Remove a reference cycle in background process (#16314)

Author: erikjohnston

changelog.d/16314.misc

@@ -0,0 +1 @@
+Remove a reference cycle for in background processes.

synapse/metrics/background_process_metrics.py

@@ -322,13 +322,21 @@ def __init__(self, name: str, instance_id: Optional[Union[int, str]] = None):
         if instance_id is None:
             instance_id = id(self)
         super().__init__("%s-%s" % (name, instance_id))
-        self._proc = _BackgroundProcess(name, self)
+        self._proc: Optional[_BackgroundProcess] = _BackgroundProcess(name, self)
 
     def start(self, rusage: "Optional[resource.struct_rusage]") -> None:
         """Log context has started running (again)."""
 
         super().start(rusage)
 
+        if self._proc is None:
+            logger.error(
+                "Background process re-entered without a proc: %s",
+                self.name,
+                stack_info=True,
+            )
+            return
+
         # We've become active again so we make sure we're in the list of active
         # procs. (Note that "start" here means we've become active, as opposed
         # to starting for the first time.)
@@ -345,10 +353,21 @@ def __exit__(
 
         super().__exit__(type, value, traceback)
 
+        if self._proc is None:
+            logger.error(
+                "Background process exited without a proc: %s",
+                self.name,
+                stack_info=True,
+            )
+            return
+
         # The background process has finished. We explicitly remove and manually
         # update the metrics here so that if nothing is scraping metrics the set
         # doesn't infinitely grow.
         with _bg_metrics_lock:
             _background_processes_active_since_last_scrape.discard(self._proc)
 
         self._proc.update_metrics()
+
+        # Set proc to None to break the reference cycle.
+        self._proc = None