Fix LaterGauge metrics to collect from all servers (#18751)

Fix `LaterGauge` metrics to collect from all servers

Follow-up to #18714

Previously, our `LaterGauge` metrics did include the `server_name` label
as expected but we were only seeing the last server being reported in
some cases. Any `LaterGauge` that we were creating multiple times was
only reporting the last instance.

This PR updates all `LaterGauge` to be created once and then we use
`LaterGauge.register_hook(...)` to add in the metric callback as before.
This works now because we store a list of callbacks instead of just one.

I noticed this problem thanks to some [tests in the Synapse Pro for
Small Hosts](element-hq/synapse-small-hosts#173)
repo that sanity check all metrics to ensure that we can see each metric
includes data from multiple servers.


### Testing strategy

1. This is only noticeable when you run multiple Synapse instances in
the same process.
 1. TODO

(see test that was added)

### Dev notes

Previous non-global `LaterGauge`:

```
synapse_federation_send_queue_xxx
synapse_federation_transaction_queue_pending_destinations
synapse_federation_transaction_queue_pending_pdus
synapse_federation_transaction_queue_pending_edus
synapse_handlers_presence_user_to_current_state_size
synapse_handlers_presence_wheel_timer_size
synapse_notifier_listeners
synapse_notifier_rooms
synapse_notifier_users
synapse_replication_tcp_resource_total_connections
synapse_replication_tcp_command_queue
synapse_background_update_status
synapse_federation_known_servers
synapse_scheduler_running_tasks
```



### Pull Request Checklist

<!-- Please read
https://element-hq.github.io/synapse/latest/development/contributing_guide.html
before submitting your pull request -->

* [x] Pull request is based on the develop branch
* [x] Pull request includes a [changelog
file](https://element-hq.github.io/synapse/latest/development/contributing_guide.html#changelog).
The entry should:
- Be a short description of your change which makes sense to users.
"Fixed a bug that prevented receiving messages from other servers."
instead of "Moved X method from `EventStore` to `EventWorkerStore`.".
  - Use markdown where necessary, mostly for `code blocks`.
  - End with either a period (.) or an exclamation mark (!).
  - Start with a capital letter.
- Feel free to credit yourself, by adding a sentence "Contributed by
@github_username." or "Contributed by [Your Name]." to the end of the
entry.
* [x] [Code
style](https://element-hq.github.io/synapse/latest/code_style.html) is
correct (run the
[linters](https://element-hq.github.io/synapse/latest/development/contributing_guide.html#run-the-linters))

Author: MadLittleMods

changelog.d/18751.misc

@@ -0,0 +1 @@
+Fix `LaterGauge` metrics to collect from all servers.

synapse/federation/send_queue.py

@@ -37,6 +37,7 @@
 """
 
 import logging
+from enum import Enum
 from typing import (
     TYPE_CHECKING,
     Dict,
@@ -67,6 +68,25 @@
 logger = logging.getLogger(__name__)
 
 
+class QueueNames(str, Enum):
+    PRESENCE_MAP = "presence_map"
+    KEYED_EDU = "keyed_edu"
+    KEYED_EDU_CHANGED = "keyed_edu_changed"
+    EDUS = "edus"
+    POS_TIME = "pos_time"
+    PRESENCE_DESTINATIONS = "presence_destinations"
+
+
+queue_name_to_gauge_map: Dict[QueueNames, LaterGauge] = {}
+
+for queue_name in QueueNames:
+    queue_name_to_gauge_map[queue_name] = LaterGauge(
+        name=f"synapse_federation_send_queue_{queue_name.value}_size",
+        desc="",
+        labelnames=[SERVER_NAME_LABEL],
+    )
+
+
 class FederationRemoteSendQueue(AbstractFederationSender):
     """A drop in replacement for FederationSender"""
 
@@ -111,23 +131,15 @@ def __init__(self, hs: "HomeServer"):
         # we make a new function, so we need to make a new function so the inner
         # lambda binds to the queue rather than to the name of the queue which
         # changes. ARGH.
-        def register(name: str, queue: Sized) -> None:
-            LaterGauge(
-                name="synapse_federation_send_queue_%s_size" % (queue_name,),
-                desc="",
-                labelnames=[SERVER_NAME_LABEL],
-                caller=lambda: {(self.server_name,): len(queue)},
+        def register(queue_name: QueueNames, queue: Sized) -> None:
+            queue_name_to_gauge_map[queue_name].register_hook(
+                lambda: {(self.server_name,): len(queue)}
             )
 
-        for queue_name in [
-            "presence_map",
-            "keyed_edu",
-            "keyed_edu_changed",
-            "edus",
-            "pos_time",
-            "presence_destinations",
-        ]:
-            register(queue_name, getattr(self, queue_name))
+        for queue_name in QueueNames:
+            queue = getattr(self, queue_name.value)
+            assert isinstance(queue, Sized)
+            register(queue_name, queue=queue)
 
         self.clock.looping_call(self._clear_queue, 30 * 1000)
 

synapse/federation/sender/__init__.py

@@ -199,6 +199,24 @@
     labelnames=[SERVER_NAME_LABEL],
 )
 
