fix: only response with html if client prefers media type

lib/plug/debugger.ex

@@ -299,8 +299,23 @@ defmodule Plug.Debugger do
 
   defp accepts_html?(_accept_header = []), do: false
 
-  defp accepts_html?(_accept_header = [header | _]),
-    do: String.contains?(header, ["*/*", "text/*", "text/html"])
+  defp accepts_html?(_accept_header = [header | _]) do
+    header
+    |> String.split(",", trim: true)
+    |> Enum.map(&Plug.Conn.Utils.media_type/1)
+    |> Enum.any?(fn
+      {:ok, type, subtype, params} ->
+        accept_html_media_type?(type, subtype, Map.get(params, "q", 1))
+
+      _ ->
+        false
+    end)
+  end
+
+  defp accept_html_media_type?("text", "html", 1), do: true
+  defp accept_html_media_type?("text", "*", 1), do: true
+  defp accept_html_media_type?("*", "*", 1), do: true
+  defp accept_html_media_type?(_type, _subtype, _q), do: false
 
   defp maybe_fetch_session(conn) do
     if conn.private[:plug_session_fetch] do

test/plug/debugger_test.exs

@@ -628,4 +628,31 @@ defmodule Plug.DebuggerTest do
       String.to_existing_atom("not_existing_atom_for_test_does_not_create_new_atom")
     end
   end
+
+  test "render markdown with text/html;q=0.5" do
+    conn =
+      conn(:get, "/")
+      |> put_req_header("accept", "text/html;q=0.5")
+      |> render([], fn -> raise "oops" end)
+
+    assert get_resp_header(conn, "content-type") == ["text/markdown; charset=utf-8"]
+  end
+
+  test "render markdown with text/*;q=0.5" do
+    conn =
+      conn(:get, "/")
+      |> put_req_header("accept", "text/*;q=0.5")
+      |> render([], fn -> raise "oops" end)
+
+    assert get_resp_header(conn, "content-type") == ["text/markdown; charset=utf-8"]
+  end
+
+  test "render markdown with */*;q=0.5" do
+    conn =
+      conn(:get, "/")
+      |> put_req_header("accept", "*/*;q=0.5")
+      |> render([], fn -> raise "oops" end)
+
+    assert get_resp_header(conn, "content-type") == ["text/markdown; charset=utf-8"]
+  end
 end