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.

2 files changed, 18 insertions, 7 deletions

diff --git a/lib/pleroma/plugs/rate_limiter/limiter_supervisor.ex b/lib/pleroma/plugs/rate_limiter/limiter_supervisor.ex
index 187582ede..884268d96 100644
--- a/lib/pleroma/plugs/rate_limiter/limiter_supervisor.ex
+++ b/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
diff --git a/lib/pleroma/plugs/rate_limiter/rate_limiter.ex b/lib/pleroma/plugs/rate_limiter/rate_limiter.ex
index 9c362a392..b9cbe9716 100644
--- a/lib/pleroma/plugs/rate_limiter/rate_limiter.ex
+++ b/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}),