Discord activity: add per-instance room limit

We send all rooms for that instance to the client every few,
seconds, so we shouldn't let the number grow without bound.

3 files changed, 58 insertions, 29 deletions

diff --git a/lib/mediasync/http_errors.ex b/lib/mediasync/http_errors.ex
index 5df85fd..30e3b58 100644
--- a/lib/mediasync/http_errors.ex
+++ b/lib/mediasync/http_errors.ex
@@ -99,7 +99,7 @@ defmodule Mediasync.HTTPErrors do
 
     conn
     |> put_json_content_type()
-    |> send_resp(400, Jason.encode!(error))
+    |> send_resp(400, Jason.encode!(error, pretty: true))
   end
 
   @bad_gateway Jason.encode!(%{"error" => "badGateway"}, pretty: true)
diff --git a/lib/mediasync/room.ex b/lib/mediasync/room.ex
index 6386893..f518220 100644
--- a/lib/mediasync/room.ex
+++ b/lib/mediasync/room.ex
@@ -51,12 +51,12 @@ defmodule Mediasync.Room do
         _ -> state
       end
 
-    Tuple.append(
-      GenServer.start_link(__MODULE__, state,
-        name: {:via, Registry, {Mediasync.RoomRegistry, state.room_id}}
-      ),
-      state.room_id
-    )
+    case GenServer.start_link(__MODULE__, state,
+           name: {:via, Registry, {Mediasync.RoomRegistry, state.room_id}}
+         ) do
+      {:ok, pid} -> {:ok, pid, state.room_id}
+      {:error, reason} -> {:error, reason}
+    end
   end
 
   @spec get_video_info(GenServer.server()) :: Mediasync.Room.VideoInfo.t()
@@ -93,22 +93,45 @@ defmodule Mediasync.Room do
   end
 
   @inactive_check_wait_milliseconds 10 * 1000
+  @discord_activity_instance_rooms_max 20
 
   @impl true
   @spec init(Mediasync.Room.State.t()) :: {:ok, Mediasync.Room.State.t()}
   def init(state = %Mediasync.Room.State{}) do
-    if state.discord_instance_id do
-      Registry.register(Mediasync.DiscordActivityInstanceRegistry, state.discord_instance_id, %{
-        host_username: state.host_username,
-        room_id: state.room_id
-      })
-    end
+    discord_activity_ok? =
+      if state.discord_instance_id do
+        instance_room_count =
+          Registry.count_match(
+            Mediasync.DiscordActivityInstanceRegistry,
+            state.discord_instance_id,
+            :_
+          )
+
+        if instance_room_count < @discord_activity_instance_rooms_max do
+          Registry.register(
+            Mediasync.DiscordActivityInstanceRegistry,
+            state.discord_instance_id,
+            %{
+              host_username: state.host_username,
+              room_id: state.room_id
+            }
+          )
+
+          true
+        else
+          false
+        end
+      end
 
-    Process.send_after(self(), :check_if_active, @inactive_check_wait_milliseconds)
+    if discord_activity_ok? != false do
+      Process.send_after(self(), :check_if_active, @inactive_check_wait_milliseconds)
 
-    Logger.info("Created room #{state.room_id}")
+      Logger.info("Created room #{state.room_id}")
 
-    {:ok, state}
+      {:ok, state}
+    else
+      {:stop, :discord_activity_instance_max_rooms_reached}
+    end
   end
 
   @impl true
diff --git a/lib/mediasync/router.ex b/lib/mediasync/router.ex
index c60ca8a..2a80882 100644
--- a/lib/mediasync/router.ex
+++ b/lib/mediasync/router.ex
@@ -85,19 +85,25 @@ defmodule Mediasync.Router do
             {"", nil, nil}
           end
 
-        {:ok, _pid, room_id} =
-          DynamicSupervisor.start_child(
-            Mediasync.RoomSupervisor,
-            {Mediasync.Room,
-             %Mediasync.Room.State{
-               video_info: video_info,
-               host_user_token_hash: get_user_token_hash!(conn),
-               host_username: host_username,
-               discord_instance_id: instance_id
-             }}
-          )
-
-        redirect(conn, status: 303, location: "/room/#{room_id}#{suffix}")
+        case DynamicSupervisor.start_child(
+               Mediasync.RoomSupervisor,
+               {Mediasync.Room,
+                %Mediasync.Room.State{
+                  video_info: video_info,
+                  host_user_token_hash: get_user_token_hash!(conn),
+                  host_username: host_username,
+                  discord_instance_id: instance_id
+                }}
+             ) do
+          {:ok, _pid, room_id} ->
+            redirect(conn, status: 303, location: "/room/#{room_id}#{suffix}")
+
+          {:error, :discord_activity_instance_max_rooms_reached} ->
+            Mediasync.HTTPErrors.send_bad_request(
+              conn,
+              message: "Cannot host more rooms in this activity instance."
+            )
+        end
     end
   end