psql: Fix \watch when using interval values less than 1ms

michaelpq · michaelpq · commit 8a6170860c83 · 2024-10-14T12:27:57.000+09:00
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 6f9ee74, 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
diff --git a/src/bin/psql/command.c b/src/bin/psql/command.c
@@ -5231,6 +5231,10 @@ 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)
@@ -5356,10 +5360,10 @@ 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 */
@@ -5380,7 +5384,8 @@ 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
@@ -352,8 +352,14 @@ sub psql_fails_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(
