Fix misidentification of SQL statement type in plpgsql's exec_stmt_execsql.

To distinguish SQL statements that are INSERT/UPDATE/DELETE from other
ones, exec_stmt_execsql looked at the post-rewrite form of the statement
rather than the original.  This is problematic because it did that only
during first execution of the statement (in a session), but the correct
answer could change later due to addition or removal of DO INSTEAD rules
during the session.  That could lead to an Assert failure, as reported
by Tushar Ahuja and Robert Haas.  In non-assert builds, there's a hazard
that we would fail to enforce STRICT behavior when we'd be expected to.
That would happen if an initially present DO INSTEAD, that replaced the
original statement with one of a different type, were removed; after that
the statement should act "normally", including strictness enforcement, but
it didn't.  (The converse case of enforcing strictness when we shouldn't
doesn't seem to be a hazard, as addition of a DO INSTEAD that changes the
statement type would always lead to acting as though the statement returned
zero rows, so that the strictness error could not fire.)

To fix, inspect the original form of the statement not the post-rewrite
form, making it valid to assume the answer can't change intra-session.
This should lead to the same answer in every case except when there is a
DO INSTEAD that changes the statement type; we will now set mod_stmt=true
anyway, while we would not have done so before.  That breaks the Assert
in the SPI_OK_REWRITTEN code path, which expected the latter behavior.
It might be all right to assert mod_stmt rather than !mod_stmt there,
but I'm not entirely convinced that that'd always hold, so just remove
the assertion altogether.

This has been broken for a long time, so back-patch to all supported
branches.

Discussion: https://postgr.es/m/CA+TgmoZUrRN4xvZe_BbBn_Xp0BDwuMEue-0OyF0fJpfvU2Yc7Q@mail.gmail.com

src/pl/plpgsql/src/pl_exec.c

@@ -4031,19 +4031,20 @@ exec_stmt_execsql(PLpgSQL_execstate *estate,
 		foreach(l, SPI_plan_get_plan_sources(expr->plan))
 		{
 			CachedPlanSource *plansource = (CachedPlanSource *) lfirst(l);
-			ListCell   *l2;
 
-			foreach(l2, plansource->query_list)
+			/*
+			 * We could look at the raw_parse_tree, but it seems simpler to
+			 * check the command tag.  Note we should *not* look at the Query
+			 * tree(s), since those are the result of rewriting and could have
+			 * been transmogrified into something else entirely.
+			 */
+			if (plansource->commandTag &&
+				(strcmp(plansource->commandTag, "INSERT") == 0 ||
+				 strcmp(plansource->commandTag, "UPDATE") == 0 ||
+				 strcmp(plansource->commandTag, "DELETE") == 0))
 			{
-				Query	   *q = lfirst_node(Query, l2);
-
-				if (q->canSetTag)
-				{
-					if (q->commandType == CMD_INSERT ||
-						q->commandType == CMD_UPDATE ||
-						q->commandType == CMD_DELETE)
-						stmt->mod_stmt = true;
-				}
+				stmt->mod_stmt = true;
+				break;
 			}
 		}
 	}
@@ -4108,12 +4109,12 @@ exec_stmt_execsql(PLpgSQL_execstate *estate,
 			break;
 
 		case SPI_OK_REWRITTEN:
-			Assert(!stmt->mod_stmt);
 
 			/*
 			 * The command was rewritten into another kind of command. It's
 			 * not clear what FOUND would mean in that case (and SPI doesn't
-			 * return the row count either), so just set it to false.
+			 * return the row count either), so just set it to false.  Note
+			 * that we can't assert anything about mod_stmt here.
 			 */
 			exec_set_found(estate, false);
 			break;