+transaction_queue_pending_destinations_gauge = LaterGauge(
+    name="synapse_federation_transaction_queue_pending_destinations",
+    desc="",
+    labelnames=[SERVER_NAME_LABEL],
+)
+
+transaction_queue_pending_pdus_gauge = LaterGauge(
+    name="synapse_federation_transaction_queue_pending_pdus",
+    desc="",
+    labelnames=[SERVER_NAME_LABEL],
+)
+
+transaction_queue_pending_edus_gauge = LaterGauge(
+    name="synapse_federation_transaction_queue_pending_edus",
+    desc="",
+    labelnames=[SERVER_NAME_LABEL],
+)
+
 # Time (in s) to wait before trying to wake up destinations that have
 # catch-up outstanding.
 # Please note that rate limiting still applies, so while the loop is
@@ -398,38 +416,28 @@ def __init__(self, hs: "HomeServer"):
         # map from destination to PerDestinationQueue
         self._per_destination_queues: Dict[str, PerDestinationQueue] = {}
 
-        LaterGauge(
-            name="synapse_federation_transaction_queue_pending_destinations",
-            desc="",
-            labelnames=[SERVER_NAME_LABEL],
-            caller=lambda: {
+        transaction_queue_pending_destinations_gauge.register_hook(
+            lambda: {
                 (self.server_name,): sum(
                     1
                     for d in self._per_destination_queues.values()
                     if d.transmission_loop_running
                 )
-            },
+            }
         )
-
-        LaterGauge(
-            name="synapse_federation_transaction_queue_pending_pdus",
-            desc="",
-            labelnames=[SERVER_NAME_LABEL],
-            caller=lambda: {
+        transaction_queue_pending_pdus_gauge.register_hook(
+            lambda: {
                 (self.server_name,): sum(
                     d.pending_pdu_count() for d in self._per_destination_queues.values()
                 )
-            },
+            }
         )
-        LaterGauge(
-            name="synapse_federation_transaction_queue_pending_edus",
-            desc="",
-            labelnames=[SERVER_NAME_LABEL],
-            caller=lambda: {
+        transaction_queue_pending_edus_gauge.register_hook(
+            lambda: {
                 (self.server_name,): sum(
                     d.pending_edu_count() for d in self._per_destination_queues.values()
                 )
-            },
+            }
         )
 
         self._is_processing = False

synapse/handlers/presence.py

@@ -173,6 +173,18 @@
     labelnames=["locality", "from", "to", SERVER_NAME_LABEL],
 )
 
+presence_user_to_current_state_size_gauge = LaterGauge(
+    name="synapse_handlers_presence_user_to_current_state_size",
+    desc="",
+    labelnames=[SERVER_NAME_LABEL],
+)
+
+presence_wheel_timer_size_gauge = LaterGauge(
+    name="synapse_handlers_presence_wheel_timer_size",
+    desc="",
+    labelnames=[SERVER_NAME_LABEL],
+)
+
 # If a user was last active in the last LAST_ACTIVE_GRANULARITY, consider them
 # "currently_active"
 LAST_ACTIVE_GRANULARITY = 60 * 1000
@@ -779,11 +791,8 @@ def __init__(self, hs: "HomeServer"):
             EduTypes.PRESENCE, self.incoming_presence
         )
 
-        LaterGauge(
-            name="synapse_handlers_presence_user_to_current_state_size",
-            desc="",
-            labelnames=[SERVER_NAME_LABEL],
-            caller=lambda: {(self.server_name,): len(self.user_to_current_state)},
+        presence_user_to_current_state_size_gauge.register_hook(
+            lambda: {(self.server_name,): len(self.user_to_current_state)}
         )
 
         # The per-device presence state, maps user to devices to per-device presence state.
@@ -882,11 +891,8 @@ def __init__(self, hs: "HomeServer"):
                 60 * 1000,
             )
 
-        LaterGauge(
-            name="synapse_handlers_presence_wheel_timer_size",
-            desc="",
-            labelnames=[SERVER_NAME_LABEL],
-            caller=lambda: {(self.server_name,): len(self.wheel_timer)},
+        presence_wheel_timer_size_gauge.register_hook(
+            lambda: {(self.server_name,): len(self.wheel_timer)}
         )
 
         # Used to handle sending of presence to newly joined users/servers

synapse/http/request_metrics.py

@@ -164,12 +164,12 @@ def _get_in_flight_counts() -> Mapping[Tuple[str, ...], int]:
     return counts
 
 
-LaterGauge(
+in_flight_requests = LaterGauge(
     name="synapse_http_server_in_flight_requests_count",
     desc="",
     labelnames=["method", "servlet", SERVER_NAME_LABEL],
-    caller=_get_in_flight_counts,
 )
+in_flight_requests.register_hook(_get_in_flight_counts)
 
 
 class RequestMetrics:

synapse/metrics/__init__.py

@@ -31,6 +31,7 @@
     Dict,
     Generic,
     Iterable,
+    List,
     Mapping,
     Optional,
     Sequence,
@@ -73,8 +74,6 @@
 
