Re: libpqrcv_PQexec() seems to violate latch protocol

On 2017-06-06 23:24:50 +0200, Petr Jelinek wrote:
> On 06/06/17 23:17, Andres Freund wrote:
> > Right. I found a couple more instance of similarly iffy, although not
> > quite as broken, patterns in launcher.c. It's easy to get this wrong,
> > but it's a lot easy if you do it differently everywhere you use a
> > latch. It's not good if code in the same file, by the same author(s),
> > has different ways of using latches.
>
> Huh? I see same pattern everywhere in launcher.c, what am I missing?

WaitForReplicationWorkerAttach:
while (...)
CHECK_FOR_INTERRUPTS();
/* other stuff including returns */
WaitLatch()
WL_POSTMASTER_DEATH
ResetLatch()

logicalrep_worker_stop loop 1:
while (...)
/* other stuff */
CHECK_FOR_INTERRUPTS()
WaitLatch()
POSTMASTER_DEATH
ResetLatch()
/* other stuff including returns */
logicalrep_worker_stop loop 1:
while (...)
/* other stuff including returns */
CHECK_FOR_INTERRUPTS();
WaitLatch()
WL_POSTMASTER_DEATH
ResetLatch()

ApplyLauncherMain:
while (!got_SIGTERM)
/* lots other stuff */
WaitLatch()
WL_POSTMASTER_DEATH
/* some other stuff */
ResetLatch()
(note no CFI)

they're not hugely different, but subtely there are differences.
Sometimes you're guaranteed to check for interrupts after resetting the
latch, in other cases not. Sometimes expensive-ish things happen before
a CFI...

Greetings,

Andres Freund