Prevent ALTER TABLE ... MODIFY ... NOT NULL on columns for which

there is a foreign key constraint ON ... SET NULL.  (Bug #25927)

dict_foreign_find_index(): Add paramettter check_null.

dict_foreign_add_to_cache(): Do not allow ON DELETE SET NULL
or ON UPDATE SET NULL if any of the referencing columns are declared NOT NULL.

dict/dict0dict.c

@@ -1970,9 +1970,12 @@ dict_foreign_find_index(
 	ulint		n_cols,	/* in: number of columns */
 	dict_index_t*	types_idx, /* in: NULL or an index to whose types the
 				   column types must match */
-	ibool		check_charsets)
+	ibool		check_charsets,
 				/* in: whether to check charsets.
 				only has an effect if types_idx != NULL */
+	ulint		check_null)
+				/* in: nonzero if none of the columns must
+				be declared NOT NULL */
 {
 	dict_index_t*	index;
 	dict_field_t*	field;
@@ -2002,6 +2005,12 @@ dict_foreign_find_index(
 					break;
 				}
 
+				if (check_null
+				    && (field->col->prtype & DATA_NOT_NULL)) {
+
+					return(NULL);
+				}
+
 				if (types_idx && !cmp_cols_are_equal(
 					    dict_index_get_nth_col(index, i),
 					    dict_index_get_nth_col(types_idx,
@@ -2118,7 +2127,7 @@ dict_foreign_add_to_cache(
 			ref_table,
 			(const char**) for_in_cache->referenced_col_names,
 			for_in_cache->n_fields, for_in_cache->foreign_index,
-			check_charsets);
+			check_charsets, FALSE);
 
 		if (index == NULL) {
 			dict_foreign_error_report(
@@ -2150,7 +2159,10 @@ dict_foreign_add_to_cache(
 			for_table,
 			(const char**) for_in_cache->foreign_col_names,
 			for_in_cache->n_fields,
-			for_in_cache->referenced_index, check_charsets);
+			for_in_cache->referenced_index, check_charsets,
+			for_in_cache->type
+			& (DICT_FOREIGN_ON_DELETE_SET_NULL
+			   | DICT_FOREIGN_ON_UPDATE_SET_NULL));
 
 		if (index == NULL) {
 			dict_foreign_error_report(
@@ -2160,7 +2172,9 @@ dict_foreign_add_to_cache(
 				"the columns as the first columns,"
 				" or the data types in the\n"
 				"table do not match"
-				" the ones in the referenced table.");
+				" the ones in the referenced table\n"
+				"or one of the ON ... SET NULL columns"
+				" is declared NOT NULL.");
 
 			if (for_in_cache == foreign) {
 				if (added_to_referenced_list) {
@@ -2966,7 +2980,8 @@ dict_create_foreign_constraints_low(
 	/* Try to find an index which contains the columns
 	as the first fields and in the right order */
 
-	index = dict_foreign_find_index(table, column_names, i, NULL, TRUE);
+	index = dict_foreign_find_index(table, column_names, i,
+					NULL, TRUE, FALSE);
 
 	if (!index) {
 		mutex_enter(&dict_foreign_err_mutex);
@@ -3237,7 +3252,8 @@ dict_create_foreign_constraints_low(
 	if (referenced_table) {
 		index = dict_foreign_find_index(referenced_table,
 						column_names, i,
-						foreign->foreign_index, TRUE);
+						foreign->foreign_index,
+						TRUE, FALSE);
 		if (!index) {
 			dict_foreign_free(foreign);
 			mutex_enter(&dict_foreign_err_mutex);