Fix and simplify some usages of TimestampDifference().

Introduce TimestampDifferenceMilliseconds() to simplify callers
that would rather have the difference in milliseconds, instead of
the select()-oriented seconds-and-microseconds format.  This gets
rid of at least one integer division per call, and it eliminates
some apparently-easy-to-mess-up arithmetic.

Two of these call sites were in fact wrong:

* pg_prewarm's autoprewarm_main() forgot to multiply the seconds
by 1000, thus ending up with a delay 1000X shorter than intended.
That doesn't quite make it a busy-wait, but close.

* postgres_fdw's pgfdw_get_cleanup_result() thought it needed to compute
microseconds not milliseconds, thus ending up with a delay 1000X longer
than intended.  Somebody along the way had noticed this problem but
misdiagnosed the cause, and imposed an ad-hoc 60-second limit rather
than fixing the units.  This was relatively harmless in context, because
we don't care that much about exactly how long this delay is; still,
it's wrong.

There are a few more callers of TimestampDifference() that don't
have a direct need for seconds-and-microseconds, but can't use
TimestampDifferenceMilliseconds() either because they do need
microsecond precision or because they might possibly deal with
intervals long enough to overflow 32-bit milliseconds.  It might be
worth inventing another API to improve that, but that seems outside
the scope of this patch; so those callers are untouched here.

Given the fact that we are fixing some bugs, and the likelihood
that future patches might want to back-patch code that uses this
new API, back-patch to all supported branches.

Alexey Kondratov and Tom Lane

Discussion: https://postgr.es/m/3b1c053a21c07c1ed5e00be3b2b855ef@postgrespro.ru

1 files changed, 5 insertions, 7 deletions

diff --git a/contrib/pg_prewarm/autoprewarm.c b/contrib/pg_prewarm/autoprewarm.c
index 3bd0010bf8b..fcbdbdd0279 100644
--- a/contrib/pg_prewarm/autoprewarm.c
+++ b/contrib/pg_prewarm/autoprewarm.c
@@ -226,18 +226,16 @@ autoprewarm_main(Datum main_arg)
 		}
 		else
 		{
-			long		delay_in_ms = 0;
-			TimestampTz next_dump_time = 0;
-			long		secs = 0;
-			int			usecs = 0;
+			TimestampTz next_dump_time;
+			long		delay_in_ms;
 
 			/* Compute the next dump time. */
 			next_dump_time =
 				TimestampTzPlusMilliseconds(last_dump_time,
 											autoprewarm_interval * 1000);
-			TimestampDifference(GetCurrentTimestamp(), next_dump_time,
-								&secs, &usecs);
-			delay_in_ms = secs + (usecs / 1000);
+			delay_in_ms =
+				TimestampDifferenceMilliseconds(GetCurrentTimestamp(),
+												next_dump_time);
 
 			/* Perform a dump if it's time. */
 			if (delay_in_ms <= 0)