Empty search_path in logical replication apply worker and walsender.

This is like CVE-2018-1058 commit
582edc369c.  Today, a malicious user of a
publisher or subscriber database can invoke arbitrary SQL functions
under an identity running replication, often a superuser.  This fix may
cause "does not exist" or "no schema has been selected to create in"
errors in a replication process.  After upgrading, consider watching
server logs for these errors.  Objects accruing schema qualification in
the wake of the earlier commit are unlikely to need further correction.
Back-patch to v10, which introduced logical replication.

Security: CVE-2020-14349

src/backend/replication/libpqwalreceiver/libpqwalreceiver.c

@@ -21,6 +21,7 @@
 
 #include "access/xlog.h"
 #include "catalog/pg_type.h"
+#include "common/connect.h"
 #include "funcapi.h"
 #include "libpq-fe.h"
 #include "mb/pg_wchar.h"
@@ -213,6 +214,22 @@ libpqrcv_connect(const char *conninfo, bool logical, const char *appname,
 		return NULL;
 	}
 
+	if (logical)
+	{
+		PGresult   *res;
+
+		res = libpqrcv_PQexec(conn->streamConn,
+							  ALWAYS_SECURE_SEARCH_PATH_SQL);
+		if (PQresultStatus(res) != PGRES_TUPLES_OK)
+		{
+			PQclear(res);
+			ereport(ERROR,
+					(errmsg("could not clear search path: %s",
+							pchomp(PQerrorMessage(conn->streamConn)))));
+		}
+		PQclear(res);
+	}
+
 	conn->logical = logical;
 
 	return conn;

src/backend/replication/logical/worker.c

@@ -2019,6 +2019,12 @@ ApplyWorkerMain(Datum main_arg)
 											  MyLogicalRepWorker->userid,
 											  0);
 
+	/*
+	 * Set always-secure search path, so malicious users can't redirect user
+	 * code (e.g. pg_index.indexprs).
+	 */
+	SetConfigOption("search_path", "", PGC_SUSET, PGC_S_OVERRIDE);
+
 	/* Load the subscription into persistent memory context. */
 	ApplyContext = AllocSetContextCreate(TopMemoryContext,
 										 "ApplyContext",

src/test/subscription/t/001_rep_changes.pl

@@ -16,6 +16,10 @@ $node_subscriber->init(allows_streaming => 'logical');
 $node_subscriber->start;
 
 # Create some preexisting content on publisher
+$node_publisher->safe_psql(
+	'postgres',
+	"CREATE FUNCTION public.pg_get_replica_identity_index(int)
+	 RETURNS regclass LANGUAGE sql AS 'SELECT 1/0'");    # shall not call
 $node_publisher->safe_psql('postgres',
 	"CREATE TABLE tab_notrep AS SELECT generate_series(1,10) AS a");
 $node_publisher->safe_psql('postgres',