implement rooms and not_rooms filters on /message

I really doubt anybody is sending /message requests with a filter that
rejects the entire request, but it's the first step in the filter
implementation.

src/api/client_server/message.rs

@@ -14,7 +14,9 @@ use ruma::{
 
 use crate::{
     service::{pdu::PduBuilder, rooms::timeline::PduCount},
-    services, utils, Ar, Error, Ra, Result,
+    services, utils,
+    utils::filter::CompiledRoomEventFilter,
+    Ar, Error, Ra, Result,
 };
 
 /// # `PUT /_matrix/client/r0/rooms/{roomId}/send/{eventType}/{txnId}`
@@ -136,6 +138,13 @@ pub(crate) async fn get_message_events_route(
     let sender_device =
         body.sender_device.as_ref().expect("user is authenticated");
 
+    let Ok(filter) = CompiledRoomEventFilter::try_from(&body.filter) else {
+        return Err(Error::BadRequest(
+            ErrorKind::InvalidParam,
+            "invalid 'filter' parameter",
+        ));
+    };
+
     let from = match body.from.clone() {
         Some(from) => PduCount::try_from_string(&from)?,
         None => match body.dir {
@@ -144,6 +153,15 @@ pub(crate) async fn get_message_events_route(
         },
     };
 
+    if !filter.room_allowed(&body.room_id) {
+        return Ok(Ra(get_message_events::v3::Response {
+            start: from.stringify(),
+            end: None,
+            chunk: vec![],
+            state: vec![],
+        }));
+    }
+
     let to = body.to.as_ref().and_then(|t| PduCount::try_from_string(t).ok());
 
     services()

src/utils.rs

@@ -1,4 +1,5 @@
 pub(crate) mod error;
+pub(crate) mod filter;
 
 use std::{
     borrow::Cow,

src/utils/filter.rs

@@ -0,0 +1,96 @@
+//! Helper tools for implementing filtering in the `/client/v3/sync` and
+//! `/client/v3/rooms/:roomId/messages` endpoints.
+//!
+//! The default strategy for filtering is to generate all events, check them
+//! against the filter, and drop events that were rejected. When significant
+//! fraction of events are rejected, this results in a large amount of wasted
+//! work computing events that will be dropped. In most cases, the structure of
+//! our database doesn't allow for anything fancier, with only a few exceptions.
+//!
+//! The first exception is room filters (`room`/`not_room` pairs in
+//! `filter.rooms` and `filter.rooms.{account_data,timeline,ephemeral,state}`).
+//! In `/messages`, if the room is rejected by the filter, we can skip the
+//! entire request.
+
+use std::{collections::HashSet, hash::Hash};
+
+use ruma::{api::client::filter::RoomEventFilter, RoomId};
+
+use crate::Error;
+
+/// Structure for testing against an allowlist and a denylist with a single
+/// `HashSet` lookup.
+///
+/// The denylist takes precedence (an item included in both the allowlist and
+/// the denylist is denied).
+pub(crate) enum AllowDenyList<'a, T: ?Sized> {
+    /// TODO: fast-paths for allow-all and deny-all?
+    Allow(HashSet<&'a T>),
+    Deny(HashSet<&'a T>),
+}
+
+impl<'a, T: ?Sized + Hash + PartialEq + Eq> AllowDenyList<'a, T> {
+    fn new<A, D>(allow: Option<A>, deny: D) -> AllowDenyList<'a, T>
+    where
+        A: Iterator<Item = &'a T>,
+        D: Iterator<Item = &'a T>,
+    {
+        let deny_set = deny.collect::<HashSet<_>>();
+        if let Some(allow) = allow {
+            AllowDenyList::Allow(
+                allow.filter(|x| !deny_set.contains(x)).collect(),
+            )
+        } else {
+            AllowDenyList::Deny(deny_set)
+        }
+    }
+
+    fn from_slices<O: AsRef<T>>(
+        allow: Option<&'a [O]>,
+        deny: &'a [O],
+    ) -> AllowDenyList<'a, T> {
+        AllowDenyList::new(
+            allow.map(|allow| allow.iter().map(AsRef::as_ref)),
+            deny.iter().map(AsRef::as_ref),
+        )
+    }
+
+    pub(crate) fn allowed(&self, value: &T) -> bool {
+        match self {
+            AllowDenyList::Allow(allow) => allow.contains(value),
+            AllowDenyList::Deny(deny) => !deny.contains(value),
+        }
+    }
+}
+
+pub(crate) struct CompiledRoomEventFilter<'a> {
+    rooms: AllowDenyList<'a, RoomId>,
+}
+
+impl<'a> TryFrom<&'a RoomEventFilter> for CompiledRoomEventFilter<'a> {
+    type Error = Error;
+
+    fn try_from(
+        source: &'a RoomEventFilter,
+    ) -> Result<CompiledRoomEventFilter<'a>, Error> {
+        Ok(CompiledRoomEventFilter {
+            rooms: AllowDenyList::from_slices(
+                source.rooms.as_deref(),
+                &source.not_rooms,
+            ),
+        })
+    }
+}
+
+impl CompiledRoomEventFilter<'_> {
+    /// Returns `true` if a room is allowed by the `rooms` and `not_rooms`
+    /// fields.
+    ///
+    /// This does *not* test the room against the top-level `rooms` filter.
+    /// It is expected that callers have already filtered rooms that are
+    /// rejected by the top-level filter using [`CompiledRoomFilter::rooms`], if
+    /// applicable.
+    pub(crate) fn room_allowed(&self, room_id: &RoomId) -> bool {
+        self.rooms.allowed(room_id)
+    }
+}