MDEV-20079 When setting back the system time while mysqld is running, NOW() and UNIX_TIMESTAMP() results get stuck

typo. system_time.start wasn't updated when system_time.sec
and system_time.sec_part were.

Author: vuvova

mysql-test/main/system_time_debug.result

@@ -0,0 +1,16 @@
+set @old_dbug=@@debug_dbug;
+select timestampdiff(minute,now(),sysdate()) as 'must be 0', (unix_timestamp(sysdate()) - unix_timestamp()) div 60 as 'must be 0';
+must be 0	must be 0
+0	0
+set @@debug_dbug='+d,system_time_plus_one_hour';
+select timestampdiff(minute,now(),sysdate()) as 'must be 0', (unix_timestamp(sysdate()) - unix_timestamp()) div 60 as 'must be 0';
+must be 0	must be 0
+0	0
+set @@debug_dbug='+d,system_time_minus_one_hour:-d,system_time_plus_one_hour';
+select timestampdiff(minute,now(),sysdate()) as 'must be 0', (unix_timestamp(sysdate()) - unix_timestamp()) div 60 as 'must be 0';
+must be 0	must be 0
+0	0
+set @@debug_dbug=@old_dbug;
+select timestampdiff(minute,now(),sysdate()) as 'must be 0', (unix_timestamp(sysdate()) - unix_timestamp()) div 60 as 'must be 0';
+must be 0	must be 0
+0	0

mysql-test/main/system_time_debug.test

@@ -0,0 +1,16 @@
+source include/have_debug.inc;
+#
+# MDEV-20079 When setting back the system time while mysqld is running, NOW() and UNIX_TIMESTAMP() results get stuck
+#
+set @old_dbug=@@debug_dbug;
+
+# because NOW() is taken at the beginning of the query and sysdate() is the actual
+# time when sysdate() was evaluated, they don't necessarily have to be equal.
+# but hopefully they're less than within a minute from each other.
+select timestampdiff(minute,now(),sysdate()) as 'must be 0', (unix_timestamp(sysdate()) - unix_timestamp()) div 60 as 'must be 0';
+set @@debug_dbug='+d,system_time_plus_one_hour';
+select timestampdiff(minute,now(),sysdate()) as 'must be 0', (unix_timestamp(sysdate()) - unix_timestamp()) div 60 as 'must be 0';
+set @@debug_dbug='+d,system_time_minus_one_hour:-d,system_time_plus_one_hour';
+select timestampdiff(minute,now(),sysdate()) as 'must be 0', (unix_timestamp(sysdate()) - unix_timestamp()) div 60 as 'must be 0';
+set @@debug_dbug=@old_dbug;
+select timestampdiff(minute,now(),sysdate()) as 'must be 0', (unix_timestamp(sysdate()) - unix_timestamp()) div 60 as 'must be 0';

mysys/my_getsystime.c

@@ -99,6 +99,8 @@ my_hrtime_t my_hrtime()
   while (gettimeofday(&t, NULL) != 0) {}
   hrtime.val= t.tv_sec*1000000ULL + t.tv_usec;
 #endif
+  DBUG_EXECUTE_IF("system_time_plus_one_hour", hrtime.val += 3600*1000000ULL;);
+  DBUG_EXECUTE_IF("system_time_minus_one_hour", hrtime.val -= 3600*1000000ULL;);
   return hrtime;
 }
 

sql/sql_class.h

@@ -3443,6 +3443,7 @@ class THD :public Statement,
     {
       system_time.sec= sec;
       system_time.sec_part= sec_part;
+      system_time.start= hrtime;
     }
     else
     {