Federation API for Space summary (#9652)

Builds on the work done in #9643 to add a federation API for space summaries.

There's a bit of refactoring of the existing client-server code first, to avoid too much duplication.

3 files changed, 197 insertions, 54 deletions

diff --git a/changelog.d/9652.feature b/changelog.d/9652.feature
new file mode 100644
index 0000000000..2f7ccedcfb
--- /dev/null
+++ b/changelog.d/9652.feature
@@ -0,0 +1 @@
+Add initial experimental support for a "space summary" API.
diff --git a/synapse/federation/transport/server.py b/synapse/federation/transport/server.py
index 2cf935f38d..84e39c5a46 100644
--- a/synapse/federation/transport/server.py
+++ b/synapse/federation/transport/server.py
@@ -18,7 +18,7 @@
 import functools
 import logging
 import re
-from typing import Optional, Tuple, Type
+from typing import Container, Mapping, Optional, Sequence, Tuple, Type
 
 import synapse
 from synapse.api.constants import MAX_GROUP_CATEGORYID_LENGTH, MAX_GROUP_ROLEID_LENGTH
@@ -29,7 +29,7 @@ from synapse.api.urls import (
     FEDERATION_V1_PREFIX,
     FEDERATION_V2_PREFIX,
 )
-from synapse.http.server import JsonResource
+from synapse.http.server import HttpServer, JsonResource
 from synapse.http.servlet import (
     parse_boolean_from_args,
     parse_integer_from_args,
@@ -44,7 +44,8 @@ from synapse.logging.opentracing import (
     whitelisted_homeserver,
 )
 from synapse.server import HomeServer
-from synapse.types import ThirdPartyInstanceID, get_domain_from_id
+from synapse.types import JsonDict, ThirdPartyInstanceID, get_domain_from_id
+from synapse.util.ratelimitutils import FederationRateLimiter
 from synapse.util.stringutils import parse_and_validate_server_name
 from synapse.util.versionstring import get_version_string
 
@@ -1376,6 +1377,40 @@ class FederationGroupsSettingJoinPolicyServlet(BaseFederationServlet):
         return 200, new_content
 
 
+class FederationSpaceSummaryServlet(BaseFederationServlet):
+    PREFIX = FEDERATION_UNSTABLE_PREFIX + "/org.matrix.msc2946"
+    PATH = "/spaces/(?P<room_id>[^/]*)"
+
+    async def on_POST(
+        self,
+        origin: str,
+        content: JsonDict,
+        query: Mapping[bytes, Sequence[bytes]],
+        room_id: str,
+    ) -> Tuple[int, JsonDict]:
+        suggested_only = content.get("suggested_only", False)
+        if not isinstance(suggested_only, bool):
+            raise SynapseError(
+                400, "'suggested_only' must be a boolean", Codes.BAD_JSON
+            )
+
+        exclude_rooms = content.get("exclude_rooms", [])
+        if not isinstance(exclude_rooms, list) or any(
+            not isinstance(x, str) for x in exclude_rooms
+        ):
+            raise SynapseError(400, "bad value for 'exclude_rooms'", Codes.BAD_JSON)
+
+        max_rooms_per_space = content.get("max_rooms_per_space")
+        if max_rooms_per_space is not None and not isinstance(max_rooms_per_space, int):
+            raise SynapseError(
+                400, "bad value for 'max_rooms_per_space'", Codes.BAD_JSON
+            )
+
+        return 200, await self.handler.federation_space_summary(
+            room_id, suggested_only, max_rooms_per_space, exclude_rooms
+        )
+
+
 class RoomComplexityServlet(BaseFederationServlet):
     """
     Indicates to other servers how complex (and therefore likely
@@ -1474,18 +1509,24 @@ DEFAULT_SERVLET_GROUPS = (
 )
 
 
-def register_servlets(hs, resource, authenticator, ratelimiter, servlet_groups=None):
+def register_servlets(
+    hs: HomeServer,
+    resource: HttpServer,
+    authenticator: Authenticator,
+    ratelimiter: FederationRateLimiter,
+    servlet_groups: Optional[Container[str]] = None,
+):
     """Initialize and register servlet classes.
 
     Will by default register all servlets. For custom behaviour, pass in
     a list of servlet_groups to register.
 
