sql: check FK back-references in ALTER COLUMN TYPE

Previously, ALTER TABLE ... ALTER COLUMN ... TYPE ... statements would
execute regardless if the column is also the target of a foreign key
constraint on another table. This commit fixes this bug.

Fixes cockroachdb#71709.

Release note (bug fix): fixes a bug in which an ALTER TABLE ... ALTER
COLUMN ... TYPE ... statement could execute despite the column being the
target of a foreign key constraint on another table.

pkg/sql/alter_column_type.go

@@ -202,7 +202,7 @@ func alterColumnTypeGeneral(
 
 	// Disallow ALTER COLUMN TYPE general for columns that have a foreign key
 	// constraint.
-	for _, fk := range tableDesc.AllActiveAndInactiveForeignKeys() {
+	checkFK := func(fk *descpb.ForeignKeyConstraint) error {
 		if fk.OriginTableID == tableDesc.GetID() {
 			for _, id := range fk.OriginColumnIDs {
 				if col.GetID() == id {
@@ -217,6 +217,16 @@ func alterColumnTypeGeneral(
 				}
 			}
 		}
+		return nil
+	}
+
+	for _, fk := range tableDesc.AllActiveAndInactiveForeignKeys() {
+		if err := checkFK(fk); err != nil {
+			return err
+		}
+	}
+	if err := tableDesc.ForeachInboundFK(checkFK); err != nil {
+		return err
 	}
 
 	// Disallow ALTER COLUMN TYPE general for columns that are

pkg/sql/logictest/testdata/logic_test/alter_column_type

@@ -390,7 +390,7 @@ statement ok
 CREATE TABLE t18 (x INT NOT NULL PRIMARY KEY);
 CREATE TABLE t19 (y INT NOT NULL REFERENCES t18 (x), INDEX(y));
 
-statement error pq: unimplemented: ALTER COLUMN TYPE requiring rewrite of on-disk data is currently not supported for columns that are part of an index
+statement error pq: unimplemented: ALTER COLUMN TYPE for a column that has a constraint is currently not supported
 ALTER TABLE t18 ALTER COLUMN x TYPE STRING
 
 statement error pq: unimplemented: ALTER COLUMN TYPE for a column that has a constraint is currently not supported
@@ -529,6 +529,9 @@ CREATE TABLE child_71089 (a INT, b INT REFERENCES parent_71089 (id) NOT NULL)
 statement ok
 ALTER TABLE child_71089 ALTER COLUMN a TYPE FLOAT;
 
+statement error pq: unimplemented: ALTER COLUMN TYPE for a column that has a constraint is currently not supported
+ALTER TABLE parent_71089 ALTER COLUMN id TYPE uuid USING gen_random_uuid();
+
 # ColumnConversionValidate should error out if the conversion is not valid.
 # STRING -> BYTES is a ColumnConversionValidate type conversion, it should
 # try the conversion and error out if the cast cannot be applied.