MDEV-33156 Crash on innodb_buf_flush_list_now=ON and innodb_force_recovery=6

srv_start(): Move a read only mode startup tweak from
innodb_init_params() to the correct location. Also if
innodb_force_recovery=6 we will disable the doublewrite buffer,
because InnoDB must run in read-only mode to prevent further corruption.

This change only affects debug checks. Whenever srv_read_only_mode holds,
the buf_pool.flush_list will be empty, that is, there will be no writes
of persistent InnoDB data pages.

Reviewed by: Thirunarayanan Balathandayuthapani

Author: dr-m

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

@@ -12,6 +12,7 @@ check table t1;
 Table	Op	Msg_type	Msg_text
 test.t1	check	Warning	InnoDB: Index 'b' contains 990 entries, should be 1024.
 test.t1	check	error	Corrupt
+SET GLOBAL innodb_buf_flush_list_now=ON;
 # restart
 SET GLOBAL innodb_fast_shutdown=0;
 # restart

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

@@ -72,6 +72,7 @@ EOF
 
 --replace_regex /contains \d+ entries/contains 990 entries/
 check table t1;
+SET GLOBAL innodb_buf_flush_list_now=ON;
 
 --source include/shutdown_mysqld.inc
 

storage/innobase/handler/ha_innodb.cc

@@ -3893,11 +3893,6 @@ static int innodb_init_params()
 	}
 #endif
 
-	if (srv_read_only_mode) {
-		ib::info() << "Started in read only mode";
-		srv_use_doublewrite_buf = FALSE;
-	}
-
 #ifdef LINUX_NATIVE_AIO
 #elif !defined _WIN32
 	/* Currently native AIO is supported only on windows and linux

storage/innobase/srv/srv0start.cc

@@ -1073,10 +1073,14 @@ dberr_t srv_start(bool create_new_db)
 	if (srv_force_recovery) {
 		ib::info() << "!!! innodb_force_recovery is set to "
 			<< srv_force_recovery << " !!!";
+		if (srv_force_recovery == SRV_FORCE_NO_LOG_REDO) {
+			srv_read_only_mode = true;
+		}
 	}
 
-	if (srv_force_recovery == SRV_FORCE_NO_LOG_REDO) {
-		srv_read_only_mode = true;
+	if (srv_read_only_mode) {
+		sql_print_information("InnoDB: Started in read only mode");
+		srv_use_doublewrite_buf = false;
 	}
 
 	high_level_read_only = srv_read_only_mode