Fix fractional vacuum_cost_delay.

Commit 4753ef37 changed vacuum_delay_point() to use the WaitLatch() API,
to fix the problem that vacuum could keep running for a very long time
after the postmaster died.

Unfortunately, that broke commit caf626b2's support for fractional
vacuum_cost_delay, which shipped in PostgreSQL 12.  WaitLatch() works in
whole milliseconds.

For now, revert the change from commit 4753ef37, but add an explicit
check for postmaster death.  That's an extra system call on systems
other than Linux and FreeBSD, but that overhead doesn't matter much
considering that we willingly went to sleep and woke up again.  (In
later work, we might add higher resolution timeouts to the latch API so
that we could do this with our standard programming pattern, but that
wouldn't be back-patched.)

Back-patch to 14, where commit 4753ef37 arrived.

Reported-by: Melanie Plageman 
Discussion: https://postgr.es/m/CAAKRu_b-q0hXCBUCAATh0Z4Zi6UkiC0k2DFgoD3nC-r3SkR3tg%40mail.gmail.com

diff --git a/src/backend/commands/vacuum.c b/src/backend/commands/vacuum.c
index 5c4bc15b441b36da99b0703500a3268086b40d9e..ac69b4dc23142d35581571a40e30774dfcd004f7 100644 (file)
--- a/src/backend/commands/vacuum.c
+++ b/src/backend/commands/vacuum.c
@@ -45,6 +45,7 @@
 #include "postmaster/bgworker_internals.h"
 #include "storage/bufmgr.h"
 #include "storage/lmgr.h"
+#include "storage/pmsignal.h"
 #include "storage/proc.h"
 #include "storage/procarray.h"
 #include "utils/acl.h"
@@ -2171,11 +2172,18 @@ vacuum_delay_point(void)
        if (msec > VacuumCostDelay * 4)
            msec = VacuumCostDelay * 4;
 
-       (void) WaitLatch(MyLatch,
-                        WL_LATCH_SET | WL_TIMEOUT | WL_EXIT_ON_PM_DEATH,
-                        msec,
-                        WAIT_EVENT_VACUUM_DELAY);
-       ResetLatch(MyLatch);
+       pgstat_report_wait_start(WAIT_EVENT_VACUUM_DELAY);
+       pg_usleep(msec * 1000);
+       pgstat_report_wait_end();
+
+       /*
+        * We don't want to ignore postmaster death during very long vacuums
+        * with vacuum_cost_delay configured.  We can't use the usual
+        * WaitLatch() approach here because we want microsecond-based sleep
+        * durations above.
+        */
+       if (IsUnderPostmaster && !PostmasterIsAlive())
+           exit(1);
 
        VacuumCostBalance = 0;