MDEV-20389 The test innodb.innodb_bug84958 fails intermittently

* Ensure no background purge is done;
* Better result showing actual number in case of failure;
* Higher and more safe difference threshold.

mysql-test/suite/innodb/r/innodb_bug84958.result

@@ -4,13 +4,18 @@
 #
 # Set up the test with a procedure and a function.
 #
+SET @saved_frequency= @@GLOBAL.innodb_purge_rseg_truncate_frequency;
+SET GLOBAL innodb_purge_rseg_truncate_frequency= 1;
+InnoDB		0 transactions not purged
 CREATE PROCEDURE insert_n(start int, end int)
 BEGIN
 DECLARE i INT DEFAULT start;
+START TRANSACTION;
 WHILE i <= end do
 INSERT INTO t1 VALUES (1, 2, 3) ON DUPLICATE KEY UPDATE c = i;
 SET i = i + 1;
 END WHILE;
+COMMIT;
 END~~
 CREATE FUNCTION num_pages_get()
 RETURNS INT
@@ -51,9 +56,9 @@ SET @num_pages_1 = num_pages_get();
 SELECT * FROM t1 force index (b);
 a	b	c
 SET @num_pages_2= num_pages_get();
-SELECT @num_pages_2 - @num_pages_1 < 500;
-@num_pages_2 - @num_pages_1 < 500
-1
+SELECT IF(@num_pages_2 - @num_pages_1 < 5000, 'OK', @num_pages_2 - @num_pages_1) num_pages_diff;
+num_pages_diff
+OK
 #
 # Commit and show the final record.
 #
@@ -76,6 +81,7 @@ test.t1	check	status	OK
 #
 disconnect con2;
 disconnect con3;
+SET GLOBAL innodb_purge_rseg_truncate_frequency= @saved_frequency;
 DROP TABLE t1;
 DROP PROCEDURE insert_n;
 DROP FUNCTION num_pages_get;

mysql-test/suite/innodb/t/innodb_bug84958.test

@@ -6,15 +6,20 @@
 --echo #
 
 --source include/have_innodb.inc
+SET @saved_frequency= @@GLOBAL.innodb_purge_rseg_truncate_frequency;
+SET GLOBAL innodb_purge_rseg_truncate_frequency= 1;
+--source include/wait_all_purged.inc
 
 DELIMITER ~~;
 CREATE PROCEDURE insert_n(start int, end int)
 BEGIN
     DECLARE i INT DEFAULT start;
+    START TRANSACTION;
     WHILE i <= end do
         INSERT INTO t1 VALUES (1, 2, 3) ON DUPLICATE KEY UPDATE c = i;
         SET i = i + 1;
     END WHILE;
+    COMMIT;
 END~~
 
 CREATE FUNCTION num_pages_get()
@@ -64,7 +69,7 @@ SET @num_pages_1 = num_pages_get();
 SELECT * FROM t1 force index (b);
 SET @num_pages_2= num_pages_get();
 
-SELECT @num_pages_2 - @num_pages_1 < 500;
+SELECT IF(@num_pages_2 - @num_pages_1 < 5000, 'OK', @num_pages_2 - @num_pages_1) num_pages_diff;
 
 --echo #
 --echo # Commit and show the final record.
@@ -81,6 +86,7 @@ CHECK TABLE t1;
 --echo #
 disconnect con2;
 disconnect con3;
+SET GLOBAL innodb_purge_rseg_truncate_frequency= @saved_frequency;
 DROP TABLE t1;
 DROP PROCEDURE insert_n;
 DROP FUNCTION num_pages_get;