Sanity-checking for metrics updates

Check that our clocks go forward.

synapse/util/logcontext.py

@@ -325,10 +325,9 @@ def stop(self):
             )
             return
 
-        usage_end = get_thread_resource_usage()
-
-        self._resource_usage.ru_utime += usage_end.ru_utime - self.usage_start.ru_utime
-        self._resource_usage.ru_stime += usage_end.ru_stime - self.usage_start.ru_stime
+        utime_delta, stime_delta = self._get_cputime()
+        self._resource_usage.ru_utime += utime_delta
+        self._resource_usage.ru_stime += stime_delta
 
         self.usage_start = None
 
@@ -346,13 +345,38 @@ def get_resource_usage(self):
         # can include resource usage so far.
         is_main_thread = threading.current_thread() is self.main_thread
         if self.alive and self.usage_start and is_main_thread:
-            current = get_thread_resource_usage()
-            res.ru_utime += current.ru_utime - self.usage_start.ru_utime
-            res.ru_stime += current.ru_stime - self.usage_start.ru_stime
+            utime_delta, stime_delta = self._get_cputime()
+            res.ru_utime += utime_delta
+            res.ru_stime += stime_delta
 
         return res
 
+    def _get_cputime(self):
+        """Get the cpu usage time so far
+
+        Returns: Tuple[float, float]: seconds in user mode, seconds in system mode
+        """
+        current = get_thread_resource_usage()
+
+        utime_delta = current.ru_utime - self.usage_start.ru_utime
+        stime_delta = current.ru_stime - self.usage_start.ru_stime
+
+        # sanity check
+        if utime_delta < 0:
+            raise ValueError("utime went backwards! %f < %f" % (
+                current.ru_utime, self.usage_start.ru_utime,
+            ))
+
+        if stime_delta < 0:
+            raise ValueError("stime went backwards! %f < %f" % (
+                current.ru_stime, self.usage_start.ru_stime,
+            ))
+
+        return utime_delta, stime_delta
+
     def add_database_transaction(self, duration_sec):
+        if duration_sec < 0:
+            raise ValueError('DB txn time can only be non-negative')
         self._resource_usage.db_txn_count += 1
         self._resource_usage.db_txn_duration_sec += duration_sec
 
@@ -363,6 +387,8 @@ def add_database_scheduled(self, sched_sec):
             sched_sec (float): number of seconds it took us to get a
                 connection
         """
+        if sched_sec < 0:
+            raise ValueError('DB scheduling time can only be non-negative')
         self._resource_usage.db_sched_duration_sec += sched_sec
 
     def record_event_fetch(self, event_count):