rate limiter: Fix a race condition

When multiple requests are processed by rate limiter plug at the same
time and the bucket is not yet initialized, both would try to initialize
the bucket resulting in an internal server error.

lib/pleroma/plugs/rate_limiter/limiter_supervisor.ex

@@ -7,8 +7,8 @@ def start_link(init_arg) do
     DynamicSupervisor.start_link(__MODULE__, init_arg, name: __MODULE__)
   end
 
-  def add_limiter(limiter_name, expiration) do
-    {:ok, _pid} =
+  def add_or_return_limiter(limiter_name, expiration) do
+    result =
       DynamicSupervisor.start_child(
         __MODULE__,
         %{
@@ -28,6 +28,12 @@ def add_limiter(limiter_name, expiration) do
              ]}
         }
       )
+
+    case result do
+      {:ok, _pid} = result -> result
+      {:error, {:already_started, pid}} -> {:ok, pid}
+      _ -> result
+    end
   end
 
   @impl true

lib/pleroma/plugs/rate_limiter/rate_limiter.ex

@@ -171,7 +171,7 @@ defp check_rate(action_settings) do
         {:error, value}
 
       {:error, :no_cache} ->
-        initialize_buckets(action_settings)
+        initialize_buckets!(action_settings)
         check_rate(action_settings)
     end
   end
@@ -250,11 +250,16 @@ defp attach_selected_params(input, %{conn_params: conn_params, opts: plug_opts})
     |> String.replace_leading(":", "")
   end
 
-  defp initialize_buckets(%{name: _name, limits: nil}), do: :ok
+  defp initialize_buckets!(%{name: _name, limits: nil}), do: :ok
 
-  defp initialize_buckets(%{name: name, limits: limits}) do
-    LimiterSupervisor.add_limiter(anon_bucket_name(name), get_scale(:anon, limits))
-    LimiterSupervisor.add_limiter(user_bucket_name(name), get_scale(:user, limits))
+  defp initialize_buckets!(%{name: name, limits: limits}) do
+    {:ok, _pid} =
+      LimiterSupervisor.add_or_return_limiter(anon_bucket_name(name), get_scale(:anon, limits))
+
+    {:ok, _pid} =
+      LimiterSupervisor.add_or_return_limiter(user_bucket_name(name), get_scale(:user, limits))
+
+    :ok
   end
 
   defp attach_identity(base, %{mode: :user, conn_info: conn_info}),

test/plugs/rate_limiter_test.exs

@@ -242,4 +242,35 @@ test "different users are counted independently" do
       refute conn_2.halted
     end
   end
+
+  test "doesn't crash due to a race condition when multiple requests are made at the same time and the bucket is not yet initialized" do
+    limiter_name = :test_race_condition
+    Pleroma.Config.put([:rate_limit, limiter_name], {1000, 5})
+    Pleroma.Config.put([Pleroma.Web.Endpoint, :http, :ip], {8, 8, 8, 8})
+
+    opts = RateLimiter.init(name: limiter_name)
+
+    conn = conn(:get, "/")
+    conn_2 = conn(:get, "/")
+
+    %Task{pid: pid1} =
+      task1 =
+      Task.async(fn ->
+        receive do
+          :process2_up ->
+            RateLimiter.call(conn, opts)
+        end
+      end)
+
+    task2 =
+      Task.async(fn ->
+        send(pid1, :process2_up)
+        RateLimiter.call(conn_2, opts)
+      end)
+
+    Task.await(task1)
+    Task.await(task2)
+
+    refute {:err, :not_found} == RateLimiter.inspect_bucket(conn, limiter_name, opts)
+  end
 end