feat(http): back off from hosts that answer 429 or 503

Object fetches and WebFinger stop sending requests to a host until
Retry-After or X-RateLimit-Reset (default 5 minutes, max 1 hour).
Ported from Akkoma's Pleroma.HTTP.Backoff.

Co-authored-by: FloatingGhost <hannah@coffee-and-dreams.uk>
Co-authored-by: Oneric <oneric@oneric.stub>

lib/mix/pleroma.ex

@@ -17,7 +17,7 @@ defmodule Mix.Pleroma do
     :oban,
     :logger_backends
   ]
-  @cachex_children ["object", "user", "scrubber", "web_resp"]
+  @cachex_children ["object", "user", "scrubber", "web_resp", "http_backoff"]
   @doc "Common functions to be reused in mix tasks"
   def start_pleroma do
     Pleroma.Config.Holder.save_default()

lib/pleroma/application.ex

@@ -198,6 +198,7 @@ defmodule Pleroma.Application do
       ),
       build_cachex("rel_me", limit: 2500),
       build_cachex("host_meta", default_ttl: :timer.minutes(120), limit: 5_000),
+      build_cachex("http_backoff", limit: 10_000),
       build_cachex("translations", default_ttl: :timer.hours(24), limit: 5_000)
     ]
   end

lib/pleroma/http/backoff.ex

@@ -0,0 +1,101 @@
+# Pleroma: A lightweight social networking server
+# Copyright © 2017-2022 Pleroma Authors <https://pleroma.social/>
+# SPDX-License-Identifier: AGPL-3.0-only
+
+defmodule Pleroma.HTTP.Backoff do
+  @moduledoc """
+  GET wrapper that stops sending requests to a host after it answers
+  429 or 503, until the time given by `Retry-After` or `X-RateLimit-Reset`.
+
+  Based on Akkoma's `Pleroma.HTTP.Backoff`.
+  """
+
+  alias Pleroma.HTTP
+
+  require Logger
+
+  @cachex Pleroma.Config.get([:cachex, :provider], Cachex)
+  @cache :http_backoff_cache
+
+  @default_seconds 5 * 60
+  @max_seconds 60 * 60
+
+  @spec get(String.t(), list(), keyword()) :: {:ok, Tesla.Env.t()} | {:error, any()}
+  def get(url, headers \\ [], options \\ []) when is_binary(url) do
+    host = URI.parse(url).host
+
+    case @cachex.get(@cache, host) do
+      {:ok, true} ->
+        {:error, :ratelimit}
+
+      _ ->
+        url
+        |> HTTP.get(headers, options)
+        |> check_backoff(host)
+    end
+  end
+
+  defp check_backoff({:ok, %{status: status, headers: headers}}, host)
+       when status in [429, 503] do
+    seconds = backoff_seconds(headers, DateTime.utc_now())
+    Logger.warning("#{host} answered #{status}, backing off for #{seconds} seconds")
+    @cachex.put(@cache, host, true, ttl: :timer.seconds(seconds))
+    {:error, :ratelimit}
+  end
+
+  defp check_backoff(response, _host), do: response
+
+  @doc """
+  Seconds to back off for, from the first valid `Retry-After` or
+  `X-RateLimit-Reset` header. Missing, invalid or past values give the
+  default of #{@default_seconds} seconds; values are capped at #{@max_seconds}.
+  """
+  @spec backoff_seconds(list(), DateTime.t()) :: pos_integer()
+  def backoff_seconds(headers, now) do
+    [
+      parse_retry_after(header(headers, "retry-after"), now),
+      parse_timestamp(header(headers, "x-ratelimit-reset"), now)
+    ]
+    |> Enum.find(@default_seconds, &is_integer/1)
+    |> min(@max_seconds)
+  end
+
+  defp header(headers, name) do
+    Enum.find_value(headers, fn
+      {key, value} when is_binary(key) and is_binary(value) ->
+        if String.downcase(key) == name, do: String.trim(value)
+
+      _ ->
+        nil
+    end)
+  end
+
+  # Retry-After is either delta-seconds or an HTTP-date (RFC 9110, 10.2.3)
+  defp parse_retry_after(nil, _now), do: nil
+
+  defp parse_retry_after(value, now) do
+    case Integer.parse(value) do
+      {seconds, ""} when seconds > 0 -> seconds
+      {_, ""} -> nil
+      _ -> parse_timestamp(value, now)
+    end
+  end
+
+  defp parse_timestamp(nil, _now), do: nil
+
+  defp parse_timestamp(value, now) do
+    with {:ok, %DateTime{} = datetime} <- parse_datetime(value),
+         seconds when seconds > 0 <- DateTime.diff(datetime, now) do
+      seconds
+    else
+      _ -> nil
+    end
+  end
+
+  defp parse_datetime(value) do
+    with {:error, _} <- Timex.parse(value, "{RFC1123}"),
+         {:ok, datetime, _offset} <- DateTime.from_iso8601(value) do
+      {:ok, datetime}
+    end
+  end
+end

lib/pleroma/object/fetcher.ex

@@ -216,7 +216,7 @@ defmodule Pleroma.Object.Fetcher do
       |> maybe_date_fetch(date)
       |> sign_fetch(id, date)
 
-    case HTTP.get(id, headers) do
+    case HTTP.Backoff.get(id, headers) do
       {:ok, %{body: body, status: code, headers: headers, url: final_url}}
       when code in 200..299 ->
         remote_host = if final_url, do: URI.parse(final_url).host, else: nil

lib/pleroma/web/web_finger.ex

@@ -158,7 +158,11 @@ defmodule Pleroma.Web.WebFinger do
   @cachex Pleroma.Config.get([:cachex, :provider], Cachex)
   def find_lrdd_template(domain) do
     @cachex.fetch!(:host_meta_cache, domain, fn _ ->
-      {:commit, fetch_lrdd_template(domain)}
+      case fetch_lrdd_template(domain) do
+        # retry once the backoff expires instead of caching the miss
+        {:error, :ratelimit} = error -> {:ignore, error}
+        result -> {:commit, result}
+      end
     end)
   rescue
     e -> {:error, "Cachex error: #{inspect(e)}"}
@@ -168,9 +172,13 @@ defmodule Pleroma.Web.WebFinger do
     # WebFinger is restricted to HTTPS - https://tools.ietf.org/html/rfc7033#section-9.1
     meta_url = "https://#{domain}/.well-known/host-meta"
 
-    with {:ok, %{status: status, body: body}} when status in 200..299 <- HTTP.get(meta_url) do
+    with {:ok, %{status: status, body: body}} when status in 200..299 <-
+           HTTP.Backoff.get(meta_url) do
       get_template_from_xml(body)
     else
+      {:error, :ratelimit} = error ->
+        error
+
       error ->
         Logger.warning("Can't find LRDD template in #{inspect(meta_url)}: #{inspect(error)}")
         {:error, :lrdd_not_found}
@@ -210,7 +218,7 @@ defmodule Pleroma.Web.WebFinger do
 
     with address when is_binary(address) <- get_address_from_domain(domain, account),
          {:ok, %{status: status, body: body, headers: headers}} when status in 200..299 <-
-           HTTP.get(
+           HTTP.Backoff.get(
              address,
              [{"accept", "application/xrd+xml,application/jrd+json"}]
            ) do