Improved the test performance_schema.func_file_io,

so that investigating test failures is easier.

Detect cases when @before_count / @after_count is NULL.

mysql-test/suite/perfschema/r/func_file_io.result

@@ -15,6 +15,9 @@ SET @before_count = (SELECT SUM(TIMER_WAIT)
 FROM performance_schema.events_waits_history_long
 WHERE (EVENT_NAME = 'wait/io/file/myisam/dfile')
 AND (OBJECT_NAME LIKE '%t1.MYD'));
+SELECT (@before_count >= 0) as have_before_count;
+have_before_count
+1
 SELECT IF(@before_count > 0, 'Success', 'Failure') has_instrumentation;
 has_instrumentation
 Success
@@ -27,6 +30,9 @@ SET @after_count = (SELECT SUM(TIMER_WAIT)
 FROM performance_schema.events_waits_history_long
 WHERE (EVENT_NAME = 'wait/io/file/myisam/dfile')
 AND (OBJECT_NAME LIKE '%t1.MYD') AND (1 = 1));
+SELECT (@after_count >= 0) as have_after_count;
+have_after_count
+1
 SELECT IF((@after_count - @before_count) > 0, 'Success', 'Failure') test_ff1_timed;
 test_ff1_timed
 Success
@@ -35,6 +41,9 @@ SET @before_count = (SELECT SUM(TIMER_WAIT)
 FROM performance_schema.events_waits_history_long
 WHERE (EVENT_NAME = 'wait/io/file/myisam/dfile')
 AND (OBJECT_NAME LIKE '%t1.MYD') AND (2 = 2));
+SELECT (@before_count >= 0) as have_before_count;
+have_before_count
+1
 SELECT * FROM t1 WHERE id < 6;
 id	b
 1	initial value
@@ -46,6 +55,9 @@ SET @after_count = (SELECT SUM(TIMER_WAIT)
 FROM performance_schema.events_waits_history_long
 WHERE (EVENT_NAME = 'wait/io/file/myisam/dfile')
 AND (OBJECT_NAME LIKE '%t1.MYD') AND (3 = 3));
+SELECT (@after_count >= 0) as have_after_count;
+have_after_count
+1
 SELECT IF((COALESCE(@after_count, 0) - COALESCE(@before_count, 0)) = 0, 'Success', 'Failure') test_ff2_timed;
 test_ff2_timed
 Success

mysql-test/suite/perfschema/t/func_file_io.test

@@ -41,6 +41,7 @@ SET @before_count = (SELECT SUM(TIMER_WAIT)
                      WHERE (EVENT_NAME = 'wait/io/file/myisam/dfile')
                        AND (OBJECT_NAME LIKE '%t1.MYD'));
 
+SELECT (@before_count >= 0) as have_before_count;
 SELECT IF(@before_count > 0, 'Success', 'Failure') has_instrumentation;
 
 SELECT * FROM t1 WHERE id < 4;
@@ -50,6 +51,7 @@ SET @after_count = (SELECT SUM(TIMER_WAIT)
                     WHERE (EVENT_NAME = 'wait/io/file/myisam/dfile')
                       AND (OBJECT_NAME LIKE '%t1.MYD') AND (1 = 1));
 
+SELECT (@after_count >= 0) as have_after_count;
 SELECT IF((@after_count - @before_count) > 0, 'Success', 'Failure') test_ff1_timed;
 
 UPDATE performance_schema.setup_instruments SET enabled='NO';
@@ -59,6 +61,7 @@ SET @before_count = (SELECT SUM(TIMER_WAIT)
                      WHERE (EVENT_NAME = 'wait/io/file/myisam/dfile')
                        AND (OBJECT_NAME LIKE '%t1.MYD') AND (2 = 2));
 
+SELECT (@before_count >= 0) as have_before_count;
 SELECT * FROM t1 WHERE id < 6;
 
 SET @after_count = (SELECT SUM(TIMER_WAIT)
@@ -66,6 +69,7 @@ SET @after_count = (SELECT SUM(TIMER_WAIT)
                     WHERE (EVENT_NAME = 'wait/io/file/myisam/dfile')
                       AND (OBJECT_NAME LIKE '%t1.MYD') AND (3 = 3));
 
+SELECT (@after_count >= 0) as have_after_count;
 SELECT IF((COALESCE(@after_count, 0) - COALESCE(@before_count, 0)) = 0, 'Success', 'Failure') test_ff2_timed;
 
 #