MDEV-33927 Wrong error message while altering the table with foreign key relationship

- Alter statement using copy algorithm throws wrong message
"Cannot delete rows from table which is parent in a foreign
key constraint" even though we expect duplicate key for the index.

copy_data_between_tables(): Remove the error condition to
throw ER_FK_CANNOT_DELETE_PARENT and make sure that ALTER IGNORE
shouldn't allow to break unique of foreign key constraints.

mysql-test/suite/innodb/r/fk_col_alter.result

@@ -132,3 +132,20 @@ ADD CONSTRAINT t2_fk FOREIGN KEY(f3) REFERENCES t1(f1);
 DROP TABLE t2, t1;
 SET SESSION FOREIGN_KEY_CHECKS = ON;
 # End of 10.4 tests
+#
+#  MDEV-33927 Wrong error message while altering the table with
+#                foreign key relationship
+#
+CREATE TABLE t1(f1 INT NOT NULL, f2 INT NOT NULL,
+f3 INT NOT NULL, PRIMARY KEY(f1),
+INDEX(f2))ENGINE=InnoDB;
+INSERT INTO t1 VALUES(1, 1, 2), (2, 2, 2);
+CREATE TABLE t2 (f1 INT NOT NULL, f2 INT NOT NULL,
+PRIMARY KEY(f1), INDEX(f2)) ENGINE=INNODB;
+INSERT INTO t2 VALUES(1, 1), (2, 2);
+ALTER TABLE t1 ADD FOREIGN KEY(f1) REFERENCES t1(f1);
+ALTER TABLE t1 ADD FOREIGN KEY (f2) REFERENCES t2 (f2),
+ADD UNIQUE INDEX(f3), ALGORITHM=COPY;
+ERROR 23000: Duplicate entry '2' for key 'f3'
+DROP TABLE t1, t2;
+# End of 10.5 tests

mysql-test/suite/innodb/t/fk_col_alter.test

@@ -168,3 +168,21 @@ ALTER TABLE t2 CHANGE COLUMN f2 f3 VARCHAR(20) NOT NULL,
 DROP TABLE t2, t1;
 SET SESSION FOREIGN_KEY_CHECKS = ON;
 --echo # End of 10.4 tests
+
+--echo #
+--echo #  MDEV-33927 Wrong error message while altering the table with
+--echo #                foreign key relationship
+--echo #
+CREATE TABLE t1(f1 INT NOT NULL, f2 INT NOT NULL,
+		f3 INT NOT NULL, PRIMARY KEY(f1),
+		INDEX(f2))ENGINE=InnoDB;
+INSERT INTO t1 VALUES(1, 1, 2), (2, 2, 2);
+CREATE TABLE t2 (f1 INT NOT NULL, f2 INT NOT NULL,
+		 PRIMARY KEY(f1), INDEX(f2)) ENGINE=INNODB;
+INSERT INTO t2 VALUES(1, 1), (2, 2);
+ALTER TABLE t1 ADD FOREIGN KEY(f1) REFERENCES t1(f1);
+--error ER_DUP_ENTRY
+ALTER TABLE t1 ADD FOREIGN KEY (f2) REFERENCES t2 (f2),
+               ADD UNIQUE INDEX(f3), ALGORITHM=COPY;
+DROP TABLE t1, t2;
+--echo # End of 10.5 tests

sql/sql_table.cc

@@ -11857,23 +11857,7 @@ copy_data_between_tables(THD *thd, TABLE *from, TABLE *to, bool ignore,
       }
       else
       {
-        /* Duplicate key error. */
-        if (unlikely(alter_ctx->fk_error_if_delete_row))
-        {
-          /*
-            We are trying to omit a row from the table which serves as parent
-            in a foreign key. This might have broken referential integrity so
-            emit an error. Note that we can't ignore this error even if we are
-            executing ALTER IGNORE TABLE. IGNORE allows to skip rows, but
-            doesn't allow to break unique or foreign key constraints,
-          */
-          my_error(ER_FK_CANNOT_DELETE_PARENT, MYF(0),
-                   alter_ctx->fk_error_id,
-                   alter_ctx->fk_error_table);
-          break;
-        }
-
-        if (ignore)
+        if (ignore && !alter_ctx->fk_error_if_delete_row)
         {
           /* This ALTER IGNORE TABLE. Simply skip row and continue. */
           to->file->restore_auto_increment(prev_insert_id);