factor canonical alias event validation into a helper function

Not all the semantics from the spec quote in the doc comment are
implemented yet.

src/api/client_server/state.rs

@@ -210,40 +210,8 @@ async fn send_state_event_for_key_helper(
 ) -> Result<Arc<EventId>> {
     let sender_user = sender;
 
-    // TODO: allow alias if it previously existed
     if event_type == &StateEventType::RoomCanonicalAlias {
-        if let Ok(canonical_alias) = serde_json::from_str::<
+        validate_canonical_alias_event(room_id, json.cast_ref())?;
-            RoomCanonicalAliasEventContent,
-        >(json.json().get())
-        {
-            let mut aliases = canonical_alias.alt_aliases.clone();
-
-            if let Some(alias) = canonical_alias.alias {
-                aliases.push(alias);
-            }
-
-            for alias in aliases {
-                if alias.server_name() != services().globals.server_name()
-                    || services()
-                        .rooms
-                        .alias
-                        .resolve_local_alias(&alias)?
-                        .filter(|room| room == room_id)
-                        .is_none()
-                {
-                    return Err(Error::BadRequest(
-                        ErrorKind::BadAlias,
-                        "You are only allowed to send canonical_alias events \
-                         when it's aliases already exists",
-                    ));
-                }
-            }
-        } else {
-            return Err(Error::BadRequest(
-                ErrorKind::InvalidParam,
-                "m.room.canonical_alias event did not match expected schema",
-            ));
-        }
     }
 
     let room_token = services()
@@ -271,3 +239,56 @@ async fn send_state_event_for_key_helper(
 
     Ok(event_id)
 }
+
+/// Checks that a new `m.room.canonical_alias` event is valid, by the spec's
+/// requirements.
+///
+/// From the [spec]:
+///
+/// > If the event type being sent is m.room.canonical_alias servers SHOULD
+/// > ensure that any new aliases being listed in the event are valid per their
+/// > grammar/syntax and that they point to the room ID where the state event
+/// > is to be sent. Servers do not validate aliases which are being removed or
+/// > are already present in the state event.
+///
+/// [spec]: https://spec.matrix.org/v1.13/client-server-api/#put_matrixclientv3roomsroomidstateeventtypestatekey
+fn validate_canonical_alias_event(
+    room_id: &RoomId,
+    json: &Raw<RoomCanonicalAliasEventContent>,
+) -> Result<()> {
+    // TODO: allow alias if it previously existed
+    if let Ok(canonical_alias) = serde_json::from_str::<
+        RoomCanonicalAliasEventContent,
+    >(json.json().get())
+    {
+        let mut aliases = canonical_alias.alt_aliases.clone();
+
+        if let Some(alias) = canonical_alias.alias {
+            aliases.push(alias);
+        }
+
+        for alias in aliases {
+            if alias.server_name() != services().globals.server_name()
+                || services()
+                    .rooms
+                    .alias
+                    .resolve_local_alias(&alias)?
+                    .filter(|room| room == room_id)
+                    .is_none()
+            {
+                return Err(Error::BadRequest(
+                    ErrorKind::BadAlias,
+                    "You are only allowed to send canonical_alias events when \
+                     it's aliases already exists",
+                ));
+            }
+        }
+    } else {
+        return Err(Error::BadRequest(
+            ErrorKind::InvalidParam,
+            "m.room.canonical_alias event did not match expected schema",
+        ));
+    }
+
+    Ok(())
+}