Prevent setting a column as identity if its not-null constraint is invalid

We don't allow null values to appear in identity-generated columns in
other ways, so we shouldn't let unvalidated not-null constraints do it
either.  Oversight in commit a379061a22.

Author: jian he <jian.universality@gmail.com>
Backpatch-through: 18
Discussion: https://postgr.es/m/CACJufxGQM_+vZoYJMaRoZfNyV=L2jxosjv_0TLAScbuLJXWRfQ@mail.gmail.com

src/backend/commands/tablecmds.c

@@ -8281,6 +8281,31 @@ ATExecAddIdentity(Relation rel, const char *colName,
 				 errmsg("column \"%s\" of relation \"%s\" must be declared NOT NULL before identity can be added",
 						colName, RelationGetRelationName(rel))));
 
+	/*
+	 * On the other hand, if a not-null constraint exists, then verify that
+	 * it's compatible.
+	 */
+	if (attTup->attnotnull)
+	{
+		HeapTuple	contup;
+		Form_pg_constraint conForm;
+
+		contup = findNotNullConstraintAttnum(RelationGetRelid(rel),
+											 attnum);
+		if (!HeapTupleIsValid(contup))
+			elog(ERROR, "cache lookup failed for not-null constraint on column \"%s\" of relation \"%s\"",
+				 colName, RelationGetRelationName(rel));
+
+		conForm = (Form_pg_constraint) GETSTRUCT(contup);
+		if (!conForm->convalidated)
+			ereport(ERROR,
+					errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
+					errmsg("incompatible NOT VALID constraint \"%s\" on relation \"%s\"",
+						   NameStr(conForm->conname), RelationGetRelationName(rel)),
+					errhint("You might need to validate it using %s.",
+							"ALTER TABLE ... VALIDATE CONSTRAINT"));
+	}
+
 	if (attTup->attidentity)
 		ereport(ERROR,
 				(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),

src/test/regress/expected/constraints.out

@@ -1404,6 +1404,10 @@ ALTER TABLE notnull_tbl1 ADD PRIMARY KEY (a);
 ERROR:  cannot create primary key on column "a"
 DETAIL:  The constraint "nn" on column "a" of table "notnull_tbl1", marked NOT VALID, is incompatible with a primary key.
 HINT:  You might need to validate it using ALTER TABLE ... VALIDATE CONSTRAINT.
+-- cannot set column as generated-as-identity if it has an invalid not-null
+ALTER TABLE notnull_tbl1 ALTER COLUMN a ADD GENERATED ALWAYS AS IDENTITY;
+ERROR:  incompatible NOT VALID constraint "nn" on relation "notnull_tbl1"
+HINT:  You might need to validate it using ALTER TABLE ... VALIDATE CONSTRAINT.
 -- ALTER column SET NOT NULL validates an invalid constraint (but this fails
 -- because of rows with null values)
 ALTER TABLE notnull_tbl1 ALTER a SET NOT NULL;

src/test/regress/sql/constraints.sql

@@ -832,6 +832,9 @@ ALTER TABLE notnull_tbl1 ADD CONSTRAINT nn NOT NULL a;
 -- cannot add primary key on a column with an invalid not-null
 ALTER TABLE notnull_tbl1 ADD PRIMARY KEY (a);
 
+-- cannot set column as generated-as-identity if it has an invalid not-null
+ALTER TABLE notnull_tbl1 ALTER COLUMN a ADD GENERATED ALWAYS AS IDENTITY;
+
 -- ALTER column SET NOT NULL validates an invalid constraint (but this fails
 -- because of rows with null values)
 ALTER TABLE notnull_tbl1 ALTER a SET NOT NULL;