Keep HTTP/1.0 CONNECT tunnel sockets open

lib/mint/http1.ex

@@ -1126,6 +1126,16 @@ defmodule Mint.HTTP1 do
     {:ok, request}
   end
 
+  # A successful CONNECT switches the connection to tunnel mode, so the
+  # response's HTTP version and Connection headers no longer determine the
+  # lifetime of the underlying socket. In particular, HTTP/1.0 responses are
+  # otherwise treated as non-persistent and would close the newly-established
+  # tunnel before the caller can use it.
+  defp request_done(%{request: %{method: "CONNECT", status: status}} = conn)
+       when status in 200..299 do
+    pop_request(conn)
+  end
+
   defp request_done(%{request: request} = conn) do
     conn = pop_request(conn)
 

test/mint/http1/conn_test.exs

@@ -165,6 +165,18 @@ defmodule Mint.HTTP1Test do
     assert conn.buffer == "XXX"
   end
 
+  test "HTTP/1.0 2xx response to CONNECT leaves the tunnel socket open", %{conn: conn} do
+    {:ok, conn, ref} = HTTP1.request(conn, "CONNECT", "example.com:443", [], nil)
+
+    assert {:ok, conn, [_status, _headers, {:done, ^ref}]} =
+             HTTP1.stream(
+               conn,
+               {:tcp, conn.socket, "HTTP/1.0 200 Connection established\r\n\r\n"}
+             )
+
+    assert HTTP1.open?(conn)
+  end
+
   test "content-length is ignored in 2xx response to CONNECT request", %{conn: conn} do
     {:ok, conn, ref} = HTTP1.request(conn, "CONNECT", "example.com:443", [], nil)
     response = "HTTP/1.1 200 OK\r\ncontent-length: 0\r\n\r\n"

test/mint/tunnel_proxy_connect_test.exs

@@ -37,6 +37,26 @@ defmodule Mint.TunnelProxyConnectTest do
     assert merge_body(rest, request) == "hello"
   end
 
+  test "tunnels through a proxy that sends an HTTP/1.0 CONNECT response" do
+    {origin_port, _origin_ref} = start_tls_origin()
+
+    {proxy_port, _proxy_ref} =
+      start_connect_proxy(
+        "HTTP/1.0 200 Connection established\r\nProxy-agent: tinyproxy/1.11.1\r\n\r\n"
+      )
+
+    assert {:ok, conn} =
+             HTTP.connect(:https, "localhost", origin_port,
+               proxy: {:http, "localhost", proxy_port, []},
+               transport_opts: [verify: :verify_none]
+             )
+
+    assert {:ok, conn, request} = HTTP.request(conn, "GET", "/", [], nil)
+    assert {:ok, _conn, responses} = receive_stream(conn)
+    assert [{:status, ^request, 200}, {:headers, ^request, _headers} | rest] = responses
+    assert merge_body(rest, request) == "hello"
+  end
+
   test "a proxy that stalls the CONNECT response yields a tunnel timeout error" do
     proxy_port = start_silent_proxy()