Explicitly signal keepalives in HTTP/1.0 requests

lib/bandit/http1/socket.ex

@@ -325,6 +325,8 @@ defmodule Bandit.HTTP1.Socket do
     def send_headers(%@for{write_state: :unsent} = socket, status, headers, body_disposition) do
       resp_line = "#{socket.version} #{status} #{Plug.Conn.Status.reason_phrase(status)}\r\n"
 
+      headers = maybe_add_keepalive_header(status, headers, socket)
+
       case body_disposition do
         :raw ->
           # This is an optimization for the common case of sending a non-encoded body (or file),
@@ -347,6 +349,13 @@ defmodule Bandit.HTTP1.Socket do
       end
     end
 
+    # RFC 9112§9.3
+    defp maybe_add_keepalive_header(status, headers, %@for{version: :"HTTP/1.0", keepalive: true})
+         when status not in 100..199,
+         do: [{"connection", "keep-alive"} | headers]
+
+    defp maybe_add_keepalive_header(_status, headers, _socket), do: headers
+
     defp encode_headers(headers) do
       headers
       |> Enum.map(fn {k, v} -> [k, ": ", v, "\r\n"] end)

test/bandit/http1/request_test.exs

@@ -297,6 +297,21 @@ defmodule HTTP1RequestTest do
       assert {:ok, "200 OK", _headers, _body} = SimpleHTTP1Client.recv_reply(client)
     end
 
+    test "keepalive are explicitly signalled in HTTP/1.0", context do
+      client = SimpleHTTP1Client.tcp_client(context)
+
+      SimpleHTTP1Client.send(
+        client,
+        "GET",
+        "/echo_components",
+        ["host: localhost", "connection: keep-alive"],
+        "1.0"
+      )
+
+      assert {:ok, "200 OK", headers, _body} = SimpleHTTP1Client.recv_reply(client)
+      assert [{:connection, "keep-alive"} | _headers] = headers
+    end
+
     test "unread content length bodies are read before starting a new request", context do
       client = SimpleHTTP1Client.tcp_client(context)