MDEV-34455 innodb_read_only=ON fails to imply innodb_doublewrite=OFF

innodb_doublewrite_update(): Disallow any change if srv_read_only_mode
holds, that is, the server was started with innodb_read_only=ON or
innodb_force_recovery=6.

This fixes up commit 1122ac97 (MDEV-33545).

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

@@ -186,6 +186,11 @@ t1	CREATE TABLE `t1` (
 CHECK TABLE t1;
 Table	Op	Msg_type	Msg_text
 test.t1	check	status	OK
+SET GLOBAL innodb_doublewrite=ON;
+SELECT @@GLOBAL.innodb_doublewrite "OFF expected";
+OFF expected
+OFF
+SET GLOBAL innodb_buf_flush_list_now=ON;
 # restart
 FTS_INDEX_1.ibd
 FTS_INDEX_2.ibd

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

@@ -87,6 +87,10 @@ SELECT * FROM t1 WHERE MATCH(b,c) AGAINST ('column');
 SHOW CREATE TABLE t1;
 CHECK TABLE t1;
 
+SET GLOBAL innodb_doublewrite=ON;
+SELECT @@GLOBAL.innodb_doublewrite "OFF expected";
+SET GLOBAL innodb_buf_flush_list_now=ON;
+
 --let $restart_parameters=
 --source include/restart_mysqld.inc
 --replace_regex /FTS_[0-9a-f]*_[0-9a-f]*/FTS/

storage/innobase/handler/ha_innodb.cc

@@ -18420,7 +18420,8 @@ static void innodb_data_file_write_through_update(THD *, st_mysql_sys_var*,
 static void innodb_doublewrite_update(THD *, st_mysql_sys_var*,
                                       void *, const void *save)
 {
-  fil_system.set_use_doublewrite(*static_cast<const ulong*>(save));
+  if (!srv_read_only_mode)
+    fil_system.set_use_doublewrite(*static_cast<const ulong*>(save));
 }
 
 static void innodb_log_file_size_update(THD *thd, st_mysql_sys_var*,