MDEV-18609 Assertion !is_string || (*af)->charset() == cf->charset failed

The Create_field::charset can contain garbage for columns
that the SQL layer does not consider as being string columns.
InnoDB considers BIT a string column for historical reasons
(and backward compatibility with old persistent InnoDB metadata),
and therefore it checked the charset.

The Field::charset() consistently is my_charset_bin for BIT,
so we can trust that one.

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

@@ -779,10 +779,22 @@ DELETE FROM t1 LIMIT 3;
 ALTER TABLE t1 MODIFY f BIT(15);
 affected rows: 1
 info: Records: 1  Duplicates: 0  Warnings: 0
+ALTER TABLE t1 MODIFY f BIT(8);
+affected rows: 1
+info: Records: 1  Duplicates: 0  Warnings: 0
 SELECT HEX(f) FROM t1;
 HEX(f)
 80
 DROP TABLE t1;
+CREATE TABLE t1 (b BIT NOT NULL) ENGINE=InnoDB ROW_FORMAT=REDUNDANT DEFAULT CHARSET utf16;
+INSERT INTO t1 SET b=b'1';
+ALTER TABLE t1 CHANGE b c BIT NOT NULL;
+affected rows: 0
+info: Records: 0  Duplicates: 0  Warnings: 0
+SELECT HEX(c) FROM t1;
+HEX(c)
+1
+DROP TABLE t1;
 CREATE TABLE t1
 (id INT PRIMARY KEY, c2 INT UNIQUE,
 c3 POINT NOT NULL DEFAULT ST_GeomFromText('POINT(3 4)'),
@@ -1508,10 +1520,22 @@ DELETE FROM t1 LIMIT 3;
 ALTER TABLE t1 MODIFY f BIT(15);
 affected rows: 1
 info: Records: 1  Duplicates: 0  Warnings: 0
+ALTER TABLE t1 MODIFY f BIT(8);
+affected rows: 1
+info: Records: 1  Duplicates: 0  Warnings: 0
 SELECT HEX(f) FROM t1;
 HEX(f)
 80
 DROP TABLE t1;
+CREATE TABLE t1 (b BIT NOT NULL) ENGINE=InnoDB ROW_FORMAT=COMPACT DEFAULT CHARSET utf16;
+INSERT INTO t1 SET b=b'1';
+ALTER TABLE t1 CHANGE b c BIT NOT NULL;
+affected rows: 0
+info: Records: 0  Duplicates: 0  Warnings: 0
+SELECT HEX(c) FROM t1;
+HEX(c)
+1
+DROP TABLE t1;
 CREATE TABLE t1
 (id INT PRIMARY KEY, c2 INT UNIQUE,
 c3 POINT NOT NULL DEFAULT ST_GeomFromText('POINT(3 4)'),
@@ -2237,10 +2261,22 @@ DELETE FROM t1 LIMIT 3;
 ALTER TABLE t1 MODIFY f BIT(15);
 affected rows: 1
 info: Records: 1  Duplicates: 0  Warnings: 0
+ALTER TABLE t1 MODIFY f BIT(8);
+affected rows: 1
+info: Records: 1  Duplicates: 0  Warnings: 0
 SELECT HEX(f) FROM t1;
 HEX(f)
 80
 DROP TABLE t1;
+CREATE TABLE t1 (b BIT NOT NULL) ENGINE=InnoDB ROW_FORMAT=DYNAMIC DEFAULT CHARSET utf16;
+INSERT INTO t1 SET b=b'1';
+ALTER TABLE t1 CHANGE b c BIT NOT NULL;
+affected rows: 0
+info: Records: 0  Duplicates: 0  Warnings: 0
+SELECT HEX(c) FROM t1;
+HEX(c)
+1
+DROP TABLE t1;
 disconnect analyze;
 SELECT variable_value-@old_instant instants
 FROM information_schema.global_status

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

@@ -668,10 +668,19 @@ ALTER TABLE t1 MODIFY f BIT(15);
 DELETE FROM t1 LIMIT 3;
 --enable_info
 ALTER TABLE t1 MODIFY f BIT(15);
+ALTER TABLE t1 MODIFY f BIT(8);
 --disable_info
 SELECT HEX(f) FROM t1;
 DROP TABLE t1;
 
+eval CREATE TABLE t1 (b BIT NOT NULL) $engine DEFAULT CHARSET utf16;
+INSERT INTO t1 SET b=b'1';
+--enable_info
+ALTER TABLE t1 CHANGE b c BIT NOT NULL;
+--disable_info
+SELECT HEX(c) FROM t1;
+DROP TABLE t1;
+
 dec $format;
 }
 disconnect analyze;

storage/innobase/handler/handler0alter.cc

@@ -9016,7 +9016,6 @@ static void get_type(const Field& f, ulint& prtype, ulint& mtype, ulint& len)
 @param table_name Table name in MySQL
 @param pos 0-based index to user_table->cols[] or user_table->v_cols[]
 @param f new column
-@param cf column modification
 @param is_v if it's a virtual column
 @retval true Failure
 @retval false Success */
@@ -9028,7 +9027,6 @@ innobase_rename_or_enlarge_column_try(
 	const char*		table_name,
 	ulint			pos,
 	const Field&		f,
-	const Create_field&	cf,
 	bool			is_v)
 {
 	dict_col_t*	col;
@@ -9052,7 +9050,7 @@ innobase_rename_or_enlarge_column_try(
 	ulint prtype, mtype, len;
 	get_type(f, prtype, mtype, len);
 	DBUG_ASSERT(!dtype_is_string_type(col->mtype)
-		    || col->mbminlen == cf.charset->mbminlen);
+		    || col->mbminlen == f.charset()->mbminlen);
 	DBUG_ASSERT(col->len <= len);
 
 #ifdef UNIV_DEBUG
@@ -9152,7 +9150,7 @@ innobase_rename_or_enlarge_columns_try(
 			if (cf->field == *fp) {
 				if (innobase_rename_or_enlarge_column_try(
 					    ctx->old_table, trx, table_name,
-					    idx, **af, *cf, is_v)) {
+					    idx, **af, is_v)) {
 					DBUG_RETURN(true);
 				}
 				break;
@@ -9208,8 +9206,6 @@ innobase_rename_or_enlarge_columns_cache(
 				->m_col
 				: dict_table_get_nth_col(user_table, col_n);
 			const bool is_string= dtype_is_string_type(col->mtype);
-			DBUG_ASSERT(!is_string
-				    || (*af)->charset() == cf->charset);
 			DBUG_ASSERT(col->mbminlen
 				    == (is_string
 					? (*af)->charset()->mbminlen : 0));
@@ -9227,7 +9223,7 @@ innobase_rename_or_enlarge_columns_cache(
 				dict_mem_table_col_rename(
 					user_table, col_n,
 					cf->field->field_name.str,
-					cf->field_name.str, is_virtual);
+					(*af)->field_name.str, is_virtual);
 			}
 
 			break;