Fix walsender error cleanup code

In commit 850196b610 I (Álvaro) failed to handle the case of walsender
shutting down on an error before setting up its 'xlogreader' pointer;
the error handling code dereferences the pointer, causing a crash.
Fix by testing the pointer before trying to dereference it.

Kyotaro authored the code fix; I adopted Nathan's test case to be used
by the TAP tests and added the necessary PostgresNode change.

Reported-by: Nathan Bossart <bossartn@amazon.com>
Author: Kyotaro Horiguchi <horikyota.ntt@gmail.com>
Author: Álvaro Herrera <alvherre@alvh.no-ip.org>
Discussion: https://postgr.es/m/C04FC24E-903D-4423-B312-6910E4D846E5@amazon.com

src/backend/replication/walsender.c

@@ -315,7 +315,7 @@ WalSndErrorCleanup(void)
 	ConditionVariableCancelSleep();
 	pgstat_report_wait_end();
 
-	if (xlogreader->seg.ws_file >= 0)
+	if (xlogreader != NULL && xlogreader->seg.ws_file >= 0)
 		wal_segment_close(xlogreader);
 
 	if (MyReplicationSlot != NULL)

src/test/perl/PostgresNode.pm

@@ -1386,6 +1386,12 @@ the B<timed_out> parameter is also given.
 If B<timeout> is set and this parameter is given, the scalar it references
 is set to true if the psql call times out.
 
+=item replication => B<value>
+
+If set, add B<replication=value> to the conninfo string.
+Passing the literal value C<database> results in a logical replication
+connection.
+
 =item extra_params => ['--single-transaction']
 
 If given, it must be an array reference containing additional parameters to B<psql>.
@@ -1414,10 +1420,17 @@ sub psql
 
 	my $stdout            = $params{stdout};
 	my $stderr            = $params{stderr};
+	my $replication       = $params{replication};
 	my $timeout           = undef;
 	my $timeout_exception = 'psql timed out';
-	my @psql_params =
-	  ('psql', '-XAtq', '-d', $self->connstr($dbname), '-f', '-');
+	my @psql_params       = (
+		'psql',
+		'-XAtq',
+		'-d',
+		$self->connstr($dbname)
+		  . (defined $replication ? " replication=$replication" : ""),
+		'-f',
+		'-');
 
 	# If the caller wants an array and hasn't passed stdout/stderr
 	# references, allocate temporary ones to capture them so we

src/test/recovery/t/006_logical_decoding.pl

@@ -7,7 +7,7 @@ use strict;
 use warnings;
 use PostgresNode;
 use TestLib;
-use Test::More tests => 12;
+use Test::More tests => 13;
 use Config;
 
 # Initialize master node
@@ -27,12 +27,21 @@ $node_master->safe_psql('postgres',
 	qq[SELECT pg_create_logical_replication_slot('test_slot', 'test_decoding');]
 );
 
+# Cover walsender error shutdown code
+my ($result, $stdout, $stderr) = $node_master->psql(
+	'template1',
+	qq[START_REPLICATION SLOT test_slot LOGICAL 0/0],
+	replication => 'database');
+ok( $stderr =~
+	  m/replication slot "test_slot" was not created in this database/,
+	"Logical decoding correctly fails to start");
+
 $node_master->safe_psql('postgres',
 	qq[INSERT INTO decoding_test(x,y) SELECT s, s::text FROM generate_series(1,10) s;]
 );
 
 # Basic decoding works
-my ($result) = $node_master->safe_psql('postgres',
+$result = $node_master->safe_psql('postgres',
 	qq[SELECT pg_logical_slot_get_changes('test_slot', NULL, NULL);]);
 is(scalar(my @foobar = split /^/m, $result),
 	12, 'Decoding produced 12 rows inc BEGIN/COMMIT');