 METRICS_PREFIX = "/_synapse/metrics"
 
-all_gauges: Dict[str, Collector] = {}
-
 HAVE_PROC_SELF_STAT = os.path.exists("/proc/self/stat")
 
 SERVER_NAME_LABEL = "server_name"
@@ -163,42 +162,47 @@ class LaterGauge(Collector):
     name: str
     desc: str
     labelnames: Optional[StrSequence] = attr.ib(hash=False)
-    # callback: should either return a value (if there are no labels for this metric),
-    # or dict mapping from a label tuple to a value
-    caller: Callable[
-        [], Union[Mapping[Tuple[str, ...], Union[int, float]], Union[int, float]]
-    ]
+    # List of callbacks: each callback should either return a value (if there are no
+    # labels for this metric), or dict mapping from a label tuple to a value
+    _hooks: List[
+        Callable[
+            [], Union[Mapping[Tuple[str, ...], Union[int, float]], Union[int, float]]
+        ]
+    ] = attr.ib(factory=list, hash=False)
 
     def collect(self) -> Iterable[Metric]:
         # The decision to add `SERVER_NAME_LABEL` is from the `LaterGauge` usage itself
         # (we don't enforce it here, one level up).
         g = GaugeMetricFamily(self.name, self.desc, labels=self.labelnames)  # type: ignore[missing-server-name-label]
 
-        try:
-            calls = self.caller()
-        except Exception:
-            logger.exception("Exception running callback for LaterGauge(%s)", self.name)
-            yield g
-            return
+        for hook in self._hooks:
+            try:
+                hook_result = hook()
+            except Exception:
+                logger.exception(
+                    "Exception running callback for LaterGauge(%s)", self.name
+                )
+                yield g
+                return
+
+            if isinstance(hook_result, (int, float)):
+                g.add_metric([], hook_result)
+            else:
+                for k, v in hook_result.items():
+                    g.add_metric(k, v)
 
-        if isinstance(calls, (int, float)):
-            g.add_metric([], calls)
-        else:
-            for k, v in calls.items():
-                g.add_metric(k, v)
+            yield g
 
-        yield g
+    def register_hook(
+        self,
+        hook: Callable[
+            [], Union[Mapping[Tuple[str, ...], Union[int, float]], Union[int, float]]
+        ],
+    ) -> None:
+        self._hooks.append(hook)
 
     def __attrs_post_init__(self) -> None:
-        self._register()
-
-    def _register(self) -> None:
-        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
 
 
 # `MetricsEntry` only makes sense when it is a `Protocol`,
@@ -250,7 +254,7 @@ def __init__(
         # Protects access to _registrations
         self._lock = threading.Lock()
 
-        self._register_with_collector()
+        REGISTRY.register(self)
 
     def register(
         self,
@@ -341,14 +345,6 @@ def collect(self) -> Iterable[Metric]:
                 gauge.add_metric(labels=key, value=getattr(metrics, name))
             yield gauge
 
-    def _register_with_collector(self) -> None:
-        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
-
 
 class GaugeHistogramMetricFamilyWithLabels(GaugeHistogramMetricFamily):
     """

synapse/notifier.py

@@ -86,6 +86,24 @@
     labelnames=["stream", SERVER_NAME_LABEL],
 )
 
+
+notifier_listeners_gauge = LaterGauge(
+    name="synapse_notifier_listeners",
+    desc="",
+    labelnames=[SERVER_NAME_LABEL],
+)
+
+notifier_rooms_gauge = LaterGauge(
+    name="synapse_notifier_rooms",
+    desc="",
+    labelnames=[SERVER_NAME_LABEL],
+)
+notifier_users_gauge = LaterGauge(
+    name="synapse_notifier_users",
+    desc="",
+    labelnames=[SERVER_NAME_LABEL],
+)
+
 T = TypeVar("T")
 
 
@@ -281,28 +299,16 @@ def count_listeners() -> Mapping[Tuple[str, ...], int]:
                 )
             }
 
-        LaterGauge(
-            name="synapse_notifier_listeners",
-            desc="",
-            labelnames=[SERVER_NAME_LABEL],
-            caller=count_listeners,
-        )
-
-        LaterGauge(
-            name="synapse_notifier_rooms",
-            desc="",
-            labelnames=[SERVER_NAME_LABEL],
-            caller=lambda: {
+        notifier_listeners_gauge.register_hook(count_listeners)
+        notifier_rooms_gauge.register_hook(
+            lambda: {
                 (self.server_name,): count(
                     bool, list(self.room_to_user_streams.values())
                 )
-            },
+            }
         )
-        LaterGauge(
-            name="synapse_notifier_users",
-            desc="",
-            labelnames=[SERVER_NAME_LABEL],
-            caller=lambda: {(self.server_name,): len(self.user_to_user_stream)},
+        notifier_users_gauge.register_hook(
+            lambda: {(self.server_name,): len(self.user_to_user_stream)}
         )
 
     def add_replication_callback(self, cb: Callable[[], None]) -> None: