Avoid picking already-bound TCP ports in kerberos and ldap test suites.

src/test/kerberos and src/test/ldap need to run a private authentication
server of the relevant type, for which they need a free TCP port.
They were just picking a random port number in 48K-64K, which works
except when something's already using the particular port.  Notably,
the probability of failure rises dramatically if one simply runs those
tests in a tight loop, because each test cycle leaves behind a bunch of
high ports that are transiently in TIME_WAIT state.

To fix, split out the code that PostgresNode.pm already had for
identifying a free TCP port number, so that it can be invoked to choose
a port for the KDC or LDAP server.  This isn't 100% bulletproof, since
conceivably something else on the machine could grab the port between
the time we check and the time we actually start the server.  But that's
a pretty short window, so in practice this should be good enough.

Back-patch to v11 where these test suites were added.

Patch by me, reviewed by Andrew Dunstan.

Discussion: https://postgr.es/m/3397.1564872168@sss.pgh.pa.us

src/test/kerberos/t/001_auth.pl

@@ -56,7 +56,7 @@ my $krb5_conf   = "${TestLib::tmp_check}/krb5.conf";
 my $kdc_conf    = "${TestLib::tmp_check}/kdc.conf";
 my $krb5_log    = "${TestLib::tmp_check}/krb5libs.log";
 my $kdc_log     = "${TestLib::tmp_check}/krb5kdc.log";
-my $kdc_port    = int(rand() * 16384) + 49152;
+my $kdc_port    = get_free_port();
 my $kdc_datadir = "${TestLib::tmp_check}/krb5kdc";
 my $kdc_pidfile = "${TestLib::tmp_check}/krb5kdc.pid";
 my $keytab      = "${TestLib::tmp_check}/krb5.keytab";

src/test/ldap/t/001_auth.pl

@@ -48,7 +48,7 @@ my $slapd_pidfile = "${TestLib::tmp_check}/slapd.pid";
 my $slapd_logfile = "${TestLib::tmp_check}/slapd.log";
 my $ldap_conf     = "${TestLib::tmp_check}/ldap.conf";
 my $ldap_server   = 'localhost';
-my $ldap_port     = int(rand() * 16384) + 49152;
+my $ldap_port     = get_free_port();
 my $ldaps_port    = $ldap_port + 1;
 my $ldap_url      = "ldap://$ldap_server:$ldap_port";
 my $ldaps_url     = "ldaps://$ldap_server:$ldaps_port";

src/test/perl/PostgresNode.pm

@@ -63,6 +63,9 @@ PostgresNode - class representing PostgreSQL server instance
   # Stop the server
   $node->stop('fast');
 
+  # Find a free, unprivileged TCP port to bind some other service to
+  my $port = get_free_port();
+
 =head1 DESCRIPTION
 
 PostgresNode contains a set of routines able to work on a PostgreSQL node,
@@ -102,6 +105,7 @@ use Scalar::Util qw(blessed);
 
 our @EXPORT = qw(
   get_new_node
+  get_free_port
 );
 
 our ($use_tcp, $test_localhost, $test_pghost, $last_host_assigned,
@@ -1022,54 +1026,21 @@ sub get_new_node
 	my $class = 'PostgresNode';
 	$class = shift if scalar(@_) % 2 != 1;
 	my ($name, %params) = @_;
-	my $port_is_forced = defined $params{port};
-	my $found          = $port_is_forced;
-	my $port = $port_is_forced ? $params{port} : $last_port_assigned;
 
