Reject cases where a query in WITH rewrites to just NOTIFY.

Since the executor can't cope with a utility statement appearing
as a node of a plan tree, we can't support cases where a rewrite
rule inserts a NOTIFY into an INSERT/UPDATE/DELETE command appearing
in a WITH clause of a larger query.  (One can imagine ways around
that, but it'd be a new feature not a bug fix, and so far there's
been no demand for it.)  RewriteQuery checked for this, but it
missed the case where the DML command rewrites to *only* a NOTIFY.
That'd lead to crashes later on in planning.  Add the missed check,
and improve the level of testing of this area.

Per bug #17094 from Yaoguang Chen.  It's been busted since WITH
was introduced, so back-patch to all supported branches.

Discussion: https://postgr.es/m/17094-bf15dff55eaf2e28@postgresql.org

1 files changed, 17 insertions, 3 deletions

diff --git a/src/backend/rewrite/rewriteHandler.c b/src/backend/rewrite/rewriteHandler.c
index fd63dd7a3bd..8f3470987ab 100644
--- a/src/backend/rewrite/rewriteHandler.c
+++ b/src/backend/rewrite/rewriteHandler.c
@@ -3366,16 +3366,30 @@ RewriteQuery(Query *parsetree, List *rewrite_events)
 
 		/*
 		 * Currently we can only handle unconditional, single-statement DO
-		 * INSTEAD rules correctly; we have to get exactly one Query out of
-		 * the rewrite operation to stuff back into the CTE node.
+		 * INSTEAD rules correctly; we have to get exactly one non-utility
+		 * Query out of the rewrite operation to stuff back into the CTE node.
 		 */
 		if (list_length(newstuff) == 1)
 		{
-			/* Push the single Query back into the CTE node */
+			/* Must check it's not a utility command */
 			ctequery = (Query *) linitial(newstuff);
 			Assert(IsA(ctequery, Query));
+			if (!(ctequery->commandType == CMD_SELECT ||
+				  ctequery->commandType == CMD_UPDATE ||
+				  ctequery->commandType == CMD_INSERT ||
+				  ctequery->commandType == CMD_DELETE))
+			{
+				/*
+				 * Currently it could only be NOTIFY; this error message will
+				 * need work if we ever allow other utility commands in rules.
+				 */
+				ereport(ERROR,
+						(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+						 errmsg("DO INSTEAD NOTIFY rules are not supported for data-modifying statements in WITH")));
+			}
 			/* WITH queries should never be canSetTag */
 			Assert(!ctequery->canSetTag);
+			/* Push the single Query back into the CTE node */
 			cte->ctequery = (Node *) ctequery;
 		}
 		else if (newstuff == NIL)