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 @@ defmodule Pleroma.Plugs.RateLimiter.LimiterSupervisor 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 @@ defmodule Pleroma.Plugs.RateLimiter.LimiterSupervisor 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 @@ defmodule Pleroma.Plugs.RateLimiter do
         {:error, value}
 
       {:error, :no_cache} ->
-        initialize_buckets(action_settings)
+        initialize_buckets!(action_settings)
         check_rate(action_settings)
     end
   end
@@ -250,11 +250,16 @@ defmodule Pleroma.Plugs.RateLimiter do
     |> 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 @@ defmodule Pleroma.Plugs.RateLimiterTest 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