     Args:
-        hs (synapse.server.HomeServer): homeserver
-        resource (JsonResource): resource class to register to
-        authenticator (Authenticator): authenticator to use
-        ratelimiter (util.ratelimitutils.FederationRateLimiter): ratelimiter to use
-        servlet_groups (list[str], optional): List of servlet groups to register.
+        hs: homeserver
+        resource: resource class to register to
+        authenticator: authenticator to use
+        ratelimiter: ratelimiter to use
+        servlet_groups: List of servlet groups to register.
             Defaults to ``DEFAULT_SERVLET_GROUPS``.
     """
     if not servlet_groups:
@@ -1500,6 +1541,14 @@ def register_servlets(hs, resource, authenticator, ratelimiter, servlet_groups=N
                 server_name=hs.hostname,
             ).register(resource)
 
+        if hs.config.experimental.spaces_enabled:
+            FederationSpaceSummaryServlet(
+                handler=hs.get_space_summary_handler(),
+                authenticator=authenticator,
+                ratelimiter=ratelimiter,
+                server_name=hs.hostname,
+            ).register(resource)
+
     if "openid" in servlet_groups:
         for servletclass in OPENID_SERVLET_CLASSES:
             servletclass(
diff --git a/synapse/handlers/space_summary.py b/synapse/handlers/space_summary.py
index 513dc0c71a..f5ead9447f 100644
--- a/synapse/handlers/space_summary.py
+++ b/synapse/handlers/space_summary.py
@@ -16,7 +16,9 @@
 import itertools
 import logging
 from collections import deque
-from typing import TYPE_CHECKING, Iterable, List, Optional, Set
+from typing import TYPE_CHECKING, Iterable, List, Optional, Sequence, Set, Tuple
+
+import attr
 
 from synapse.api.constants import EventContentFields, EventTypes, HistoryVisibility
 from synapse.api.errors import AuthError
@@ -54,7 +56,7 @@ class SpaceSummaryHandler:
         max_rooms_per_space: Optional[int] = None,
     ) -> JsonDict:
         """
-        Implementation of the space summary API
+        Implementation of the space summary C-S API
 
         Args:
             requester:  user id of the user making this request
@@ -66,7 +68,7 @@ class SpaceSummaryHandler:
 
             max_rooms_per_space: an optional limit on the number of child rooms we will
                 return. This does not apply to the root room (ie, room_id), and
-                is overridden by ROOMS_PER_SPACE_LIMIT.
+                is overridden by MAX_ROOMS_PER_SPACE.
 
         Returns:
             summary dict to return
@@ -76,67 +78,153 @@ class SpaceSummaryHandler:
         await self._auth.check_user_in_room_or_world_readable(room_id, requester)
 
         # the queue of rooms to process
-        room_queue = deque((room_id,))
+        room_queue = deque((_RoomQueueEntry(room_id),))
 
         processed_rooms = set()  # type: Set[str]
 
         rooms_result = []  # type: List[JsonDict]
         events_result = []  # type: List[JsonDict]
 
-        now = self._clock.time_msec()
+        while room_queue and len(rooms_result) < MAX_ROOMS:
+            queue_entry = room_queue.popleft()
+            room_id = queue_entry.room_id
+            logger.debug("Processing room %s", room_id)
+            processed_rooms.add(room_id)
+
+            # The client-specified max_rooms_per_space limit doesn't apply to the
+            # room_id specified in the request, so we ignore it if this is the
+            # first room we are processing.
+            max_children = max_rooms_per_space if processed_rooms else None
+
+            rooms, events = await self._summarize_local_room(
+                requester, room_id, suggested_only, max_children
+            )
+
+            rooms_result.extend(rooms)
+            events_result.extend(events)
+
+            # add any children that we haven't already processed to the queue
+            for edge_event in events:
+                if edge_event["state_key"] not in processed_rooms:
+                    room_queue.append(_RoomQueueEntry(edge_event["state_key"]))
+
+        return {"rooms": rooms_result, "events": events_result}
+
+    async def federation_space_summary(
+        self,
+        room_id: str,
+        suggested_only: bool,
+        max_rooms_per_space: Optional[int],
+        exclude_rooms: Iterable[str],
+    ) -> JsonDict:
+        """
+        Implementation of the space summary Federation API
+
+        Args:
+            room_id: room id to start the summary at
+
+            suggested_only: whether we should only return children with the "suggested"
+                flag set.
+
+            max_rooms_per_space: an optional limit on the number of child rooms we will
+                return. Unlike the C-S API, this applies to the root room (room_id).
+                It is clipped to MAX_ROOMS_PER_SPACE.
+
+            exclude_rooms: a list of rooms to skip over (presumably because the
+                calling server has already seen them).
+
+        Returns:
+            summary dict to return
+        """
+        # the queue of rooms to process
+        room_queue = deque((room_id,))
+
+        # the set of rooms that we should not walk further. Initialise it with the
+        # excluded-rooms list; we will add other rooms as we process them so that
+        # we do not loop.
+        processed_rooms = set(exclude_rooms)  # type: Set[str]
+
+        rooms_result = []  # type: List[JsonDict]
+        events_result = []  # type: List[JsonDict]
 
         while room_queue and len(rooms_result) < MAX_ROOMS:
             room_id = room_queue.popleft()
             logger.debug("Processing room %s", room_id)
             processed_rooms.add(room_id)
 
