From f1d5c2f269ddb7141d72763f2d3eb24c6e204081 Mon Sep 17 00:00:00 2001
From: Eric Eastwood <erice@element.io>
Date: Tue, 9 Nov 2021 15:07:57 -0600
Subject: [PATCH] Split out federated PDU retrieval into a non-cached version
 (#11242)

Context: https://github.com/matrix-org/synapse/pull/11114/files#r741643968
---
 changelog.d/11242.misc                  |  1 +
 synapse/federation/federation_client.py | 80 ++++++++++++++++++-------
 2 files changed, 59 insertions(+), 22 deletions(-)
 create mode 100644 changelog.d/11242.misc

diff --git a/changelog.d/11242.misc b/changelog.d/11242.misc
new file mode 100644
index 000000000..3a98259ed
--- /dev/null
+++ b/changelog.d/11242.misc
@@ -0,0 +1 @@
+Split out federated PDU retrieval function into a non-cached version.
diff --git a/synapse/federation/federation_client.py b/synapse/federation/federation_client.py
index 670186f54..3b85b135e 100644
--- a/synapse/federation/federation_client.py
+++ b/synapse/federation/federation_client.py
@@ -277,6 +277,58 @@ class FederationClient(FederationBase):
 
         return pdus
 
+    async def get_pdu_from_destination_raw(
+        self,
+        destination: str,
+        event_id: str,
+        room_version: RoomVersion,
+        outlier: bool = False,
+        timeout: Optional[int] = None,
+    ) -> Optional[EventBase]:
+        """Requests the PDU with given origin and ID from the remote home
+        server. Does not have any caching or rate limiting!
+
+        Args:
+            destination: Which homeserver to query
+            event_id: event to fetch
+            room_version: version of the room
+            outlier: Indicates whether the PDU is an `outlier`, i.e. if
+                it's from an arbitrary point in the context as opposed to part
+                of the current block of PDUs. Defaults to `False`
+            timeout: How long to try (in ms) each destination for before
+                moving to the next destination. None indicates no timeout.
+
+        Returns:
+            The requested PDU, or None if we were unable to find it.
+
+        Raises:
+            SynapseError, NotRetryingDestination, FederationDeniedError
+        """
+        transaction_data = await self.transport_layer.get_event(
+            destination, event_id, timeout=timeout
+        )
+
+        logger.debug(
+            "retrieved event id %s from %s: %r",
+            event_id,
+            destination,
+            transaction_data,
+        )
+
+        pdu_list: List[EventBase] = [
+            event_from_pdu_json(p, room_version, outlier=outlier)
+            for p in transaction_data["pdus"]
+        ]
+
+        if pdu_list and pdu_list[0]:
+            pdu = pdu_list[0]
+
+            # Check signatures are correct.
+            signed_pdu = await self._check_sigs_and_hash(room_version, pdu)
+            return signed_pdu
+
+        return None
+
     async def get_pdu(
         self,
         destinations: Iterable[str],
@@ -321,30 +373,14 @@ class FederationClient(FederationBase):
                 continue
 
             try:
-                transaction_data = await self.transport_layer.get_event(
-                    destination, event_id, timeout=timeout
+                signed_pdu = await self.get_pdu_from_destination_raw(
+                    destination=destination,
+                    event_id=event_id,
+                    room_version=room_version,
+                    outlier=outlier,
+                    timeout=timeout,
                 )
 
-                logger.debug(
-                    "retrieved event id %s from %s: %r",
-                    event_id,
-                    destination,
-                    transaction_data,
-                )
-
-                pdu_list: List[EventBase] = [
-                    event_from_pdu_json(p, room_version, outlier=outlier)
-                    for p in transaction_data["pdus"]
-                ]
-
-                if pdu_list and pdu_list[0]:
-                    pdu = pdu_list[0]
-
-                    # Check signatures are correct.
-                    signed_pdu = await self._check_sigs_and_hash(room_version, pdu)
-
-                    break
-
                 pdu_attempts[destination] = now
 
             except SynapseError as e: