Fix restore of not-null constraints with inheritance

In tables with primary keys, pg_dump creates tables with primary keys by
initially dumping them with throw-away not-null constraints (marked "no
inherit" so that they don't create problems elsewhere), to later drop
them once the primary key is restored.  Because of a unrelated
consideration, on tables with children we add not-null constraints to
all columns of the primary key when it is created.

If both a table and its child have primary keys, and pg_dump happens to
emit the child table first (and its throw-away not-null) and later its
parent table, the creation of the parent's PK will fail because the
throw-away not-null constraint collides with the permanent not-null
constraint that the PK wants to add, so the dump fails to restore.

We can work around this problem by letting the primary key "take over"
the child's not-null.  This requires no changes to pg_dump, just two
changes to ALTER TABLE: first, the ability to convert a no-inherit
not-null constraint into a regular inheritable one (including recursing
down to children, if there are any); second, the ability to "drop" a
constraint that is defined both directly in the table and inherited from
a parent (which simply means to mark it as no longer having a local
definition).

Secondarily, change ATPrepAddPrimaryKey() to acquire locks all the way
down the inheritance hierarchy, in case we need to recurse when
propagating constraints.

These two changes allow pg_dump to reproduce more cases involving
inheritance from versions 16 and older.

Lastly, make two changes to pg_dump: 1) do not try to drop a not-null
constraint that's marked as inherited; this allows a dump to restore
with no errors if a table with a PK inherits from another which also has
a PK; 2) avoid giving inherited constraints throwaway names, for the
rare cases where such a constraint survives after the restore.

Reported-by: Andrew Bille <andrewbille@gmail.com>
Reported-by: Justin Pryzby <pryzby@telsasoft.com>
Discussion: https://postgr.es/m/CAJnzarwkfRu76_yi3dqVF_WL-MpvT54zMwAxFwJceXdHB76bOA@mail.gmail.com
Discussion: https://postgr.es/m/Zh0aAH7tbZb-9HbC@pryzbyj2023

1 files changed, 30 insertions, 13 deletions

diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c
index 604280d322e..778b7c381df 100644
--- a/src/backend/catalog/pg_constraint.c
+++ b/src/backend/catalog/pg_constraint.c
@@ -565,8 +565,8 @@ ChooseConstraintName(const char *name1, const char *name2,
 }
 
 /*
- * Find and return the pg_constraint tuple that implements a validated
- * not-null constraint for the given column of the given relation.
+ * Find and return a copy of the pg_constraint tuple that implements a
+ * validated not-null constraint for the given column of the given relation.
  *
  * XXX This would be easier if we had pg_attribute.notnullconstr with the OID
  * of the constraint that implements the not-null constraint for that column.
@@ -709,37 +709,54 @@ extractNotNullColumn(HeapTuple constrTup)
  * AdjustNotNullInheritance1
  *		Adjust inheritance count for a single not-null constraint
  *
- * Adjust inheritance count, and possibly islocal status, for the not-null
- * constraint row of the given column, if it exists, and return true.
- * If no not-null constraint is found for the column, return false.
+ * If no not-null constraint is found for the column, return 0.
+ * Caller can create one.
+ * If the constraint does exist and it's inheritable, adjust its
+ * inheritance count (and possibly islocal status) and return 1.
+ * No further action needs to be taken.
+ * If the constraint exists but is marked NO INHERIT, adjust it as above
+ * and reset connoinherit to false, and return -1.  Caller is
+ * responsible for adding the same constraint to the children, if any.
  */
-bool
+int
 AdjustNotNullInheritance1(Oid relid, AttrNumber attnum, int count,
 						  bool is_no_inherit)
 {
 	HeapTuple	tup;
 
+	Assert(count >= 0);
+
 	tup = findNotNullConstraintAttnum(relid, attnum);
 	if (HeapTupleIsValid(tup))
 	{
 		Relation	pg_constraint;
 		Form_pg_constraint conform;
+		int			retval = 1;
 
 		pg_constraint = table_open(ConstraintRelationId, RowExclusiveLock);
 		conform = (Form_pg_constraint) GETSTRUCT(tup);
 
 		/*
-		 * Don't let the NO INHERIT status change (but don't complain
-		 * unnecessarily.)  In the future it might be useful to let an
-		 * inheritable constraint replace a non-inheritable one, but we'd need
-		 * to recurse to children to get it added there.
+		 * If we're asked for a NO INHERIT constraint and this relation
+		 * already has an inheritable one, throw an error.
 		 */
-		if (is_no_inherit != conform->connoinherit)
+		if (is_no_inherit && !conform->connoinherit)
 			ereport(ERROR,
 					errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
 					errmsg("cannot change NO INHERIT status of inherited NOT NULL constraint \"%s\" on relation \"%s\"",
 						   NameStr(conform->conname), get_rel_name(relid)));
 
+		/*
+		 * If the constraint already exists in this relation but it's marked
+		 * NO INHERIT, we can just remove that flag, and instruct caller to
+		 * recurse to add the constraint to children.
+		 */
+		if (!is_no_inherit && conform->connoinherit)
+		{
+			conform->connoinherit = false;
+			retval = -1;		/* caller must add constraint on child rels */
+		}
+
 		if (count > 0)
 			conform->coninhcount += count;
 
@@ -761,10 +778,10 @@ AdjustNotNullInheritance1(Oid relid, AttrNumber attnum, int count,
 
 		table_close(pg_constraint, RowExclusiveLock);
 
-		return true;
+		return retval;
 	}
 
-	return false;
+	return 0;
 }
 
 /*