From f965bf2b6aea6e06e32a72ec273782004ef4098f Mon Sep 17 00:00:00 2001
From: Lambda <csgl@xiretza.xyz>
Date: Fri, 30 Aug 2024 22:11:29 +0000
Subject: [PATCH 1/2] Implement MSC4185

---
 Cargo.toml                                |  5 +-
 src/api/client_server.rs                  |  2 +
 src/api/client_server/event_visibility.rs | 60 +++++++++++++++++++++++
 src/api/client_server/unversioned.rs      |  8 +--
 src/main.rs                               |  3 +-
 5 files changed, 72 insertions(+), 6 deletions(-)
 create mode 100644 src/api/client_server/event_visibility.rs

diff --git a/Cargo.toml b/Cargo.toml
index d7682651..b099c632 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -147,7 +147,10 @@ xdg = "2.5.2"
 nix = { version = "0.29", features = ["resource"] }
 
 [features]
-default = ["rocksdb", "sqlite", "systemd"]
+# needed for ruma Request/Response derives
+client = []
+server = []
+default = ["rocksdb", "sqlite", "systemd", "server"]
 
 # Keep sorted
 jemalloc = ["dep:tikv-jemallocator"]
diff --git a/src/api/client_server.rs b/src/api/client_server.rs
index 65c5985d..1a595289 100644
--- a/src/api/client_server.rs
+++ b/src/api/client_server.rs
@@ -6,6 +6,7 @@ mod config;
 mod context;
 mod device;
 mod directory;
+mod event_visibility;
 mod filter;
 mod keys;
 mod media;
@@ -40,6 +41,7 @@ pub(crate) use config::*;
 pub(crate) use context::*;
 pub(crate) use device::*;
 pub(crate) use directory::*;
+pub(crate) use event_visibility::*;
 pub(crate) use filter::*;
 pub(crate) use keys::*;
 pub(crate) use media::*;
diff --git a/src/api/client_server/event_visibility.rs b/src/api/client_server/event_visibility.rs
new file mode 100644
index 00000000..36b1d9b2
--- /dev/null
+++ b/src/api/client_server/event_visibility.rs
@@ -0,0 +1,60 @@
+//! Implementation of [MSC4185](https://github.com/matrix-org/matrix-spec-proposals/pull/4185)
+
+use ruma::{
+    api::{request, response, Metadata},
+    metadata, OwnedEventId, OwnedRoomId, OwnedUserId,
+};
+
+use crate::{services, Ar, Ra, Result};
+
+const METADATA: Metadata = metadata! {
+    method: GET,
+    rate_limited: true,
+    authentication: AccessToken,
+
+    history: {
+        unstable => "/_matrix/client/unstable/net.tadzik/can_user_see_event/:room_id/:user_id/:event_id",
+    },
+};
+
+#[request]
+#[allow(clippy::struct_field_names)]
+pub(crate) struct Request {
+    #[ruma_api(path)]
+    pub(crate) room_id: OwnedRoomId,
+
+    #[ruma_api(path)]
+    pub(crate) user_id: OwnedUserId,
+
+    #[ruma_api(path)]
+    pub(crate) event_id: OwnedEventId,
+}
+
+#[response]
+pub(crate) struct Response {
+    #[ruma_api(body)]
+    pub(crate) is_visible: bool,
+}
+
+pub(crate) async fn can_user_see_event_route(
+    body: Ar<Request>,
+) -> Result<Ra<Response>> {
+    let Ok(true) = services().rooms.state_accessor.user_can_see_event(
+        body.sender_user.as_ref().expect("user is authenticated"),
+        &body.room_id,
+        &body.event_id,
+    ) else {
+        return Err(crate::Error::BadRequest(
+            ruma::api::client::error::ErrorKind::Unauthorized,
+            "event is not visible to requesting user",
+        ));
+    };
+
+    Ok(Ra(Response {
+        is_visible: services().rooms.state_accessor.user_can_see_event(
+            &body.user_id,
+            &body.room_id,
+            &body.event_id,
+        )?,
+    }))
+}
diff --git a/src/api/client_server/unversioned.rs b/src/api/client_server/unversioned.rs
index 8ea8136c..78d30a9d 100644
--- a/src/api/client_server/unversioned.rs
+++ b/src/api/client_server/unversioned.rs
@@ -29,10 +29,10 @@ pub(crate) async fn get_supported_versions_route(
             "v1.4".to_owned(),
             "v1.5".to_owned(),
         ],
-        unstable_features: BTreeMap::from_iter([(
-            "org.matrix.e2e_cross_signing".to_owned(),
-            true,
-        )]),
+        unstable_features: BTreeMap::from_iter([
+            ("org.matrix.e2e_cross_signing".to_owned(), true),
+            ("org.matrix.msc4185".to_owned(), true),
+        ]),
     };
 
     Ok(Ra(resp))
diff --git a/src/main.rs b/src/main.rs
index 69ba7023..bb78fbd0 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -414,7 +414,8 @@ fn routes(config: &Config) -> Router {
         .ruma_route(c2s::get_relating_events_with_rel_type_and_event_type_route)
         .ruma_route(c2s::get_relating_events_with_rel_type_route)
         .ruma_route(c2s::get_relating_events_route)
-        .ruma_route(c2s::get_hierarchy_route);
+        .ruma_route(c2s::get_hierarchy_route)
+        .ruma_route(c2s::can_user_see_event_route);
 
     // Ruma doesn't have support for multiple paths for a single endpoint yet,
     // and these routes share one Ruma request / response type pair with

From 1ba96b95340559d405155f47cf41c0a8928d0935 Mon Sep 17 00:00:00 2001
From: Lambda <csgl@xiretza.xyz>
Date: Fri, 30 Aug 2024 22:18:50 +0000
Subject: [PATCH 2/2] Mark events as not visible when they're not found or in
 the wrong room

---
 src/service/rooms/state_accessor.rs | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/src/service/rooms/state_accessor.rs b/src/service/rooms/state_accessor.rs
index b9e9f702..5a15718e 100644
--- a/src/service/rooms/state_accessor.rs
+++ b/src/service/rooms/state_accessor.rs
@@ -215,6 +215,14 @@ impl Service {
     ) -> Result<bool> {
         let lookup = Lookup::VisibilityForUser;
 
+        let Some(pdu) = services().rooms.timeline.get_pdu(event_id)? else {
+            return Ok(false);
+        };
+
+        if pdu.room_id() != room_id {
+            return Ok(false);
+        }
+
         let Some(shortstatehash) = self.pdu_shortstatehash(event_id)? else {
             return Ok(true);
         };