Register the historic snapshot before use.

This is necessary due to the commit 8076c00592 in PG core.

Author: Antonin Houska

pg_rewrite.c

@@ -1024,8 +1024,10 @@ rewrite_table_impl(char *relschema_src, char *relname_src,
 	/*
 	 * The historic snapshot is used to retrieve data w/o concurrent changes.
 	 */
+	snap_hist = RegisterSnapshot(snap_hist);
 	perform_initial_load(estate, mtstate, proute, rel_src, snap_hist, rel_dst,
 						 partitions, ctx, conv_map);
+	UnregisterSnapshot(snap_hist);
 
 	/*
 	 * We no longer need to preserve the rows processed during the initial
@@ -1040,11 +1042,6 @@ rewrite_table_impl(char *relschema_src, char *relname_src,
 	slot->effective_xmin = InvalidTransactionId;
 	SpinLockRelease(&slot->mutex);
 
-	/*
-	 * The historic snapshot won't be needed anymore.
-	 */
-	pfree(snap_hist);
-
 	/*
 	 * This is rather paranoia than anything else --- perform_initial_load()
 	 * uses each snapshot to access different table, and it does not cause