Kill dead-end children when there's nothing else left

Previously, the postmaster would never try to kill dead-end child
processes, even if there were no other processes left. A dead-end
backend will eventually exit, when authentication_timeout expires, but
if a dead-end backend is the only thing that's preventing the server
from shutting down, it seems better to kill it immediately. It's
particularly important, if there was a bug in the early startup code
that prevented a dead-end child from timing out and exiting normally.

Includes a test for that case where a dead-end backend previously
prevented the server from shutting down.

Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://www.postgresql.org/message-id/a102f15f-eac4-4ff2-af02-f9ff209ec66f@iki.fi

src/backend/postmaster/postmaster.c

@@ -2985,10 +2985,11 @@ PostmasterStateMachine(void)
 			if (Shutdown >= ImmediateShutdown || FatalError)
 			{
 				/*
-				 * Start waiting for dead_end children to die.  This state
-				 * change causes ServerLoop to stop creating new ones.
+				 * Stop any dead_end children and stop creating new ones.
 				 */
 				pmState = PM_WAIT_DEAD_END;
+				ConfigurePostmasterWaitSet(false);
+				SignalChildren(SIGQUIT, btmask(B_DEAD_END_BACKEND));
 
 				/*
 				 * We already SIGQUIT'd the archiver and stats processes, if
@@ -3027,9 +3028,10 @@ PostmasterStateMachine(void)
 					 */
 					FatalError = true;
 					pmState = PM_WAIT_DEAD_END;
+					ConfigurePostmasterWaitSet(false);
 
 					/* Kill the walsenders and archiver too */
-					SignalChildren(SIGQUIT, btmask_all_except(B_DEAD_END_BACKEND));
+					SignalChildren(SIGQUIT, BTYPE_MASK_ALL);
 					if (PgArchPID != 0)
 						signal_child(PgArchPID, SIGQUIT);
 				}
@@ -3048,14 +3050,13 @@ PostmasterStateMachine(void)
 		if (PgArchPID == 0 && CountChildren(btmask_all_except(B_DEAD_END_BACKEND)) == 0)
 		{
 			pmState = PM_WAIT_DEAD_END;
+			ConfigurePostmasterWaitSet(false);
+			SignalChildren(SIGTERM, BTYPE_MASK_ALL);
 		}
 	}
 
 	if (pmState == PM_WAIT_DEAD_END)
 	{
-		/* Don't allow any new socket connection events. */
-		ConfigurePostmasterWaitSet(false);
-
 		/*
 		 * PM_WAIT_DEAD_END state ends when the BackendList is entirely empty
 		 * (ie, no dead_end children remain), and the archiver is gone too.
@@ -3381,12 +3382,12 @@ SignalChildren(int signal, BackendTypeMask targetMask)
 
 /*
  * Send a termination signal to children.  This considers all of our children
- * processes, except syslogger and dead_end backends.
+ * processes, except syslogger.
  */
 static void
 TerminateChildren(int signal)
 {
-	SignalChildren(signal, btmask_all_except(B_DEAD_END_BACKEND));
+	SignalChildren(signal, BTYPE_MASK_ALL);
 	if (StartupPID != 0)
 	{
 		signal_child(StartupPID, signal);

src/test/perl/PostgreSQL/Test/Cluster.pm

@@ -1194,6 +1194,9 @@ this to fail.  Otherwise, tests might fail to detect server crashes.
 With optional extra param fail_ok => 1, returns 0 for failure
 instead of bailing out.
 
+The optional extra param timeout can be used to pass the pg_ctl
+--timeout option.
+
 =cut
 
 sub stop
@@ -1209,8 +1212,11 @@ sub stop
 	return 1 unless defined $self->{_pid};
 
 	print "### Stopping node \"$name\" using mode $mode\n";
-	$ret = PostgreSQL::Test::Utils::system_log('pg_ctl', '-D', $pgdata,
-		'-m', $mode, 'stop');
+	my @cmd = ('pg_ctl', '-D', $pgdata, '-m', $mode, 'stop');
+	if ($params{timeout}) {
+		push(@cmd, ('--timeout', $params{timeout}));
+	}
+	$ret = PostgreSQL::Test::Utils::system_log(@cmd);
 
