Allow to configure whether all errors should be logged (#68)

lib/plug/cowboy.ex

@@ -98,6 +98,16 @@ defmodule Plug.Cowboy do
   - Your app is running in production without a reverse proxy, using Cowboy's
   SSL support.
 
+  ## Logging
+
+  You can configure which exceptions are logged via `:log_exceptions_with_status_code`
+  application environment variable. If the status code returned by `Plug.Exception.status/1`
+  for the exception falls into any of the configured ranges, the exception is logged.
+  By default it's set to `[500..599]`.
+
+      config :plug_cowboy,
+        log_exceptions_with_status_code: [400..599]
+
   ## Instrumentation
 
   Plug.Cowboy uses the `:telemetry` library for instrumentation. The following

lib/plug/cowboy/translator.ex

@@ -28,9 +28,7 @@ defmodule Plug.Cowboy.Translator do
          {reason, {mod, :call, [%Plug.Conn{} = conn, _opts]}},
          _stack
        ) do
-    if non_500_exception?(reason) do
-      :skip
-    else
+    if log_exception?(reason) do
       {:ok,
        [
          inspect(pid),
@@ -41,6 +39,8 @@ defmodule Plug.Cowboy.Translator do
          conn_info(min_level, conn)
          | Exception.format(:exit, reason, [])
        ], conn: conn, crash_reason: reason, domain: [:cowboy]}
+    else
+      :skip
     end
   end
 
@@ -57,11 +57,16 @@ defmodule Plug.Cowboy.Translator do
      ], crash_reason: reason, domain: [:cowboy]}
   end
 
-  defp non_500_exception?({%{__exception__: true} = exception, _}) do
-    Plug.Exception.status(exception) < 500
+  defp log_exception?({%{__exception__: true} = exception, _}) do
+    status_ranges =
+      Application.get_env(:plug_cowboy, :log_exceptions_with_status_code, [500..599])
+
+    status = Plug.Exception.status(exception)
+
+    Enum.any?(status_ranges, &(status in &1))
   end
 
-  defp non_500_exception?(_), do: false
+  defp log_exception?(_), do: true
 
   defp conn_info(_min_level, conn) do
     [server_info(conn), request_info(conn)]

test/plug/cowboy/translator_test.exs

@@ -56,6 +56,37 @@ defmodule Plug.Cowboy.TranslatorTest do
     refute output =~ "** (exit) an exception was raised:"
   end
 
+  test "ranch/cowboy logs configured statuses" do
+    Application.put_env(:plug_cowboy, :log_exceptions_with_status_code, [400..499])
+    on_exit(fn -> Application.delete_env(:plug_cowboy, :log_exceptions_with_status_code) end)
+
+    {:ok, _pid} = Plug.Cowboy.http(__MODULE__, [], port: 9002)
+
+    output =
+      capture_log(fn ->
+        :hackney.get("http://127.0.0.1:9002/warn", [], "", [])
+        Plug.Cowboy.shutdown(__MODULE__.HTTP)
+      end)
+
+    assert output =~ ~r"#PID<0\.\d+\.0> running Plug\.Cowboy\.TranslatorTest \(.*\) terminated"
+    assert output =~ "Server: 127.0.0.1:9002 (http)"
+    assert output =~ "Request: GET /"
+    assert output =~ "** (exit) an exception was raised:"
+    assert output =~ "** (Plug.Parsers.UnsupportedMediaTypeError) unsupported media type foo/bar"
+
+    output =
+      capture_log(fn ->
+        :hackney.get("http://127.0.0.1:9002/error", [], "", [])
+        Plug.Cowboy.shutdown(__MODULE__.HTTP)
+      end)
+
+    refute output =~ ~r"#PID<0\.\d+\.0> running Plug\.Cowboy\.TranslatorTest \(.*\) terminated"
+    refute output =~ "Server: 127.0.0.1:9001 (http)"
+    refute output =~ "Request: GET /"
+    refute output =~ "** (exit) an exception was raised:"
+    refute output =~ "** (RuntimeError) oops"
+  end
+
   test "ranch/cowboy linked logs" do
     {:ok, _pid} = Plug.Cowboy.http(__MODULE__, [], port: 9003)