MDEV-4578 information_schema.processlist reports incorrect value for Time (2147483647)

SHOW PROCESSLIST might see a thread that started executing a query *after*
processlist has started. Don't show a negative or huge wrapped-around query execution time.

mysql-test/r/processlist.result

+SET DEBUG_SYNC = 'dispatch_command_before_set_time WAIT_FOR do_set_time';
+SELECT 1;
+SET DEBUG_SYNC = 'fill_schema_processlist_after_unow SIGNAL do_set_time WAIT_FOR fill_schema_proceed';
+SELECT INFO,TIME,TIME_MS FROM INFORMATION_SCHEMA.PROCESSLIST WHERE INFO IS NULL;
+1
+1
+SET DEBUG_SYNC = 'now SIGNAL fill_schema_proceed';
+INFO	TIME	TIME_MS
+NULL	0	0.000

mysql-test/t/processlist.test

+#
+# MDEV-4578 information_schema.processlist reports incorrect value for Time (2147483647)
+#
+
+source include/have_debug_sync.inc;
+
+SET DEBUG_SYNC = 'dispatch_command_before_set_time WAIT_FOR do_set_time';
+send SELECT 1;
+
+connect (con1,localhost,root,,);
+
+SET DEBUG_SYNC = 'fill_schema_processlist_after_unow SIGNAL do_set_time WAIT_FOR fill_schema_proceed';
+send SELECT INFO,TIME,TIME_MS FROM INFORMATION_SCHEMA.PROCESSLIST WHERE INFO IS NULL;
+
+connection default;
+reap;
+SET DEBUG_SYNC = 'now SIGNAL fill_schema_proceed';
+
+connection con1;
+reap;
+disconnect con1;
+

sql/sql_parse.cc

@@ -914,6 +914,9 @@ bool dispatch_command(enum enum_server_command command, THD *thd,
   thd->enable_slow_log= TRUE;
   thd->query_plan_flags= QPLAN_INIT;
   thd->lex->sql_command= SQLCOM_END; /* to avoid confusing VIEW detectors */
+
+  DEBUG_SYNC(thd,"dispatch_command_before_set_time");
+
   thd->set_time();
   thd->set_query_id(get_query_id());
   if (!(server_command_flags[command] & CF_SKIP_QUERY_ID))

sql/sql_show.cc

@@ -2297,6 +2297,8 @@ int fill_schema_processlist(THD* thd, TABLE_LIST* tables, COND* cond)
   my_hrtime_t unow= my_hrtime();
   DBUG_ENTER("fill_schema_processlist");
 
+  DEBUG_SYNC(thd,"fill_schema_processlist_after_unow");
+
   user= thd->security_ctx->master_access & PROCESS_ACL ?
         NullS : thd->security_ctx->priv_user;
 
@@ -2355,9 +2357,8 @@ int fill_schema_processlist(THD* thd, TABLE_LIST* tables, COND* cond)
         table->field[4]->store(command_name[tmp->command].str,
                                command_name[tmp->command].length, cs);
       /* MYSQL_TIME */
-      const ulonglong utime= (tmp->start_time ?
-                              (unow.val - tmp->start_time * HRTIME_RESOLUTION -
-                               tmp->start_time_sec_part) : 0);
+      ulonglong start_utime= tmp->start_time * HRTIME_RESOLUTION + tmp->start_time_sec_part;
+      ulonglong utime= start_utime < unow.val ? unow.val - start_utime : 0;
       table->field[5]->store(utime / HRTIME_RESOLUTION, TRUE);
       /* STATE */
       if ((val= thread_state_info(tmp)))