sql: Unset RBRUsingConstraint during rollback

Previously, if a rollback dropped a constraint due to a FK validation
error, it would never update the RBRUsingConstraint field even if the
constraint being rolled back was referenced by that field.

Now the field is cleared properly when needed.

Release note (bug fix): Fixed a bug that could cause a schema change to
be stuck in the reverting state if the infer_rbr_region_col_using_constraint
storage parameter was being set at the same time as adding a constraint
that had a foreign key violation.

Author: rafiss

pkg/ccl/logictestccl/testdata/logic_test/regional_by_row_foreign_key

@@ -2237,3 +2237,67 @@ DROP FUNCTION display_child;
 DROP TABLE child;
 
 subtest end
+
+# ==============================================================================
+# Verify that a foreign key violation causes the addition of the
+# rbr_using_constraint to be reverted properly.
+# ==============================================================================
+
+subtest revert_fk_violation
+
+statement ok
+SET create_table_with_schema_locked = false;
+
+statement ok
+CREATE TABLE a (
+  id uuid NOT NULL PRIMARY KEY
+) LOCALITY REGIONAL BY ROW
+
+statement ok
+CREATE TABLE b (
+  id uuid NOT NULL,
+  a_id uuid NULL,
+  CONSTRAINT b_pkey PRIMARY KEY (id ASC),
+  CONSTRAINT a_id_fk FOREIGN KEY (a_id) REFERENCES a (id)
+) LOCALITY REGIONAL BY ROW
+
+# Insert data that will cause a foreign key violation when we try to add the composite FK.
+statement ok
+INSERT INTO a (id, crdb_region) VALUES ('4227eceb-71d6-422c-808b-6a12ec8a1e54', 'us-east-1')
+
+statement ok
+INSERT INTO b (id, a_id, crdb_region) VALUES ('8f6ce660-423e-4823-8e41-bf001a007b46', '4227eceb-71d6-422c-808b-6a12ec8a1e54', 'ca-central-1')
+
+# This statement should fail with a foreign key violation, and the schema change
+# should properly revert without causing infinite retries due to the missing RBRUsingConstraint.
+# Prior to the bugfix, the rollback logic would would drop the constraint but
+# not clear the RBRUsingConstraint field, causing validation to fail during
+# the rollback.
+statement error pgcode 23503 pq: foreign key violation: "b" row crdb_region='ca-central-1', a_id='4227eceb-71d6-422c-808b-6a12ec8a1e54', id='8f6ce660-423e-4823-8e41-bf001a007b46' has no match in "a"
+ALTER TABLE b
+  DROP CONSTRAINT a_id_fk,
+  ADD CONSTRAINT a_crdb_region_id_fk
+    FOREIGN KEY (crdb_region, a_id) REFERENCES a (crdb_region, id) ON UPDATE CASCADE ON DELETE CASCADE,
+  SET (infer_rbr_region_col_using_constraint = a_crdb_region_id_fk)
+
+# Verify the tables are still functional after the failed schema change.
+query I
+SELECT count(*) FROM a
+----
+1
+
+query I
+SELECT count(*) FROM b
+----
+1
+
+statement ok
+DROP TABLE b CASCADE
+
+statement ok
+DROP TABLE a CASCADE
+
+statement ok
+RESET create_table_with_schema_locked;
+
+subtest end

pkg/sql/schema_changer.go

@@ -2531,6 +2531,11 @@ func (sc *SchemaChanger) maybeDropValidatingConstraint(
 			constraint.GetName(),
 		)
 	} else if constraint.AsForeignKey() != nil {
+		// If the constraint being dropped is the regional-by-row constraint,
+		// we must clear the reference to it.
+		if desc.RBRUsingConstraint == constraint.GetConstraintID() {
+			desc.RBRUsingConstraint = descpb.ConstraintID(0)
+		}
 		for i, fk := range desc.OutboundFKs {
 			if fk.Name == constraint.GetName() {
 				desc.OutboundFKs = append(desc.OutboundFKs[:i], desc.OutboundFKs[i+1:]...)