Fix error handling in libpqrcv_connect()

When libpqrcv_connect (also known as walrcv_connect()) failed, it leaked the
libpq connection. In most paths that's fairly harmless, as the calling process
will exit soon after. But e.g. CREATE SUBSCRIPTION could lead to a somewhat
longer lived leak.

Fix by releasing resources, including the libpq connection, on error.

Add a test exercising the error code path. To make it reliable and safe, the
test tries to connect to port=-1, which happens to fail during connection
establishment, rather than during connection string parsing.

Reviewed-by: Noah Misch <noah@leadboat.com>
Discussion: https://postgr.es/m/20230121011237.q52apbvlarfv6jm6@awork3.anarazel.de
Backpatch: 11-

src/backend/replication/libpqwalreceiver/libpqwalreceiver.c

@@ -175,10 +175,7 @@ libpqrcv_connect(const char *conninfo, bool logical, const char *appname,
 	conn->streamConn = PQconnectStartParams(keys, vals,
 											 /* expand_dbname = */ true);
 	if (PQstatus(conn->streamConn) == CONNECTION_BAD)
-	{
-		*err = pchomp(PQerrorMessage(conn->streamConn));
-		return NULL;
-	}
+		goto bad_connection_errmsg;
 
 	/*
 	 * Poll connection until we have OK or FAILED status.
@@ -220,10 +217,7 @@ libpqrcv_connect(const char *conninfo, bool logical, const char *appname,
 	} while (status != PGRES_POLLING_OK && status != PGRES_POLLING_FAILED);
 
 	if (PQstatus(conn->streamConn) != CONNECTION_OK)
-	{
-		*err = pchomp(PQerrorMessage(conn->streamConn));
-		return NULL;
-	}
+		goto bad_connection_errmsg;
 
 	if (logical)
 	{
@@ -234,9 +228,9 @@ libpqrcv_connect(const char *conninfo, bool logical, const char *appname,
 		if (PQresultStatus(res) != PGRES_TUPLES_OK)
 		{
 			PQclear(res);
-			ereport(ERROR,
-					(errmsg("could not clear search path: %s",
-							pchomp(PQerrorMessage(conn->streamConn)))));
+			*err = psprintf(_("could not clear search path: %s"),
+							pchomp(PQerrorMessage(conn->streamConn)));
+			goto bad_connection;
 		}
 		PQclear(res);
 	}
@@ -244,6 +238,16 @@ libpqrcv_connect(const char *conninfo, bool logical, const char *appname,
 	conn->logical = logical;
 
 	return conn;
+
+	/* error path, using libpq's error message */
+bad_connection_errmsg:
+	*err = pchomp(PQerrorMessage(conn->streamConn));
+
+	/* error path, error already set */
+bad_connection:
+	PQfinish(conn->streamConn);
+	pfree(conn);
+	return NULL;
 }
 
 /*

src/test/regress/expected/subscription.out

@@ -130,7 +130,15 @@ ALTER SUBSCRIPTION regress_testsub4 SET (origin = any);
 
 DROP SUBSCRIPTION regress_testsub3;
 DROP SUBSCRIPTION regress_testsub4;
--- fail - invalid connection string
+-- fail, connection string does not parse
+CREATE SUBSCRIPTION regress_testsub5 CONNECTION 'i_dont_exist=param' PUBLICATION testpub;
+ERROR:  invalid connection string syntax: invalid connection option "i_dont_exist"
+
+-- fail, connection string parses, but doesn't work (and does so without
+-- connecting, so this is reliable and safe)
+CREATE SUBSCRIPTION regress_testsub5 CONNECTION 'port=-1' PUBLICATION testpub;
+ERROR:  could not connect to the publisher: invalid port number: "-1"
+-- fail - invalid connection string during ALTER
 ALTER SUBSCRIPTION regress_testsub CONNECTION 'foobar';
 ERROR:  invalid connection string syntax: missing "=" after "foobar" in connection info string
 

src/test/regress/sql/subscription.sql

@@ -77,7 +77,14 @@ ALTER SUBSCRIPTION regress_testsub4 SET (origin = any);
 DROP SUBSCRIPTION regress_testsub3;
 DROP SUBSCRIPTION regress_testsub4;
 
--- fail - invalid connection string
+-- fail, connection string does not parse
+CREATE SUBSCRIPTION regress_testsub5 CONNECTION 'i_dont_exist=param' PUBLICATION testpub;
+
+-- fail, connection string parses, but doesn't work (and does so without
+-- connecting, so this is reliable and safe)
+CREATE SUBSCRIPTION regress_testsub5 CONNECTION 'port=-1' PUBLICATION testpub;
+
+-- fail - invalid connection string during ALTER
 ALTER SUBSCRIPTION regress_testsub CONNECTION 'foobar';
 
 \dRs+