Spaces summary: call out to other servers (#9653)

When we hit an unknown room in the space tree, see if there are other servers that we might be able to poll to get the data.

Fixes: #9447

changelog.d/9653.feature

@@ -0,0 +1 @@
+Add initial experimental support for a "space summary" API.

synapse/federation/federation_client.py

@@ -27,11 +27,13 @@
     List,
     Mapping,
     Optional,
+    Sequence,
     Tuple,
     TypeVar,
     Union,
 )
 
+import attr
 from prometheus_client import Counter
 
 from twisted.internet import defer
@@ -455,6 +457,7 @@ async def _try_destination_list(
         description: str,
         destinations: Iterable[str],
         callback: Callable[[str], Awaitable[T]],
+        failover_on_unknown_endpoint: bool = False,
     ) -> T:
         """Try an operation on a series of servers, until it succeeds
 
@@ -474,6 +477,10 @@ async def _try_destination_list(
                 next server tried. Normally the stacktrace is logged but this is
                 suppressed if the exception is an InvalidResponseError.
 
+            failover_on_unknown_endpoint: if True, we will try other servers if it looks
+                like a server doesn't support the endpoint. This is typically useful
+                if the endpoint in question is new or experimental.
+
         Returns:
             The result of callback, if it succeeds
 
@@ -493,16 +500,31 @@ async def _try_destination_list(
             except UnsupportedRoomVersionError:
                 raise
             except HttpResponseException as e:
-                if not 500 <= e.code < 600:
-                    raise e.to_synapse_error()
-                else:
-                    logger.warning(
-                        "Failed to %s via %s: %i %s",
-                        description,
-                        destination,
-                        e.code,
-                        e.args[0],
-                    )
+                synapse_error = e.to_synapse_error()
+                failover = False
+
+                if 500 <= e.code < 600:
+                    failover = True
+
+                elif failover_on_unknown_endpoint:
+                    # there is no good way to detect an "unknown" endpoint. Dendrite
+                    # returns a 404 (with no body); synapse returns a 400
+                    # with M_UNRECOGNISED.
+                    if e.code == 404 or (
+                        e.code == 400 and synapse_error.errcode == Codes.UNRECOGNIZED
+                    ):
+                        failover = True
+
+                if not failover:
+                    raise synapse_error from e
+
+                logger.warning(
+                    "Failed to %s via %s: %i %s",
+                    description,
+                    destination,
+                    e.code,
+                    e.args[0],
+                )
             except Exception:
                 logger.warning(
                     "Failed to %s via %s", description, destination, exc_info=True
@@ -1042,3 +1064,141 @@ async def get_room_complexity(
         # If we don't manage to find it, return None. It's not an error if a
         # server doesn't give it to us.
         return None
+
+    async def get_space_summary(
+        self,
+        destinations: Iterable[str],
+        room_id: str,
+        suggested_only: bool,
+        max_rooms_per_space: Optional[int],
+        exclude_rooms: List[str],
+    ) -> "FederationSpaceSummaryResult":
+        """
+        Call other servers to get a summary of the given space
+
+
+        Args:
+            destinations: The remote servers. We will try them in turn, omitting any
+                that have been blacklisted.
+
+            room_id: ID of the space to be queried
+
+            suggested_only:  If true, ask the remote server to only return children
+                with the "suggested" flag set
+
+            max_rooms_per_space: A limit on the number of children to return for each
+                space
+
+            exclude_rooms: A list of room IDs to tell the remote server to skip
+
+        Returns:
+            a parsed FederationSpaceSummaryResult
+
+        Raises:
+            SynapseError if we were unable to get a valid summary from any of the
+               remote servers
+        """
+
+        async def send_request(destination: str) -> FederationSpaceSummaryResult:
+            res = await self.transport_layer.get_space_summary(
+                destination=destination,
+                room_id=room_id,
+                suggested_only=suggested_only,
+                max_rooms_per_space=max_rooms_per_space,
+                exclude_rooms=exclude_rooms,
+            )
+
+            try:
+                return FederationSpaceSummaryResult.from_json_dict(res)
+            except ValueError as e:
+                raise InvalidResponseError(str(e))
+
+        return await self._try_destination_list(
+            "fetch space summary",
+            destinations,
+            send_request,
+            failover_on_unknown_endpoint=True,
+        )
+
+
+@attr.s(frozen=True, slots=True)
+class FederationSpaceSummaryEventResult:
+    """Represents a single event in the result of a successful get_space_summary call.
+
+    It's essentially just a serialised event object, but we do a bit of parsing and
+    validation in `from_json_dict` and store some of the validated properties in
+    object attributes.
+    """
+
+    event_type = attr.ib(type=str)
+    state_key = attr.ib(type=str)
+    via = attr.ib(type=Sequence[str])
+
+    # the raw data, including the above keys
+    data = attr.ib(type=JsonDict)
+
+    @classmethod
+    def from_json_dict(cls, d: JsonDict) -> "FederationSpaceSummaryEventResult":
+        """Parse an event within the result of a /spaces/ request
+
+        Args:
+            d: json object to be parsed
+
+        Raises:
+            ValueError if d is not a valid event
+        """
+
+        event_type = d.get("type")
+        if not isinstance(event_type, str):
+            raise ValueError("Invalid event: 'event_type' must be a str")
+
+        state_key = d.get("state_key")
+        if not isinstance(state_key, str):
+            raise ValueError("Invalid event: 'state_key' must be a str")
+
+        content = d.get("content")
+        if not isinstance(content, dict):
+            raise ValueError("Invalid event: 'content' must be a dict")
+
+        via = content.get("via")
+        if not isinstance(via, Sequence):
+            raise ValueError("Invalid event: 'via' must be a list")
+        if any(not isinstance(v, str) for v in via):
+            raise ValueError("Invalid event: 'via' must be a list of strings")
+
+        return cls(event_type, state_key, via, d)
+
+
+@attr.s(frozen=True, slots=True)
+class FederationSpaceSummaryResult:
+    """Represents the data returned by a successful get_space_summary call."""
+
+    rooms = attr.ib(type=Sequence[JsonDict])
+    events = attr.ib(type=Sequence[FederationSpaceSummaryEventResult])
+
+    @classmethod
+    def from_json_dict(cls, d: JsonDict) -> "FederationSpaceSummaryResult":
+        """Parse the result of a /spaces/ request
+
+        Args:
+            d: json object to be parsed
+
+        Raises:
+            ValueError if d is not a valid /spaces/ response
+        """
+        rooms = d.get("rooms")
+        if not isinstance(rooms, Sequence):
+            raise ValueError("'rooms' must be a list")
+        if any(not isinstance(r, dict) for r in rooms):
+            raise ValueError("Invalid room in 'rooms' list")
+
+        events = d.get("events")
+        if not isinstance(events, Sequence):
+            raise ValueError("'events' must be a list")
+        if any(not isinstance(e, dict) for e in events):
+            raise ValueError("Invalid event in 'events' list")
+        parsed_events = [
+            FederationSpaceSummaryEventResult.from_json_dict(e) for e in events
+        ]
+
+        return cls(rooms, parsed_events)

