Empty search_path in Autovacuum and non-psql/pgbench clients.

This makes the client programs behave as documented regardless of the
connect-time search_path and regardless of user-created objects.  Today,
a malicious user with CREATE permission on a search_path schema can take
control of certain of these clients' queries and invoke arbitrary SQL
functions under the client identity, often a superuser.  This is
exploitable in the default configuration, where all users have CREATE
privilege on schema "public".

This changes behavior of user-defined code stored in the database, like
pg_index.indexprs and pg_extension_config_dump().  If they reach code
bearing unqualified names, "does not exist" or "no schema has been
selected to create in" errors might appear.  Users may fix such errors
by schema-qualifying affected names.  After upgrading, consider watching
server logs for these errors.

The --table arguments of src/bin/scripts clients have been lax; for
example, "vacuumdb -Zt pg_am\;CHECKPOINT" performed a checkpoint.  That
now fails, but for now, "vacuumdb -Zt 'pg_am(amname);CHECKPOINT'" still
performs a checkpoint.

Back-patch to 9.3 (all supported versions).

Reviewed by Tom Lane, though this fix strategy was not his first choice.
Reported by Arseniy Sharoglazov.

Security: CVE-2018-1058

src/bin/pg_basebackup/streamutil.c

@@ -24,6 +24,7 @@
 #include "access/xlog_internal.h"
 #include "common/fe_memutils.h"
 #include "datatype/timestamp.h"
+#include "fe_utils/connect.h"
 #include "port/pg_bswap.h"
 #include "pqexpbuffer.h"
 
@@ -208,6 +209,23 @@ GetConnection(void)
 	if (conn_opts)
 		PQconninfoFree(conn_opts);
 
+	/* Set always-secure search path, so malicious users can't get control. */
+	if (dbname != NULL)
+	{
+		PGresult   *res;
+
+		res = PQexec(tmpconn, ALWAYS_SECURE_SEARCH_PATH_SQL);
+		if (PQresultStatus(res) != PGRES_TUPLES_OK)
+		{
+			fprintf(stderr, _("%s: could not clear search_path: %s\n"),
+					progname, PQerrorMessage(tmpconn));
+			PQclear(res);
+			PQfinish(tmpconn);
+			exit(1);
+		}
+		PQclear(res);
+	}
+
 	/*
 	 * Ensure we have the same value of integer_datetimes (now always "on") as
 	 * the server we are connecting to.