Fix ALTER TABLE ONLY .. DROP CONSTRAINT.

When I consolidated two copies of the HOT-chain search logic in commit
4da99ea4, I introduced a behavior
change: the old code wouldn't necessarily traverse the entire chain,
if the most recently returned tuple were updated while the HOT chain
traversal is in progress.  The new behavior seems more correct, but
unfortunately, the code here relies on a scan with SnapshotNow failing
to see its own updates.  That seems pretty shaky even with the old HOT
chain traversal behavior, since there's no guarantee that these
updates will always be HOT, but it's trivial to broke a failure with
the new HOT search logic.  Fix by updating just the first matching
pg_constraint tuple, rather than all of them, since there should be
only one anyway.  But since nobody has reproduced this failure on older
versions, no back-patch for now.

Report and test case by Alex Hunsaker; tablecmds.c changes by me.

src/backend/commands/tablecmds.c

@@ -6734,6 +6734,7 @@ ATExecDropConstraint(Relation rel, const char *constrName,
 	{
 		Oid			childrelid = lfirst_oid(child);
 		Relation	childrel;
+		HeapTuple	copy_tuple;
 
 		/* find_inheritance_children already got lock */
 		childrel = heap_open(childrelid, NoLock);
@@ -6746,30 +6747,36 @@ ATExecDropConstraint(Relation rel, const char *constrName,
 		scan = systable_beginscan(conrel, ConstraintRelidIndexId,
 								  true, SnapshotNow, 1, &key);
 
-		found = false;
-
+		/* scan for matching tuple - there should only be one */
 		while (HeapTupleIsValid(tuple = systable_getnext(scan)))
 		{
-			HeapTuple	copy_tuple;
-
 			con = (Form_pg_constraint) GETSTRUCT(tuple);
 
 			/* Right now only CHECK constraints can be inherited */
 			if (con->contype != CONSTRAINT_CHECK)
 				continue;
 
-			if (strcmp(NameStr(con->conname), constrName) != 0)
-				continue;
+			if (strcmp(NameStr(con->conname), constrName) == 0)
+				break;
+		}
 
-			found = true;
+		if (!HeapTupleIsValid(tuple))
+			ereport(ERROR,
+					(errcode(ERRCODE_UNDEFINED_OBJECT),
+				errmsg("constraint \"%s\" of relation \"%s\" does not exist",
+					   constrName,
+					   RelationGetRelationName(childrel))));
+
+		copy_tuple = heap_copytuple(tuple);
+
+		systable_endscan(scan);
+
+		con = (Form_pg_constraint) GETSTRUCT(copy_tuple);
 
 		if (con->coninhcount <= 0)	/* shouldn't happen */
 			elog(ERROR, "relation %u has non-inherited constraint \"%s\"",
 				 childrelid, constrName);
 
-			copy_tuple = heap_copytuple(tuple);
-			con = (Form_pg_constraint) GETSTRUCT(copy_tuple);
-
 		if (recurse)
 		{
 			/*
@@ -6813,16 +6820,6 @@ ATExecDropConstraint(Relation rel, const char *constrName,
 		}
 
 		heap_freetuple(copy_tuple);
-		}
-
-		systable_endscan(scan);
-
-		if (!found)
-			ereport(ERROR,
-					(errcode(ERRCODE_UNDEFINED_OBJECT),
-				errmsg("constraint \"%s\" of relation \"%s\" does not exist",
-					   constrName,
-					   RelationGetRelationName(childrel))));
 
 		heap_close(childrel, NoLock);
 	}

src/test/regress/expected/alter_table.out

@@ -2103,3 +2103,12 @@ ALTER TABLE tt7 NOT OF;
  x      | integer      | 
  y      | numeric(8,2) | 
 
+-- make sure we can drop a constraint on the parent but it remains on the child
+CREATE TABLE test_drop_constr_parent (c text CHECK (c IS NOT NULL));
+CREATE TABLE test_drop_constr_child () INHERITS (test_drop_constr_parent);
+ALTER TABLE ONLY test_drop_constr_parent DROP CONSTRAINT "test_drop_constr_parent_c_check";
+-- should fail
+INSERT INTO test_drop_constr_child (c) VALUES (NULL);
+ERROR:  new row for relation "test_drop_constr_child" violates check constraint "test_drop_constr_parent_c_check"
+DROP TABLE test_drop_constr_parent CASCADE;
+NOTICE:  drop cascades to table test_drop_constr_child

src/test/regress/sql/alter_table.sql

@@ -1456,3 +1456,11 @@ CREATE TYPE tt_t1 AS (x int, y numeric(8,2));
 ALTER TABLE tt7 OF tt_t1;			-- reassign an already-typed table
 ALTER TABLE tt7 NOT OF;
 \d tt7
+
+-- make sure we can drop a constraint on the parent but it remains on the child
+CREATE TABLE test_drop_constr_parent (c text CHECK (c IS NOT NULL));
+CREATE TABLE test_drop_constr_child () INHERITS (test_drop_constr_parent);
+ALTER TABLE ONLY test_drop_constr_parent DROP CONSTRAINT "test_drop_constr_parent_c_check";
+-- should fail
+INSERT INTO test_drop_constr_child (c) VALUES (NULL);
+DROP TABLE test_drop_constr_parent CASCADE;