 	if ($ret != 0)
 	{

src/test/postmaster/meson.build

@@ -7,6 +7,7 @@ tests += {
   'tap': {
     'tests': [
       't/001_connection_limits.pl',
+      't/002_start_stop.pl',
     ],
   },
 }

src/test/postmaster/t/002_start_stop.pl

@@ -0,0 +1,98 @@
+
+# Copyright (c) 2021-2024, PostgreSQL Global Development Group
+
+# Test postmaster start and stop state machine.
+
+use strict;
+use warnings FATAL => 'all';
+use PostgreSQL::Test::Cluster;
+use PostgreSQL::Test::Utils;
+use Test::More;
+
+#
+# Test that dead-end backends don't prevent the server from shutting
+# down.
+#
+# Dead-end backends can linger until they reach
+# authentication_timeout. We use a long authentication_timeout and a
+# much shorter timeout for the "pg_ctl stop" operation, to test that
+# if dead-end backends are killed at fast shut down. If they're not,
+# "pg_ctl stop" will error out before the authentication timeout kicks
+# in and cleans up the dead-end backends.
+my $authentication_timeout = $PostgreSQL::Test::Utils::timeout_default;
+my $stop_timeout = $authentication_timeout / 2;
+
+# Initialize the server with low connection limits, to test dead-end backends
+my $node = PostgreSQL::Test::Cluster->new('main');
+$node->init;
+$node->append_conf('postgresql.conf', "max_connections = 5");
+$node->append_conf('postgresql.conf', "max_wal_senders = 0");
+$node->append_conf('postgresql.conf', "autovacuum_max_workers = 1");
+$node->append_conf('postgresql.conf', "max_worker_processes = 1");
+$node->append_conf('postgresql.conf', "log_connections = on");
+$node->append_conf('postgresql.conf', "log_min_messages = debug2");
+$node->append_conf('postgresql.conf',
+	"authentication_timeout = '$authentication_timeout s'");
+$node->append_conf('postgresql.conf', 'trace_connection_negotiation=on');
+$node->start;
+
+if (!$node->raw_connect_works())
+{
+	plan skip_all => "this test requires working raw_connect()";
+}
+
+my @raw_connections = ();
+
+# Open a lot of TCP (or Unix domain socket) connections to use up all
+# the connection slots. Beyond a certain number (roughly 2x
+# max_connections), they will be "dead-end backends".
+for (my $i = 0; $i <= 20; $i++)
+{
+	my $sock = $node->raw_connect();
+
+	# On a busy system, the server might reject connections if
+	# postmaster cannot accept() them fast enough. The exact limit
+	# and behavior depends on the platform. To make this reliable,
+	# we attempt SSL negotiation on each connection before opening
+	# next one. The server will reject the SSL negotations, but
+	# when it does so, we know that the backend has been launched
+	# and we should be able to open another connection.
+
+	# SSLRequest packet consists of packet length followed by
+	# NEGOTIATE_SSL_CODE.
+	my $negotiate_ssl_code = pack("Nnn", 8, 1234, 5679);
+	my $sent = $sock->send($negotiate_ssl_code);
+
+	# Read reply. We expect the server to reject it with 'N'
+	my $reply = "";
+	$sock->recv($reply, 1);
+	is($reply, "N", "dead-end connection $i");
+
+	push(@raw_connections, $sock);
+}
+
+# When all the connection slots are in use, new connections will fail
+# before even looking up the user. Hence you now get "sorry, too many
+# clients already" instead of "role does not exist" error. Test that
+# to ensure that we have used up all the slots.
+$node->connect_fails("dbname=postgres user=invalid_user",
+	"connect ",
+	expected_stderr => qr/FATAL:  sorry, too many clients already/);
+
+# Open one more connection, to really ensure that we have at least one
+# dead-end backend.
+my $sock = $node->raw_connect();
+
+# Test that the dead-end backends don't prevent the server from stopping.
+$node->stop('fast', timeout => $stop_timeout);
+
+$node->start();
+$node->connect_ok("dbname=postgres", "works after restart");
+
+# Clean up
+foreach my $socket (@raw_connections)
+{
+	$socket->close();
+}
+
+done_testing();