bot: setup autojoin

Currently broken on v0.4.0, unfortunately. Investigating.

src/bot/handlers/autojoin.rs

@@ -0,0 +1,68 @@
+use std::time::Duration;
+
+use matrix_sdk::{
+    room::Room,
+    ruma::{
+        events::{room::member::MemberEventContent, StrippedStateEvent},
+        RoomId,
+    },
+    Client,
+};
+use tokio::time::sleep;
+use tracing::{debug, error, info, warn};
+
+pub async fn autojoin_authorized_rooms(
+    room_member: StrippedStateEvent<MemberEventContent>,
+    client: Client,
+    room: Room,
+    authorized_rooms: Vec<RoomId>,
+) {
+    if room_member.state_key != client.user_id().await.unwrap() {
+        return;
+    }
+
+    if let Room::Invited(room) = room {
+        let room_id = room.room_id();
+        let room_name = room
+            .display_name()
+            .await
+            .expect("couldn't get joined room name!");
+        info!(
+            "Received invitation for room `{}`: `{}`",
+            room_id, room_name
+        );
+
+        if authorized_rooms.contains(room_id) {
+            warn!(
+                "Bot isn't authorized to join room `{}`, declining invitation",
+                room_id
+            );
+            room.reject_invitation().await.unwrap();
+            return;
+        }
+
+        debug!("Autojoining room {}", room.room_id());
+        let mut delay = 2;
+
+        while let Err(err) = room.accept_invitation().await {
+            // retry autojoin due to synapse sending invites, before the
+            // invited user can join for more information see
+            // https://github.com/matrix-org/synapse/issues/4345
+            warn!(
+                "Failed to join room {} ({:?}), retrying in {}s",
+                room.room_id(),
+                err,
+                delay
+            );
+
+            sleep(Duration::from_secs(delay)).await;
+            delay *= 2;
+
+            if delay > 3600 {
+                error!("Can't join room {} ({:?})", room.room_id(), err);
+                break;
+            }
+        }
+        info!("Successfully joined room {}", room.room_id());
+    }
+}