synapse/federation/transport/client.py

@@ -16,7 +16,7 @@
 
 import logging
 import urllib
-from typing import Any, Dict, Optional
+from typing import Any, Dict, List, Optional
 
 from synapse.api.constants import Membership
 from synapse.api.errors import Codes, HttpResponseException, SynapseError
@@ -26,6 +26,7 @@
     FEDERATION_V2_PREFIX,
 )
 from synapse.logging.utils import log_function
+from synapse.types import JsonDict
 
 logger = logging.getLogger(__name__)
 
@@ -978,6 +979,38 @@ def get_room_complexity(self, destination, room_id):
 
         return self.client.get_json(destination=destination, path=path)
 
+    async def get_space_summary(
+        self,
+        destination: str,
+        room_id: str,
+        suggested_only: bool,
+        max_rooms_per_space: Optional[int],
+        exclude_rooms: List[str],
+    ) -> JsonDict:
+        """
+        Args:
+            destination: The remote server
+            room_id: The room ID to ask about.
+            suggested_only: if True, only suggested rooms will be returned
+            max_rooms_per_space: an optional limit to the number of children to be
+               returned per space
+            exclude_rooms: a list of any rooms we can skip
+        """
+        path = _create_path(
+            FEDERATION_UNSTABLE_PREFIX, "/org.matrix.msc2946/spaces/%s", room_id
+        )
+
+        params = {
+            "suggested_only": suggested_only,
+            "exclude_rooms": exclude_rooms,
+        }
+        if max_rooms_per_space is not None:
+            params["max_rooms_per_space"] = max_rooms_per_space
+
+        return await self.client.post_json(
+            destination=destination, path=path, data=params
+        )
+
 
 def _create_path(federation_prefix, path, *args):
     """