MDEV-19092 Server crash when renaming the column when

		FOREIGN_KEY_CHECKS is disabled

- Referenced index can be null While renaming the referenced column name.
In that case, rename the referenced column name in dict_foreign_t and
find the equivalent referenced index.

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

@@ -119,4 +119,14 @@ ALTER TABLE t1 ADD FOREIGN KEY (a) REFERENCES tx(x);
 ALTER TABLE t1 DROP KEY idx;
 ALTER TABLE t1 CHANGE a c INT;
 DROP TABLE t1;
+CREATE TABLE t1 (f1 INT, f2 INT, f3 INT, KEY idx(f1)) ENGINE=InnoDB;
+ALTER TABLE t1 ADD FOREIGN KEY (f2) REFERENCES t1 (f1);
+ALTER TABLE t1 ADD COLUMN f INT;
+SET FOREIGN_KEY_CHECKS= OFF;
+ALTER TABLE t1 DROP KEY idx;
+ALTER TABLE t1 ADD KEY idx (f1);
+SET FOREIGN_KEY_CHECKS= ON;
+ALTER TABLE t1 DROP f3;
+ALTER TABLE t1 CHANGE f f3 INT;
+DROP TABLE t1;
 SET FOREIGN_KEY_CHECKS=1;

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

@@ -144,4 +144,17 @@ ALTER TABLE t1 DROP KEY idx;
 ALTER TABLE t1 CHANGE a c INT;
 # Cleanup
 DROP TABLE t1;
+
+CREATE TABLE t1 (f1 INT, f2 INT, f3 INT, KEY idx(f1)) ENGINE=InnoDB;
+ALTER TABLE t1 ADD FOREIGN KEY (f2) REFERENCES t1 (f1);
+ALTER TABLE t1 ADD COLUMN f INT;
+SET FOREIGN_KEY_CHECKS= OFF;
+ALTER TABLE t1 DROP KEY idx;
+ALTER TABLE t1 ADD KEY idx (f1);
+SET FOREIGN_KEY_CHECKS= ON;
+ALTER TABLE t1 DROP f3;
+ALTER TABLE t1 CHANGE f f3 INT;
+# Cleanup
+DROP TABLE t1;
+
 SET FOREIGN_KEY_CHECKS=1;

storage/innobase/dict/dict0mem.cc

@@ -415,17 +415,15 @@ dict_mem_table_col_rename_low(
 				}
 			}
 
-			dict_index_t* new_index = dict_foreign_find_index(
+			/* New index can be null if InnoDB already dropped
+			the foreign index when FOREIGN_KEY_CHECKS is
+			disabled */
+			foreign->foreign_index = dict_foreign_find_index(
 				foreign->foreign_table, NULL,
 				foreign->foreign_col_names,
 				foreign->n_fields, NULL, true, false,
 				NULL, NULL, NULL);
 
-			/* New index can be null if InnoDB already dropped
-			the foreign index when FOREIGN_KEY_CHECKS is
-			disabled */
-			foreign->foreign_index = new_index;
-
 		} else {
 
 			for (unsigned f = 0; f < foreign->n_fields; f++) {
@@ -447,7 +445,41 @@ dict_mem_table_col_rename_low(
 
 		foreign = *it;
 
-		ut_ad(foreign->referenced_index != NULL);
+		if (!foreign->referenced_index) {
+			/* Referenced index could have been dropped
+			when foreign_key_checks is disabled. In that case,
+			rename the corresponding referenced_col_names and
+			find the equivalent referenced index also */
+			for (unsigned f = 0; f < foreign->n_fields; f++) {
+
+				const char*& rc =
+					foreign->referenced_col_names[f];
+				if (strcmp(rc, from)) {
+					continue;
+				}
+
+				if (to_len <= strlen(rc)) {
+					memcpy(const_cast<char*>(rc), to,
+					       to_len + 1);
+				} else {
+					rc = static_cast<char*>(
+						mem_heap_dup(
+							foreign->heap,
+							to, to_len + 1));
+				}
+			}
+
+			/* New index can be null if InnoDB already dropped
+			the referenced index when FOREIGN_KEY_CHECKS is
+			disabled */
+			foreign->referenced_index = dict_foreign_find_index(
+				foreign->referenced_table, NULL,
+				foreign->referenced_col_names,
+				foreign->n_fields, NULL, true, false,
+				NULL, NULL, NULL);
+			return;
+		}
+
 
 		for (unsigned f = 0; f < foreign->n_fields; f++) {
 			/* foreign->referenced_col_names[] need to be

storage/xtradb/dict/dict0mem.cc

@@ -416,17 +416,15 @@ dict_mem_table_col_rename_low(
 				}
 			}
 
-			dict_index_t* new_index = dict_foreign_find_index(
+			/* New index can be null if XtraDB already dropped
+			the foreign index when FOREIGN_KEY_CHECKS is
+			disabled */
+			foreign->foreign_index = dict_foreign_find_index(
 				foreign->foreign_table, NULL,
 				foreign->foreign_col_names,
 				foreign->n_fields, NULL, true, false,
 				NULL, NULL, NULL);
 
-			/* New index can be null if XtraDB already dropped
-			the foreign index when FOREIGN_KEY_CHECKS is
-			disabled */
-			foreign->foreign_index = new_index;
-
 		} else {
 
 			for (unsigned f = 0; f < foreign->n_fields; f++) {
@@ -448,7 +446,41 @@ dict_mem_table_col_rename_low(
 
 		foreign = *it;
 
-		ut_ad(foreign->referenced_index != NULL);
+		if (!foreign->referenced_index) {
+			/* Referenced index could have been dropped
+			when foreign_key_checks is disabled. In that case,
+			rename the corresponding referenced_col_names and
+			find the equivalent referenced index also */
+			for (unsigned f = 0; f < foreign->n_fields; f++) {
+
+				const char*& rc =
+					foreign->referenced_col_names[f];
+
+				if (strcmp(rc, from)) {
+					continue;
+				}
+
+				if (to_len <= strlen(rc)) {
+					memcpy(const_cast<char*>(rc), to,
+					       to_len + 1);
+				} else {
+					rc = static_cast<char*>(
+						mem_heap_dup(
+							foreign->heap,
+							to, to_len + 1));
+				}
+			}
+
+			/* New index can be null if InnoDB already dropped
+			the referenced index when FOREIGN_KEY_CHECKS is
+			disabled */
+			foreign->referenced_index = dict_foreign_find_index(
+				foreign->referenced_table, NULL,
+				foreign->referenced_col_names,
+				foreign->n_fields, NULL, true, false,
+				NULL, NULL, NULL);
+			return;
+		}
 
 		for (unsigned f = 0; f < foreign->n_fields; f++) {
 			/* foreign->referenced_col_names[] need to be