Do not send a fallback if the plug has already sent a response (#289)

* Do not send a fallback if the plug has already sent a response

lib/bandit/http1/handler.ex

@@ -4,6 +4,8 @@ defmodule Bandit.HTTP1.Handler do
 
   use ThousandIsland.Handler
 
+  @already_sent {:plug_conn, :sent}
+
   @impl ThousandIsland.Handler
   def handle_data(data, socket, state) do
     connection_span = ThousandIsland.Socket.telemetry_span(socket)
@@ -109,9 +111,17 @@ defmodule Bandit.HTTP1.Handler do
   defp code_for_reason(_), do: 400
 
   defp attempt_to_send_fallback(req, code) do
-    Bandit.HTTP1.Adapter.send_resp(req, code, [], <<>>)
-  rescue
-    _ -> :ok
+    receive do
+      @already_sent ->
+        send(self(), @already_sent)
+    after
+      0 ->
+        try do
+          Bandit.HTTP1.Adapter.send_resp(req, code, [], <<>>)
+        rescue
+          _ -> :ok
+        end
+    end
   end
 
   defp maybe_keepalive(req, state) do

test/bandit/http1/request_test.exs

@@ -1393,6 +1393,20 @@ defmodule HTTP1RequestTest do
       raise "boom"
     end
 
+    @tag capture_log: true
+    test "does not send an error response if the plug has already sent one before raising",
+         context do
+      client = SimpleHTTP1Client.tcp_client(context)
+      SimpleHTTP1Client.send(client, "GET", "/send_and_raise_error", ["host: banana"])
+      assert {:ok, "200 OK", _headers, _} = SimpleHTTP1Client.recv_reply(client)
+      assert SimpleHTTP1Client.connection_closed_for_reading?(client)
+    end
+
+    def send_and_raise_error(conn) do
+      send_resp(conn, 200, "OK")
+      raise "boom"
+    end
+
     @tag capture_log: true
     test "returns a 500 if the plug does not return anything", context do
       response = Req.get!(context.req, url: "/noop")