Type hints for the remaining two files in synapse.http. (#11164)

* Teach MyPy that the sentinel context is False

This means that if `ctx: LoggingContextOrSentinel`
then `bool(ctx)` narrows us to `ctx:LoggingContext`, which is a really
neat find!

* Annotate RequestMetrics

- Raise errors for sentry if we use the sentinel context
- Ensure we don't raise an error and carry on, but not recording stats
- Include stack trace in the error case to lower Sean's blood pressure

* Make mypy pass for synapse.http.request_metrics

* Make synapse.http.connectproxyclient pass mypy

Co-authored-by: reivilibre <oliverw@matrix.org>

synapse/http/request_metrics.py

@@ -15,6 +15,8 @@
 
 import logging
 import threading
+import traceback
+from typing import Dict, Mapping, Set, Tuple
 
 from prometheus_client.core import Counter, Histogram
 
@@ -105,19 +107,14 @@ in_flight_requests_db_sched_duration = Counter(
     ["method", "servlet"],
 )
 
-# The set of all in flight requests, set[RequestMetrics]
-_in_flight_requests = set()
+_in_flight_requests: Set["RequestMetrics"] = set()
 
 # Protects the _in_flight_requests set from concurrent access
 _in_flight_requests_lock = threading.Lock()
 
 
-def _get_in_flight_counts():
-    """Returns a count of all in flight requests by (method, server_name)
-
-    Returns:
-        dict[tuple[str, str], int]
-    """
+def _get_in_flight_counts() -> Mapping[Tuple[str, ...], int]:
+    """Returns a count of all in flight requests by (method, server_name)"""
     # Cast to a list to prevent it changing while the Prometheus
     # thread is collecting metrics
     with _in_flight_requests_lock:
@@ -127,8 +124,9 @@ def _get_in_flight_counts():
         rm.update_metrics()
 
     # Map from (method, name) -> int, the number of in flight requests of that
-    # type
-    counts = {}
+    # type. The key type is Tuple[str, str], but we leave the length unspecified
+    # for compatability with LaterGauge's annotations.
+    counts: Dict[Tuple[str, ...], int] = {}
     for rm in reqs:
         key = (rm.method, rm.name)
         counts[key] = counts.get(key, 0) + 1
@@ -145,15 +143,21 @@ LaterGauge(
 
 
 class RequestMetrics:
-    def start(self, time_sec, name, method):
-        self.start = time_sec
+    def start(self, time_sec: float, name: str, method: str) -> None:
+        self.start_ts = time_sec
         self.start_context = current_context()
         self.name = name
         self.method = method
 
-        # _request_stats records resource usage that we have already added
-        # to the "in flight" metrics.
-        self._request_stats = self.start_context.get_resource_usage()
+        if self.start_context:
+            # _request_stats records resource usage that we have already added
+            # to the "in flight" metrics.
+            self._request_stats = self.start_context.get_resource_usage()
+        else:
+            logger.error(
+                "Tried to start a RequestMetric from the sentinel context.\n%s",
+                "".join(traceback.format_stack()),
+            )
 
         with _in_flight_requests_lock:
             _in_flight_requests.add(self)
@@ -169,12 +173,18 @@ class RequestMetrics:
             tag = context.tag
 
             if context != self.start_context:
-                logger.warning(
+                logger.error(
                     "Context have unexpectedly changed %r, %r",
                     context,
                     self.start_context,
                 )
                 return
+        else:
+            logger.error(
+                "Trying to stop RequestMetrics in the sentinel context.\n%s",
+                "".join(traceback.format_stack()),
+            )
+            return
 
         response_code = str(response_code)
 
@@ -183,7 +193,7 @@ class RequestMetrics:
         response_count.labels(self.method, self.name, tag).inc()
 
         response_timer.labels(self.method, self.name, tag, response_code).observe(
-            time_sec - self.start
+            time_sec - self.start_ts
         )
 
         resource_usage = context.get_resource_usage()
@@ -213,6 +223,12 @@ class RequestMetrics:
 
     def update_metrics(self):
         """Updates the in flight metrics with values from this request."""
+        if not self.start_context:
+            logger.error(
+                "Tried to update a RequestMetric from the sentinel context.\n%s",
+                "".join(traceback.format_stack()),
+            )
+            return
         new_stats = self.start_context.get_resource_usage()
 
         diff = new_stats - self._request_stats