From 9f34cae1426fe5ab915c95b7e26b1efffbb2ca0f Mon Sep 17 00:00:00 2001
From: Michael Paquier <michael@paquier.xyz>
Date: Mon, 14 Oct 2024 12:27:51 +0900
Subject: [PATCH] psql: Fix \watch when using interval values less than 1ms

Attempting to use an interval of time less than 1ms would cause \watch
to hang.  This was confusing, so let's change the logic so as an
interval lower than 1ms behaves the same as 0.

Comments are added to mention that the internals of do_watch() had
better rely on "sleep_ms", the interval value in milliseconds.  While on
it, this commit adds a test to check the behavior of interval values
less than 1ms.

\watch hanging for interval values less than 1ms existed before
6f9ee74d45aa, that has changed the code to support an interval value of
0.

Reported-by: Heikki Linnakangas
Author: Andrey M. Borodin, Michael Paquier
Discussion: https://postgr.es/m/88445e0e-3156-4b9d-afae-9a1a7b1631f6@iki.fi
Backpatch-through: 16
---
 src/bin/psql/command.c      | 11 ++++++++---
 src/bin/psql/t/001_basic.pl | 10 ++++++++--
 2 files changed, 16 insertions(+), 5 deletions(-)

diff --git a/src/bin/psql/command.c b/src/bin/psql/command.c
index 2bb8789750..328d78c73f 100644
--- a/src/bin/psql/command.c
+++ b/src/bin/psql/command.c
@@ -5359,6 +5359,10 @@ do_shell(const char *command)
  *
  * We break this out of exec_command to avoid having to plaster "volatile"
  * onto a bunch of exec_command's variables to silence stupider compilers.
+ *
+ * "sleep" is the amount of time to sleep during each loop, measured in
+ * seconds.  The internals of this function should use "sleep_ms" for
+ * precise sleep time calculations.
  */
 static bool
 do_watch(PQExpBuffer query_buf, double sleep, int iter, int min_rows)
@@ -5484,10 +5488,10 @@ do_watch(PQExpBuffer query_buf, double sleep, int iter, int min_rows)
 
 		if (user_title)
 			snprintf(title, title_len, _("%s\t%s (every %gs)\n"),
-					 user_title, timebuf, sleep);
+					 user_title, timebuf, sleep_ms / 1000.0);
 		else
 			snprintf(title, title_len, _("%s (every %gs)\n"),
-					 timebuf, sleep);
+					 timebuf, sleep_ms / 1000.0);
 		myopt.title = title;
 
 		/* Run the query and print out the result */
@@ -5508,7 +5512,8 @@ do_watch(PQExpBuffer query_buf, double sleep, int iter, int min_rows)
 		if (pagerpipe && ferror(pagerpipe))
 			break;
 
-		if (sleep == 0)
+		/* Tight loop, no wait needed */
+		if (sleep_ms == 0)
 			continue;
 
 #ifdef WIN32
diff --git a/src/bin/psql/t/001_basic.pl b/src/bin/psql/t/001_basic.pl
index 5f2f4541af..bd4fdd2030 100644
--- a/src/bin/psql/t/001_basic.pl
+++ b/src/bin/psql/t/001_basic.pl
@@ -352,8 +352,14 @@ psql_like(
 
 # Check \watch
 # Note: the interval value is parsed with locale-aware strtod()
-psql_like($node, sprintf('SELECT 1 \watch c=3 i=%g', 0.01),
-	qr/1\n1\n1/, '\watch with 3 iterations');
+psql_like(
+	$node, sprintf('SELECT 1 \watch c=3 i=%g', 0.01),
+	qr/1\n1\n1/, '\watch with 3 iterations, interval of 0.01');
+
+# Sub-millisecond wait works, equivalent to 0.
+psql_like(
+	$node, sprintf('SELECT 1 \watch c=3 i=%g', 0.0001),
+	qr/1\n1\n1/, '\watch with 3 iterations, interval of 0.0001');
 
 # Check \watch minimum row count
 psql_fails_like(