MDEV-23855 follow-up: Make innodb.doublewrite more stable

The test innodb.doublewrite could occasionally fail with 64KiB page size
because the page 0 would no longer be in the doublewrite buffer.
Let us stop purge before the server is killed, and ensure that
the entire buffer pool will be flushed before we initiate an extra
write of page 0.

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

@@ -14,6 +14,9 @@ innodb_fil_make_page_dirty_debug	0
 show variables like 'innodb_saved_page_number_debug';
 Variable_name	Value
 innodb_saved_page_number_debug	0
+connect  stop_purge,localhost,root,,;
+START TRANSACTION WITH CONSISTENT SNAPSHOT;
+connection default;
 create table t1 (f1 int primary key, f2 blob) engine=innodb;
 start transaction;
 insert into t1 values(1, repeat('#',12));
@@ -29,17 +32,17 @@ select space from information_schema.innodb_sys_tables
 where name = 'test/t1' into @space_id;
 Warnings:
 Warning	1287	'<select expression> INTO <destination>;' is deprecated and will be removed in a future release. Please use 'SELECT <select list> INTO <destination> FROM...' instead
-# Ensure that dirty pages of table t1 is flushed.
-flush tables t1 for export;
-unlock tables;
 begin;
 insert into t1 values (6, repeat('%', 12));
+# Ensure that dirty pages of table t1 are flushed.
+set global innodb_buf_flush_list_now = 1;
 # Make the first page dirty for table t1
 set global innodb_saved_page_number_debug = 0;
 set global innodb_fil_make_page_dirty_debug = @space_id;
 # Ensure that dirty pages of table t1 are flushed.
 set global innodb_buf_flush_list_now = 1;
 # Kill the server
+disconnect stop_purge;
 # Make the first page (page_no=0) of the user tablespace
 # full of zeroes.
 #

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

@@ -34,6 +34,10 @@ show variables like 'innodb_doublewrite';
 show variables like 'innodb_fil_make_page_dirty_debug';
 show variables like 'innodb_saved_page_number_debug';
 
+connect (stop_purge,localhost,root,,);
+START TRANSACTION WITH CONSISTENT SNAPSHOT;
+connection default;
+
 create table t1 (f1 int primary key, f2 blob) engine=innodb;
 
 start transaction;
@@ -51,12 +55,10 @@ commit work;
 select space from information_schema.innodb_sys_tables
 where name = 'test/t1' into @space_id;
 
---echo # Ensure that dirty pages of table t1 is flushed.
-flush tables t1 for export;
-unlock tables;
-
 begin;
 insert into t1 values (6, repeat('%', 12));
+--echo # Ensure that dirty pages of table t1 are flushed.
+set global innodb_buf_flush_list_now = 1;
 
 --source ../include/no_checkpoint_start.inc
 
@@ -69,6 +71,7 @@ set global innodb_buf_flush_list_now = 1;
 
 --let CLEANUP_IF_CHECKPOINT=drop table t1;
 --source ../include/no_checkpoint_end.inc
+disconnect stop_purge;
 
 --echo # Make the first page (page_no=0) of the user tablespace
 --echo # full of zeroes.