MDEV-21127 Assertion in key_text::key_text()

Unsigned type while RHS expression could be less than 0.

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

@@ -190,3 +190,12 @@ bor#oda	CREATE TABLE `bor#oda` (
   CONSTRAINT `bor#oda_ibfk_2` FOREIGN KEY (`b`) REFERENCES `bor#oda` (`id`)
 ) ENGINE=InnoDB DEFAULT CHARSET=utf8
 DROP TABLE `bor#oda`;
+#
+# MDEV-21127 Assertion `(size_t)(ptr - buf) < MAX_TEXT - 4' failed in key_text::key_text
+#
+CREATE TABLE t1 (
+a012345678901234567890123456789012345678901 char(255),
+b char(255),
+FOREIGN KEY ( a012345678901234567890123456789012345678901, b ) REFERENCES tx (ax, bx)
+) ENGINE=InnoDB;
+ERROR HY000: Can't create table `test`.`t1` (errno: 150 "Foreign key constraint is incorrectly formed")

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

@@ -226,3 +226,13 @@ ADD FOREIGN KEY (`b`) REFERENCES `bor#oda`(`id`);
 
 SHOW CREATE TABLE `bor#oda`;
 DROP TABLE `bor#oda`;
+
+--echo #
+--echo # MDEV-21127 Assertion `(size_t)(ptr - buf) < MAX_TEXT - 4' failed in key_text::key_text
+--echo #
+--error ER_CANT_CREATE_TABLE
+CREATE TABLE t1 (
+    a012345678901234567890123456789012345678901 char(255),
+    b char(255),
+    FOREIGN KEY ( a012345678901234567890123456789012345678901, b ) REFERENCES tx (ax, bx)
+) ENGINE=InnoDB;

storage/innobase/handler/ha_innodb.cc

@@ -12208,9 +12208,9 @@ class key_text
 			/* 3 is etc continuation ("...");
 			   2 is comma separator (", ") in case of next exists;
 			   1 is terminating ')' */
-			if ((size_t)(ptr - buf) < MAX_TEXT
-				- (it.peek() ? 3 + 2 + 1 : 3 + 1)
-				- k->field_name.length) {
+			if (MAX_TEXT - (size_t)(ptr - buf)
+				>= (it.peek() ? 3 + 2 + 1 : 3 + 1)
+				+ k->field_name.length) {
 				memcpy(ptr, k->field_name.str,
 				       k->field_name.length);
 				ptr += k->field_name.length;