MDEV-22771 Instant extension of CHAR column is wrongly allowed

commit 854c219 (MDEV-17301)
broke a constraint: Fixed-length columns cannot be extended in InnoDB
without rebuilding the table.

ha_innobase::can_convert_string(): Correct the condition. We must
not allow any instantaneous change to the length of CHAR columns
measured in characters. For any format other than ROW_FORMAT=REDUNDANT,
we can allow the length in bytes to be extended if mbminlen<mbmaxlen held
before the change of the character set.

mysql-test/suite/innodb/r/instant_alter_extend,utf8.rdiff

@@ -1,29 +1,38 @@
 --- instant_alter_convert.result
 +++ instant_alter_convert,utf8.result
-@@ -37,7 +37,7 @@
- test.t	check	status	OK
+@@ -38,7 +38,7 @@
+ best.t	check	status	OK
  call check_table('t');
  name	mtype	prtype	len
 -a	2	800FE	200
 +a	13	2100FE	600
  # CHAR enlargement
- alter table t modify a char(220), algorithm=instant;
- select count(a) from t where a = @bigval;
-@@ -51,7 +51,7 @@
- test.t	check	status	OK
+ alter table t modify a char(220);
+ affected rows: 2
+@@ -54,7 +54,7 @@
+ best.t	check	status	OK
  call check_table('t');
  name	mtype	prtype	len
 -a	2	800FE	220
 +a	13	2100FE	660
+ ALTER TABLE t CHANGE COLUMN a a CHAR(230) BINARY;
+ affected rows: 2
+ info: Records: 2  Duplicates: 0  Warnings: 0
+@@ -69,7 +69,7 @@
+ best.t	check	status	OK
+ call check_table('t');
+ name	mtype	prtype	len
+-a	13	2F00FE	230
++a	13	5300FE	690
  # Convert from VARCHAR to a bigger CHAR
- alter table t modify a varchar(200), algorithm=instant;
- ERROR 0A000: ALGORITHM=INSTANT is not supported. Reason: Cannot change column type. Try ALGORITHM=COPY
-@@ -72,7 +72,7 @@
- test.t	check	status	OK
+ alter table t modify a varchar(200);
+ affected rows: 2
+@@ -92,7 +92,7 @@
+ best.t	check	status	OK
  call check_table('t');
  name	mtype	prtype	len
 -a	2	800FE	255
 +a	13	2100FE	765
  # BINARY/VARBINARY test
  create or replace table t (a varbinary(300));
- alter table t modify a binary(255), algorithm=instant;
+ insert into t values(NULL);

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

@@ -64,6 +64,15 @@ select a, length(a) from t where a = 'z';
 check table t extended;
 call check_table('t');
 
+--enable_info
+ALTER TABLE t CHANGE COLUMN a a CHAR(230) BINARY;
+ALTER TABLE t ADD COLUMN b INT FIRST;
+ALTER TABLE t DROP b;
+--disable_info
+
+check table t extended;
+call check_table('t');
+
 --echo # Convert from VARCHAR to a bigger CHAR
 --enable_info
 alter table t modify a varchar(200);

storage/innobase/handler/ha_innodb.cc

@@ -20986,43 +20986,35 @@ is_part_of_a_primary_key(const Field* field)
 	       && field->part_of_key.is_set(s->primary_key);
 }
 
-bool
-ha_innobase::can_convert_string(const Field_string* field,
-				const Column_definition& new_type) const
+bool ha_innobase::can_convert_string(const Field_string *field,
+                                     const Column_definition &new_type) const
 {
-	DBUG_ASSERT(!field->compression_method());
-	if (new_type.type_handler() != field->type_handler()) {
-		return false;
-	}
+  DBUG_ASSERT(!field->compression_method());
+  if (new_type.type_handler() != field->type_handler())
+    return false;
 
-	if (new_type.char_length < field->char_length()) {
-		return false;
-	}
+  if (new_type.char_length != field->char_length())
+    return false;
 
-	if (new_type.charset != field->charset()) {
-		if (new_type.length != field->max_display_length()
-		    && !m_prebuilt->table->not_redundant()) {
-			return IS_EQUAL_NO;
-		}
+  const Charset field_cs(field->charset());
 
-		Charset field_cs(field->charset());
-		if (!field_cs.encoding_allows_reinterpret_as(
-			new_type.charset)) {
-			return false;
-		}
+  if (new_type.length != field->max_display_length() &&
+      (!m_prebuilt->table->not_redundant() ||
+       field_cs.mbminlen() == field_cs.mbmaxlen()))
+    return false;
 
-		if (!field_cs.eq_collation_specific_names(new_type.charset)) {
-			return !is_part_of_a_primary_key(field);
-		}
+  if (new_type.charset != field->charset())
+  {
+    if (!field_cs.encoding_allows_reinterpret_as(new_type.charset))
+      return false;
 
-		return true;
-	}
+    if (!field_cs.eq_collation_specific_names(new_type.charset))
+      return !is_part_of_a_primary_key(field);
 
-	if (new_type.length != field->max_display_length()) {
-		return false;
-	}
+    return true;
+  }
 
-	return true;
+  return true;
 }
 
 static bool