-            try:
-                await self._auth.check_user_in_room_or_world_readable(
-                    room_id, requester
-                )
-            except AuthError:
-                logger.info(
-                    "user %s cannot view room %s, omitting from summary",
-                    requester,
-                    room_id,
-                )
-                continue
+            rooms, events = await self._summarize_local_room(
+                None, room_id, suggested_only, max_rooms_per_space
+            )
 
-            room_entry = await self._build_room_entry(room_id)
-            rooms_result.append(room_entry)
+            rooms_result.extend(rooms)
+            events_result.extend(events)
 
-            # look for child rooms/spaces.
-            child_events = await self._get_child_events(room_id)
+            # add any children that we haven't already processed to the queue
+            for edge_event in events:
+                if edge_event["state_key"] not in processed_rooms:
+                    room_queue.append(edge_event["state_key"])
 
-            if suggested_only:
-                # we only care about suggested children
-                child_events = filter(_is_suggested_child_event, child_events)
+        return {"rooms": rooms_result, "events": events_result}
 
-            # The client-specified max_rooms_per_space limit doesn't apply to the
-            # room_id specified in the request, so we ignore it if this is the
-            # first room we are processing. Otherwise, apply any client-specified
-            # limit, capping to our built-in limit.
-            if max_rooms_per_space is not None and len(processed_rooms) > 1:
-                max_rooms = min(MAX_ROOMS_PER_SPACE, max_rooms_per_space)
-            else:
-                max_rooms = MAX_ROOMS_PER_SPACE
-
-            for edge_event in itertools.islice(child_events, max_rooms):
-                edge_room_id = edge_event.state_key
-
-                events_result.append(
-                    await self._event_serializer.serialize_event(
-                        edge_event,
-                        time_now=now,
-                        event_format=format_event_for_client_v2,
-                    )
+    async def _summarize_local_room(
+        self,
+        requester: Optional[str],
+        room_id: str,
+        suggested_only: bool,
+        max_children: Optional[int],
+    ) -> Tuple[Sequence[JsonDict], Sequence[JsonDict]]:
+        if not await self._is_room_accessible(room_id, requester):
+            return (), ()
+
+        room_entry = await self._build_room_entry(room_id)
+
+        # look for child rooms/spaces.
+        child_events = await self._get_child_events(room_id)
+
+        if suggested_only:
+            # we only care about suggested children
+            child_events = filter(_is_suggested_child_event, child_events)
+
+        if max_children is None or max_children > MAX_ROOMS_PER_SPACE:
+            max_children = MAX_ROOMS_PER_SPACE
+
+        now = self._clock.time_msec()
+        events_result = []  # type: List[JsonDict]
+        for edge_event in itertools.islice(child_events, max_children):
+            events_result.append(
+                await self._event_serializer.serialize_event(
+                    edge_event,
+                    time_now=now,
+                    event_format=format_event_for_client_v2,
                 )
+            )
+        return (room_entry,), events_result
 
-                # if we haven't yet visited the target of this link, add it to the queue
-                if edge_room_id not in processed_rooms:
-                    room_queue.append(edge_room_id)
+    async def _is_room_accessible(self, room_id: str, requester: Optional[str]) -> bool:
+        # if we have an authenticated requesting user, first check if they are in the
+        # room
+        if requester:
+            try:
+                await self._auth.check_user_in_room(room_id, requester)
+                return True
+            except AuthError:
+                pass
 
-        return {"rooms": rooms_result, "events": events_result}
+        # otherwise, check if the room is peekable
+        hist_vis_ev = await self._state_handler.get_current_state(
+            room_id, EventTypes.RoomHistoryVisibility, ""
+        )
+        if hist_vis_ev:
+            hist_vis = hist_vis_ev.content.get("history_visibility")
+            if hist_vis == HistoryVisibility.WORLD_READABLE:
+                return True
+
+        logger.info(
+            "room %s is unpeekable and user %s is not a member, omitting from summary",
+            room_id,
+            requester,
+        )
+        return False
 
     async def _build_room_entry(self, room_id: str) -> JsonDict:
         """Generate en entry suitable for the 'rooms' list in the summary response"""
@@ -191,6 +279,11 @@ class SpaceSummaryHandler:
         return (e for e in events if e.content.get("via"))
 
 
+@attr.s(frozen=True, slots=True)
+class _RoomQueueEntry:
+    room_id = attr.ib(type=str)
+
+
 def _is_suggested_child_event(edge_event: EventBase) -> bool:
     suggested = edge_event.content.get("suggested")
     if isinstance(suggested, bool) and suggested: