1 files changed, 107 insertions, 1 deletions
diff --git a/synapse/metrics/__init__.py b/synapse/metrics/__init__.py
index 550f8443f7..59900aa5d1 100644
--- a/synapse/metrics/__init__.py
+++ b/synapse/metrics/__init__.py
@@ -18,8 +18,11 @@ import gc
 import logging
 import os
 import platform
+import threading
 import time
 
+import six
+
 import attr
 from prometheus_client import Counter, Gauge, Histogram
 from prometheus_client.core import REGISTRY, GaugeMetricFamily
@@ -68,7 +71,7 @@ class LaterGauge(object):
             return
 
         if isinstance(calls, dict):
-            for k, v in calls.items():
+            for k, v in six.iteritems(calls):
                 g.add_metric(k, v)
         else:
             g.add_metric([], calls)
@@ -87,6 +90,109 @@ class LaterGauge(object):
         all_gauges[self.name] = self
 
 
+class InFlightGauge(object):
+    """Tracks number of things (e.g. requests, Measure blocks, etc) in flight
+    at any given time.
+
+    Each InFlightGauge will create a metric called `<name>_total` that counts
+    the number of in flight blocks, as well as a metrics for each item in the
+    given `sub_metrics` as `<name>_<sub_metric>` which will get updated by the
+    callbacks.
+
+    Args:
+        name (str)
+        desc (str)
+        labels (list[str])
+        sub_metrics (list[str]): A list of sub metrics that the callbacks
+            will update.
+    """
+
+    def __init__(self, name, desc, labels, sub_metrics):
+        self.name = name
+        self.desc = desc
+        self.labels = labels
+        self.sub_metrics = sub_metrics
+
+        # Create a class which have the sub_metrics values as attributes, which
+        # default to 0 on initialization. Used to pass to registered callbacks.
+        self._metrics_class = attr.make_class(
+            "_MetricsEntry",
+            attrs={x: attr.ib(0) for x in sub_metrics},
+            slots=True,
+        )
+
+        # Counts number of in flight blocks for a given set of label values
+        self._registrations = {}
+
+        # Protects access to _registrations
+        self._lock = threading.Lock()
+
+        self._register_with_collector()
+
+    def register(self, key, callback):
+        """Registers that we've entered a new block with labels `key`.
+
+        `callback` gets called each time the metrics are collected. The same
+        value must also be given to `unregister`.
+
+        `callback` gets called with an object that has an attribute per
+        sub_metric, which should be updated with the necessary values. Note that
+        the metrics object is shared between all callbacks registered with the
+        same key.
+
+        Note that `callback` may be called on a separate thread.
+        """
+        with self._lock:
+            self._registrations.setdefault(key, set()).add(callback)
+
+    def unregister(self, key, callback):
+        """Registers that we've exited a block with labels `key`.
+        """
+
+        with self._lock:
+            self._registrations.setdefault(key, set()).discard(callback)
+
+    def collect(self):
+        """Called by prometheus client when it reads metrics.
+
+        Note: may be called by a separate thread.
+        """
+        in_flight = GaugeMetricFamily(self.name + "_total", self.desc, labels=self.labels)
+
+        metrics_by_key = {}
+
+        # We copy so that we don't mutate the list while iterating
+        with self._lock:
+            keys = list(self._registrations)
+
+        for key in keys:
+            with self._lock:
+                callbacks = set(self._registrations[key])
+
+            in_flight.add_metric(key, len(callbacks))
+
+            metrics = self._metrics_class()
+            metrics_by_key[key] = metrics
+            for callback in callbacks:
+                callback(metrics)
+
+        yield in_flight
+
+        for name in self.sub_metrics:
+            gauge = GaugeMetricFamily("_".join([self.name, name]), "", labels=self.labels)
+            for key, metrics in six.iteritems(metrics_by_key):
+                gauge.add_metric(key, getattr(metrics, name))
+            yield gauge
+
+    def _register_with_collector(self):
+        if self.name in all_gauges.keys():
+            logger.warning("%s already registered, reregistering" % (self.name,))
+            REGISTRY.unregister(all_gauges.pop(self.name))
+
+        REGISTRY.register(self)
+        all_gauges[self.name] = self
+
+
 #
 # Detailed CPU metrics
 #