MDEV-17815 Assertion failed in btr_node_ptr_max_size for CHAR(0)

btr_node_ptr_max_size(): Treat CHAR(0) from SQL as a special case.
The InnoDB internal SQL parser maps the type "CHAR" to DATA_VARCHAR,
but MariaDB does allow CHAR(0) with an empty value, and does enforce
the length limitation.

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

@@ -75,10 +75,13 @@ t1_VARCHAR_10_BINARY VARCHAR(10) BINARY,
 t1_VARCHAR_500 VARCHAR(500),
 t1_VARCHAR_500_BINARY VARCHAR(500) BINARY,
 t1_YEAR_2 YEAR(2),
-t1_YEAR_4 YEAR(4)
+t1_YEAR_4 YEAR(4),
+t1_CHAR_0 CHAR(0),
+t1_MYSQL_0 CHAR(0) CHARACTER SET utf8
 ) ENGINE=InnoDB;
 Warnings:
 Note	1287	'YEAR(2)' is deprecated and will be removed in a future release. Please use YEAR(4) instead
+INSERT INTO t1 () VALUES ();
 SELECT
 name,
 CASE mtype
@@ -109,6 +112,7 @@ t1_BINARY_100	DATA_FIXBINARY
 t1_BIT_2	DATA_FIXBINARY	UNSIGNED
 t1_BIT_20	DATA_FIXBINARY	UNSIGNED
 t1_BLOB	DATA_BLOB	
+t1_CHAR_0	DATA_CHAR	
 t1_CHAR_100	DATA_CHAR	
 t1_CHAR_100_BINARY	DATA_MYSQL	
 t1_DATE	DATA_INT	
@@ -131,6 +135,7 @@ t1_MEDIUMBLOB	DATA_BLOB
 t1_MEDIUMINT	DATA_INT	
 t1_MEDIUMINT_UNSIGNED	DATA_INT	UNSIGNED
 t1_MEDIUMTEXT	DATA_BLOB	
+t1_MYSQL_0	DATA_MYSQL	
 t1_SET	DATA_INT	UNSIGNED
 t1_SET_9	DATA_INT	UNSIGNED
 t1_SET_BINARY	DATA_INT	UNSIGNED
@@ -153,3 +158,9 @@ t1_VARCHAR_500_BINARY	DATA_VARMYSQL
 t1_YEAR_2	DATA_INT	UNSIGNED
 t1_YEAR_4	DATA_INT	UNSIGNED
 DROP TABLE t1;
+#
+# MDEV-17815 Assertion failed in btr_node_ptr_max_size for CHAR(0)
+#
+CREATE TABLE t1 (c CHAR(0), KEY(c)) ENGINE=InnoDB;
+INSERT INTO t1 VALUES ('');
+DROP TABLE t1;

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

@@ -88,9 +88,13 @@ CREATE TABLE t1
   t1_VARCHAR_500 VARCHAR(500),
   t1_VARCHAR_500_BINARY VARCHAR(500) BINARY,
   t1_YEAR_2 YEAR(2),
-  t1_YEAR_4 YEAR(4)
+  t1_YEAR_4 YEAR(4),
+  t1_CHAR_0 CHAR(0),
+  t1_MYSQL_0 CHAR(0) CHARACTER SET utf8
 ) ENGINE=InnoDB;
 
+INSERT INTO t1 () VALUES ();
+
 SELECT
   name,
   CASE mtype
@@ -116,3 +120,10 @@ WHERE name LIKE "t1\_%"
 ORDER BY name;
 
 DROP TABLE t1;
+
+--echo #
+--echo # MDEV-17815 Assertion failed in btr_node_ptr_max_size for CHAR(0)
+--echo #
+CREATE TABLE t1 (c CHAR(0), KEY(c)) ENGINE=InnoDB;
+INSERT INTO t1 VALUES ('');
+DROP TABLE t1;

storage/innobase/btr/btr0cur.cc

@@ -753,6 +753,25 @@ static ulint btr_node_ptr_max_size(const dict_index_t* index)
 
 		field_max_size = dict_col_get_max_size(col);
 		if (UNIV_UNLIKELY(!field_max_size)) {
+			switch (col->mtype) {
+			case DATA_CHAR:
+			case DATA_MYSQL:
+				/* CHAR(0) is a possible data type.
+				The InnoDB internal SQL parser maps
+				CHAR to DATA_VARCHAR, so DATA_CHAR (or
+				DATA_MYSQL) is only coming from the
+				MariaDB SQL layer. */
+				if (comp) {
+					/* Add a length byte, because
+					fixed-length empty field are
+					encoded as variable-length.
+					For ROW_FORMAT=REDUNDANT,
+					these bytes were added to
+					rec_max_size before this loop. */
+					rec_max_size++;
+				}
+				continue;
+			}
 			/* SYS_FOREIGN.ID is defined as CHAR in the
 			InnoDB internal SQL parser, which translates
 			into the incorrect VARCHAR(0).  InnoDB does