From dd40a3e3c28c69466bc2e8c2a223608ac51e05b7 Mon Sep 17 00:00:00 2001
From: Thomas Munro <thomas.munro@gmail.com>
Date: Fri, 10 Mar 2023 16:19:42 +1300
Subject: [PATCH 3/3] Use microsecond-based naps in walreceiver.

Since anything based on timestamp differences is really in microseconds
under the covers, we might as well use the new higher resolution API for
waiting.

XXX For illustration; there would be many other places that could change
like this
---
 src/backend/replication/walreceiver.c | 16 ++++++++--------
 src/backend/utils/adt/timestamp.c     | 20 ++++++++++++++++++++
 src/include/utils/timestamp.h         |  2 ++
 3 files changed, 30 insertions(+), 8 deletions(-)

diff --git a/src/backend/replication/walreceiver.c b/src/backend/replication/walreceiver.c
index f6446da2d6..18c66c0c63 100644
--- a/src/backend/replication/walreceiver.c
+++ b/src/backend/replication/walreceiver.c
@@ -445,7 +445,7 @@ WalReceiverMain(void)
 				pgsocket	wait_fd = PGINVALID_SOCKET;
 				int			rc;
 				TimestampTz nextWakeup;
-				long		nap;
+				int64		nap;
 
 				/*
 				 * Exit walreceiver if we're not in recovery. This should not
@@ -530,7 +530,7 @@ WalReceiverMain(void)
 
 				/* Calculate the nap time, clamping as necessary. */
 				now = GetCurrentTimestamp();
-				nap = TimestampDifferenceMilliseconds(now, nextWakeup);
+				nap = TimestampDifferenceMicroseconds(now, nextWakeup);
 
 				/*
 				 * Ideally we would reuse a WaitEventSet object repeatedly
@@ -544,12 +544,12 @@ WalReceiverMain(void)
 				 * avoiding some system calls.
 				 */
 				Assert(wait_fd != PGINVALID_SOCKET);
-				rc = WaitLatchOrSocket(MyLatch,
-									   WL_EXIT_ON_PM_DEATH | WL_SOCKET_READABLE |
-									   WL_TIMEOUT | WL_LATCH_SET,
-									   wait_fd,
-									   nap,
-									   WAIT_EVENT_WAL_RECEIVER_MAIN);
+				rc = WaitLatchOrSocketUs(MyLatch,
+										 WL_EXIT_ON_PM_DEATH | WL_SOCKET_READABLE |
+										 WL_TIMEOUT | WL_LATCH_SET,
+										 wait_fd,
+										 nap,
+										 WAIT_EVENT_WAL_RECEIVER_MAIN);
 				if (rc & WL_LATCH_SET)
 				{
 					ResetLatch(MyLatch);
diff --git a/src/backend/utils/adt/timestamp.c b/src/backend/utils/adt/timestamp.c
index de93db89d4..52f6568397 100644
--- a/src/backend/utils/adt/timestamp.c
+++ b/src/backend/utils/adt/timestamp.c
@@ -1719,6 +1719,26 @@ TimestampDifferenceMilliseconds(TimestampTz start_time, TimestampTz stop_time)
 		return (long) ((diff + 999) / 1000);
 }
 
+/*
+ * TimestampDifferenceMicroseconds -- convert the difference between two
+ * 		timestamps into microseconds
+ *
+ * Compute a wait time for WaitLatchUs().
+ */
+int64
+TimestampDifferenceMicroseconds(TimestampTz start_time, TimestampTz stop_time)
+{
+	TimestampTz diff;
+
+	/* Deal with zero or negative elapsed time quickly. */
+	if (start_time >= stop_time)
+		return 0;
+	/* To not fail with timestamp infinities, we must detect overflow. */
+	if (pg_sub_s64_overflow(stop_time, start_time, &diff))
+		return PG_INT64_MAX;
+	return diff;
+}
+
 /*
  * TimestampDifferenceExceeds -- report whether the difference between two
  *		timestamps is >= a threshold (expressed in milliseconds)
diff --git a/src/include/utils/timestamp.h b/src/include/utils/timestamp.h
index edd59dc432..1caa15221d 100644
--- a/src/include/utils/timestamp.h
+++ b/src/include/utils/timestamp.h
@@ -100,6 +100,8 @@ extern void TimestampDifference(TimestampTz start_time, TimestampTz stop_time,
 								long *secs, int *microsecs);
 extern long TimestampDifferenceMilliseconds(TimestampTz start_time,
 											TimestampTz stop_time);
+extern int64 TimestampDifferenceMicroseconds(TimestampTz start_time,
+											 TimestampTz stop_time);
 extern bool TimestampDifferenceExceeds(TimestampTz start_time,
 									   TimestampTz stop_time,
 									   int msec);
-- 
2.39.2