From 6744818fe556f07fe6c004320fa18a15fbee98fd 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 3/4] 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.
---
 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