro@gmail.com> wrote:
> On Fri, Mar 12, 2021 at 7:55 PM Thomas Munro <thomas.munro@gmail.com> wrote:
> > On Thu, Mar 11, 2021 at 7:34 PM Michael Paquier <michael@paquier.xyz> wrote:
> > > Wow.  This probably means that we would be able to get rid of
> > > USE_POSTMASTER_DEATH_SIGNAL?
> >
> > We'll still need it, because there'd still be systems with no signal:
> > NetBSD, OpenBSD, AIX, HPUX, illumos.
>
> Erm, and of course macOS.
>
> There is actually something we could do here: ioctl(I_SETSIG) for
> SysV-derived systems and fcntl(O_ASYNC) for BSD-derived systems seems
> like a promising way to get a SIGIO signal when the postmaster goes
> away and the pipe becomes readable.  Previously I'd discounted this,
> because it's not in POSIX and I doubted it would work well on other
> systems.  But I was flicking through Stevens' UNIX book while trying
> to figure out that POLLHUP stuff from a nearby thread (though it's
> useless for that purpose) and I learned from section 12.6 that SIGIO
> is fairly ancient, originating in 4.2BSD, adopted by SVR4, so it's
> likely present in quite a few systems, maybe even all of our support
> platforms if we're prepared to do it two different ways.  Just a
> thought.

Alright, here's a proof-of-concept patch that does that.  Adding to the next CF.

This seems to work on Linux, macOS, FreeBSD and OpenBSD (and I assume
any other BSD).  Can anyone tell me if it works on illumos, AIX or
HPUX, and if not, how to fix it or disable the feature gracefully?
For now the patch assumes that if you have SIGIO then you can do this;
perhaps it should also test for O_ASYNC?  Perhaps HPUX has the signal
but requires a different incantation with I_SETSIG?

Full disclosure: The reason for my interest in this subject is that I
have a work-in-progress patch set to make latches, locks and condition
variables more efficient using futexes on several OSes, but it needs a
signal to wake on postmaster death.

On 09/04/2021 07:01, Thomas Munro wrote:
> On Wed, Mar 31, 2021 at 7:02 PM Thomas Munro <thomas.munro@gmail.com> wrote:
>> On Fri, Mar 12, 2021 at 7:55 PM Thomas Munro <thomas.munro@gmail.com> wrote:
>>> On Thu, Mar 11, 2021 at 7:34 PM Michael Paquier <michael@paquier.xyz> wrote:
>>>> Wow.  This probably means that we would be able to get rid of
>>>> USE_POSTMASTER_DEATH_SIGNAL?
>>>
>>> We'll still need it, because there'd still be systems with no signal:
>>> NetBSD, OpenBSD, AIX, HPUX, illumos.
>>
>> Erm, and of course macOS.
>>
>> There is actually something we could do here: ioctl(I_SETSIG) for
>> SysV-derived systems and fcntl(O_ASYNC) for BSD-derived systems seems
>> like a promising way to get a SIGIO signal when the postmaster goes
>> away and the pipe becomes readable.  Previously I'd discounted this,
>> because it's not in POSIX and I doubted it would work well on other
>> systems.  But I was flicking through Stevens' UNIX book while trying
>> to figure out that POLLHUP stuff from a nearby thread (though it's
>> useless for that purpose) and I learned from section 12.6 that SIGIO
>> is fairly ancient, originating in 4.2BSD, adopted by SVR4, so it's
>> likely present in quite a few systems, maybe even all of our support
>> platforms if we're prepared to do it two different ways.  Just a
>> thought.
> 
> Alright, here's a proof-of-concept patch that does that.  Adding to the next CF.

Looks good to me.

> This seems to work on Linux, macOS, FreeBSD and OpenBSD (and I assume
> any other BSD).  Can anyone tell me if it works on illumos, AIX or
> HPUX, and if not, how to fix it or disable the feature gracefully?
> For now the patch assumes that if you have SIGIO then you can do this;
> perhaps it should also test for O_ASYNC?  Perhaps HPUX has the signal
> but requires a different incantation with I_SETSIG?

I think it would be OK to just commit this (after REL_14_STABLE has been 
created) and see what breaks in the buildfarm. Then we'll at least know 
if we need more autoconf checks or something to disable this.

- Heikki

Heikki Linnakangas <hlinnaka@iki.fi> writes:
> On 09/04/2021 07:01, Thomas Munro wrote:
>> This seems to work on Linux, macOS, FreeBSD and OpenBSD (and I assume
>> any other BSD).  Can anyone tell me if it works on illumos, AIX or
>> HPUX, and if not, how to fix it or disable the feature gracefully?
>> For now the patch assumes that if you have SIGIO then you can do this;
>> perhaps it should also test for O_ASYNC?  Perhaps HPUX has the signal
>> but requires a different incantation with I_SETSIG?

I took a look on HPUX 10.20 (gaur's host):

* SIGIO exists, but signal.h only defines it with
-D_INCLUDE_HPUX_SOURCE which we don't use.

* I found I_SETSIG, but ...

$ grep -r SETSIG /usr/include
/usr/include/sys/stropts.h:#define      I_SETSIG        _IO('S', 9) /* request SIGPOLL signal on events */

stropts.h seems to be for a feature called "streams", which is
probably nonstandard enough that we don't want to deal with it.

So I think the short answer on this platform is that if you conditionalize
on #ifdef SIGIO then it will just not do it, and we should be fine.

Can't say about HPUX 11.

            regards, tom lane

On Fri, Jun 11, 2021 at 1:18 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Heikki Linnakangas <hlinnaka@iki.fi> writes:
> > On 09/04/2021 07:01, Thomas Munro wrote:
> >> This seems to work on Linux, macOS, FreeBSD and OpenBSD (and I assume
> >> any other BSD).  Can anyone tell me if it works on illumos, AIX or
> >> HPUX, and if not, how to fix it or disable the feature gracefully?
> >> For now the patch assumes that if you have SIGIO then you can do this;
> >> perhaps it should also test for O_ASYNC?  Perhaps HPUX has the signal
> >> but requires a different incantation with I_SETSIG?
>
> I took a look on HPUX 10.20 (gaur's host):

Thanks both for looking!

Unfortunately I'll have to withdraw this patch in its current form.
On closer inspection, only the last backend to start up and do
F_SETOWN on the pipe receives the signal.  We'd probably have to
create a separate pipe for each backend, or something like that, which
seems unwarranted so far.

> * I found I_SETSIG, but ...
>
> $ grep -r SETSIG /usr/include
> /usr/include/sys/stropts.h:#define      I_SETSIG        _IO('S', 9) /* request SIGPOLL signal on events */
>
> stropts.h seems to be for a feature called "streams", which is
> probably nonstandard enough that we don't want to deal with it.

Agreed.  It is technically POSIX, but optional and marked obsolescent.
It's annoying to see that I_SETSIG did allow multiple processes to
register to receive signals for the same event on the same underlying
stream, unlike F_SETOWN.  Oh well.