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>

test/pleroma/http/backoff_test.exs

@@ -0,0 +1,154 @@
+# Pleroma: A lightweight social networking server
+# Copyright © 2017-2022 Pleroma Authors <https://pleroma.social/>
+# SPDX-License-Identifier: AGPL-3.0-only
+
+# Based on Akkoma's test/pleroma/http/backoff_test.exs
+defmodule Pleroma.HTTP.BackoffTest do
+  use Pleroma.DataCase, async: false
+
+  alias Pleroma.HTTP.Backoff
+
+  @cache :http_backoff_cache
+  @url "https://ratelimited.example.com/users/alice"
+  @host "ratelimited.example.com"
+
+  defp mock_response(status, headers \\ []) do
+    test_pid = self()
+
+    Tesla.Mock.mock(fn %{method: :get} ->
+      send(test_pid, :request)
+      %Tesla.Env{status: status, headers: headers, body: ""}
+    end)
+  end
+
+  defp ttl_seconds do
+    {:ok, ttl} = Cachex.ttl(@cache, @host)
+    div(ttl, 1000)
+  end
+
+  defp assert_ttl_about(expected) do
+    ttl = ttl_seconds()
+    assert ttl in (expected - 15)..expected//1, "expected a TTL near #{expected}, got #{ttl}"
+  end
+
+  describe "get/3" do
+    test "returns the response when the host is not backed off" do
+      mock_response(200)
+
+      assert {:ok, %Tesla.Env{status: 200}} = Backoff.get(@url)
+      assert {:ok, nil} = Cachex.get(@cache, @host)
+    end
+
+    test "does not send a request while the host is backed off" do
+      Cachex.put(@cache, @host, true)
+      mock_response(200)
+
+      assert {:error, :ratelimit} = Backoff.get(@url)
+      refute_received :request
+    end
+
+    for status <- [429, 503] do
+      test "backs off from the host after a #{status} response" do
+        mock_response(unquote(status))
+
+        assert {:error, :ratelimit} = Backoff.get(@url)
+        assert_received :request
+        assert {:ok, true} = Cachex.get(@cache, @host)
+
+        assert {:error, :ratelimit} = Backoff.get(@url <> "/other")
+        refute_received :request
+      end
+    end
+
+    test "uses a default backoff without headers" do
+      mock_response(429)
+
+      assert {:error, :ratelimit} = Backoff.get(@url)
+      assert_ttl_about(300)
+    end
+
+    test "only backs off from the host that answered" do
+      mock_response(429)
+      assert {:error, :ratelimit} = Backoff.get(@url)
+
+      mock_response(200)
+      assert {:ok, _} = Backoff.get("https://other.example.com/users/bob")
+    end
+  end
+
+  describe "backoff headers" do
+    test "parses Retry-After in seconds" do
+      mock_response(429, [{"retry-after", "600"}])
+
+      assert {:error, :ratelimit} = Backoff.get(@url)
+      assert_ttl_about(600)
+    end
+
+    test "parses Retry-After as an HTTP-date" do
+      date =
+        DateTime.utc_now()
+        |> DateTime.add(600)
+        |> Timex.format!("{WDshort}, {0D} {Mshort} {YYYY} {h24}:{m}:{s} GMT")
+
+      mock_response(503, [{"retry-after", date}])
+
+      assert {:error, :ratelimit} = Backoff.get(@url)
+      assert_ttl_about(600)
+    end
+
+    test "parses Retry-After as an ISO 8601 timestamp" do
+      date = DateTime.utc_now() |> DateTime.add(600) |> DateTime.to_iso8601()
+      mock_response(429, [{"retry-after", date}])
+
+      assert {:error, :ratelimit} = Backoff.get(@url)
+      assert_ttl_about(600)
+    end
+
+    test "parses X-RateLimit-Reset as an ISO 8601 timestamp" do
+      date = DateTime.utc_now() |> DateTime.add(600) |> DateTime.to_iso8601()
+      mock_response(429, [{"x-ratelimit-reset", date}])
+
+      assert {:error, :ratelimit} = Backoff.get(@url)
+      assert_ttl_about(600)
+    end
+
+    test "matches header names case-insensitively" do
+      mock_response(429, [{"Retry-After", "600"}])
+
+      assert {:error, :ratelimit} = Backoff.get(@url)
+      assert_ttl_about(600)
+    end
+
+    test "prefers Retry-After over X-RateLimit-Reset" do
+      date = DateTime.utc_now() |> DateTime.add(1200) |> DateTime.to_iso8601()
+      mock_response(429, [{"x-ratelimit-reset", date}, {"retry-after", "600"}])
+
+      assert {:error, :ratelimit} = Backoff.get(@url)
+      assert_ttl_about(600)
+    end
+
+    test "falls back to the next header when one is invalid" do
+      date = DateTime.utc_now() |> DateTime.add(600) |> DateTime.to_iso8601()
+      mock_response(429, [{"retry-after", "soon"}, {"x-ratelimit-reset", date}])
+
+      assert {:error, :ratelimit} = Backoff.get(@url)
+      assert_ttl_about(600)
+    end
+
+    for value <- ["soon", "-5", "0", "12.5", "", "Wed, 21 Oct 2015 07:28:00 GMT"] do
+      test "ignores the invalid or past Retry-After value #{inspect(value)}" do
+        mock_response(429, [{"retry-after", unquote(value)}])
+
+        assert {:error, :ratelimit} = Backoff.get(@url)
+        assert_ttl_about(300)
+      end
+    end
+
+    test "caps absurd values at one hour" do
+      mock_response(429, [{"retry-after", "99999999999"}])
+
+      assert {:error, :ratelimit} = Backoff.get(@url)
+      assert_ttl_about(3600)
+    end
+  end
+end