libpqrcv_PQexec() seems to violate latch protocol

Hi,

The function in $subject does:
while (PQisBusy(streamConn))
{
int rc;

/*
* We don't need to break down the sleep into smaller increments,
* since we'll get interrupted by signals and can either handle
* interrupts here or elog(FATAL) within SIGTERM signal handler if
* the signal arrives in the middle of establishment of
* replication connection.
*/
ResetLatch(&MyProc->procLatch);
rc = WaitLatchOrSocket(&MyProc->procLatch,
WL_POSTMASTER_DEATH | WL_SOCKET_READABLE |
WL_LATCH_SET,
PQsocket(streamConn),
0,
WAIT_EVENT_LIBPQWALRECEIVER);
if (rc & WL_POSTMASTER_DEATH)
exit(1);
/* interrupted */
if (rc & WL_LATCH_SET)
{
CHECK_FOR_INTERRUPTS();
continue;
}

Doing ResetLatch();WaitLatch() like that makes it possible to miss a the
latch being set, e.g. if it happens just after WaitLatchOrSocket()
returns.

Afaict, the ResetLatch() really should just instead be in the if (rc & WL_LATCH_SET)
block.

Unless somebody protests, I'll make it so.

Greetings,

Andres Freund