Add ability to discard events under load

fixes #848

lib/sentry/logger_handler.ex

@@ -87,6 +87,15 @@ defmodule Sentry.LoggerHandler do
       want to use sync mode, set this option to `0`. This option effectively implements
       **overload protection**.
       """
+    ],
+    discard_threshold: [
+      type: :non_neg_integer,
+      default: 500,
+      doc: """
+      *since v10.8.2* - The number of queued events after which this handler will start
+      to discard events. If you don't want to discard, set this option to `0`. This option
+      effectively implements **load shedding**.
+      """
     ]
   ]
 
@@ -231,7 +240,8 @@ defmodule Sentry.LoggerHandler do
     :tags_from_metadata,
     :capture_log_messages,
     :rate_limiting,
-    :sync_threshold
+    :sync_threshold,
+    :discard_threshold
   ]
 
   ## Logger handler callbacks
@@ -319,6 +329,10 @@ defmodule Sentry.LoggerHandler do
       config.rate_limiting && RateLimiter.increment(handler_id) == :rate_limited ->
         :ok
 
+      config.discard_threshold > 0 &&
+          SenderPool.get_queued_events_counter() >= config.discard_threshold ->
+        :ok
+
       true ->
         # Logger handlers run in the process that logs, so we already read all the
         # necessary Sentry context from the process dictionary (when creating the event).

test/sentry/logger_handler_test.exs

@@ -601,14 +601,61 @@ defmodule Sentry.LoggerHandlerTest do
     end
   end
 
-  defp register_before_send(_context) do
+  describe "discard threshold" do
+    @tag handler_config: %{
+           discard_threshold: 2,
+           capture_log_messages: true
+         },
+         send_request: true
+
+    test "discards logged messages", %{sender_ref: ref} do
+      register_delay()
+
+      Logger.error("First")
+      assert_receive {^ref, %{message: %{formatted: "First"}}}
+
+      Logger.error("Second")
+      assert_receive {^ref, %{message: %{formatted: "Second"}}}
+
+      Logger.error("Third")
+      refute_receive {^ref, _event}, 100
+
+      Process.sleep(300)
+      Logger.error("Fourth")
+      assert_receive {^ref, %{message: %{formatted: "Fourth"}}}
+    end
+  end
+
+  defp register_delay do
+    bypass = Bypass.open()
+
+    put_test_config(
+      dsn: "http://public:secret@localhost:#{bypass.port}/1",
+      dedup_events: false,
+      hackney_opts: [recv_timeout: 500, pool: :sentry_pool]
+    )
+
+    Bypass.expect(bypass, fn conn ->
+      assert conn.request_path == "/api/1/envelope/"
+      assert conn.method == "POST"
+      Process.sleep(150)
+      Plug.Conn.resp(conn, 200, ~s<{"id": "340"}>)
+    end)
+  end
+
+  defp register_before_send(context) do
     pid = self()
     ref = make_ref()
 
     put_test_config(
       before_send: fn event ->
         send(pid, {ref, event})
-        false
+
+        if Map.get(context, :send_request, false) do
+          event
+        else
+          false
+        end
       end,
       dsn: "http://public:secret@localhost:9392/1"
     )