-	while ($found == 0)
+	# Select a port.
+	my $port;
+	if (defined $params{port})
 	{
-
+		$port = $params{port};
-		# advance $port, wrapping correctly around range end
+	}
-		$port = 49152 if ++$port >= 65536;
+	else
-		print "# Checking port $port\n";
+	{
-
+		# When selecting a port, we look for an unassigned TCP port number,
-		# Check first that candidate port number is not included in
+		# even if we intend to use only Unix-domain sockets.  This is clearly
-		# the list of already-registered nodes.
+		# necessary on $use_tcp (Windows) configurations, and it seems like a
-		$found = 1;
+		# good idea on Unixen as well.
-		foreach my $node (@all_nodes)
+		$port = get_free_port();
-		{
-			$found = 0 if ($node->port == $port);
-		}
-
-		# Check to see if anything else is listening on this TCP port.  This
-		# is *necessary* on $use_tcp (Windows) configurations.  Seek a port
-		# available for all possible listen_addresses values, for own_host
-		# nodes and so the caller can harness this port for the widest range
-		# of purposes.  The 0.0.0.0 test achieves that for post-2006 Cygwin,
-		# which automatically sets SO_EXCLUSIVEADDRUSE.  The same holds for
-		# MSYS (a Cygwin fork).  Testing 0.0.0.0 is insufficient for Windows
-		# native Perl (https://stackoverflow.com/a/14388707), so we also test
-		# individual addresses.
-		#
-		# This seems like a good idea on Unixen as well, even though we don't
-		# ask the postmaster to open a TCP port on Unix.  On Non-Linux,
-		# non-Windows kernels, binding to 127.0.0.1/24 addresses other than
-		# 127.0.0.1 might fail with EADDRNOTAVAIL.  Binding to 0.0.0.0 is
-		# unnecessary on non-Windows systems.
-		#
-		# XXX A port available now may become unavailable by the time we start
-		# the postmaster.
-		if ($found == 1)
-		{
-			foreach my $addr (qw(127.0.0.1),
-				$use_tcp ? qw(127.0.0.2 127.0.0.3 0.0.0.0) : ())
-			{
-				can_bind($addr, $port) or $found = 0;
-			}
-		}
 	}
-
-	print "# Found port $port\n";
 
 	# Select a host.
 	my $host = $test_pghost;
@@ -1094,12 +1065,79 @@ sub get_new_node
 	# Add node to list of nodes
 	push(@all_nodes, $node);
 
-	# And update port for next time
-	$port_is_forced or $last_port_assigned = $port;
-
 	return $node;
 }
 
+=pod
+
+=item get_free_port()
+
+Locate an unprivileged (high) TCP port that's not currently bound to
+anything.  This is used by get_new_node, and is also exported for use
+by test cases that need to start other, non-Postgres servers.
+
+Ports assigned to existing PostgresNode objects are automatically
+excluded, even if those servers are not currently running.
+
+XXX A port available now may become unavailable by the time we start
+the desired service.
+
+=cut
+
+sub get_free_port
+{
+	my $found = 0;
+	my $port  = $last_port_assigned;
+
+	while ($found == 0)
+	{
+
+		# advance $port, wrapping correctly around range end
+		$port = 49152 if ++$port >= 65536;
+		print "# Checking port $port\n";
+
+		# Check first that candidate port number is not included in
+		# the list of already-registered nodes.
+		$found = 1;
+		foreach my $node (@all_nodes)
+		{
+			$found = 0 if ($node->port == $port);
+		}
+
+		# Check to see if anything else is listening on this TCP port.
+		# Seek a port available for all possible listen_addresses values,
+		# so callers can harness this port for the widest range of purposes.
+		# The 0.0.0.0 test achieves that for post-2006 Cygwin, which
+		# automatically sets SO_EXCLUSIVEADDRUSE.  The same holds for MSYS (a
+		# Cygwin fork).  Testing 0.0.0.0 is insufficient for Windows native
+		# Perl (https://stackoverflow.com/a/14388707), so we also test
+		# individual addresses.
+		#
+		# On non-Linux, non-Windows kernels, binding to 127.0.0/24 addresses
+		# other than 127.0.0.1 might fail with EADDRNOTAVAIL.  Binding to
+		# 0.0.0.0 is unnecessary on non-Windows systems.
+		if ($found == 1)
+		{
+			foreach my $addr (qw(127.0.0.1),
+				$use_tcp ? qw(127.0.0.2 127.0.0.3 0.0.0.0) : ())
+			{
+				if (!can_bind($addr, $port))
+				{
+					$found = 0;
+					last;
+				}
+			}
+		}
+	}
+
+	print "# Found port $port\n";
+
+	# Update port for next time
+	$last_port_assigned = $port;
+
+	return $port;
+}
+
 # Internal routine to check whether a host:port is available to bind
 sub can_bind
 {