Add assign_as option for Plug.RequestId (#1172)

- `:assign_as` - The name of the key that will be used to store the
  discovered or generated request id in `conn.private`. If not provided,
  the request id will not be stored.

  ```elixir
  plug Plug.RequestId, assign_as: :plug_request_id
  ```

Resolves: #1171

lib/plug/request_id.ex

@@ -34,6 +34,12 @@ defmodule Plug.RequestId do
 
  plug Plug.RequestId, http_header: "custom-request-id"
 
+ * `:assign_as` - The name of the key that will be used to store the
+ discovered or generated request id in `conn.assigns`. If not provided,
+ the request id will not be stored.
+
+ plug Plug.RequestId, assign_as: :plug_request_id
+
  """
 
  require Logger
@@ -42,14 +48,17 @@ defmodule Plug.RequestId do
 
  @impl true
  def init(opts) do
- Keyword.get(opts, :http_header, "x-request-id")
+ {
+ Keyword.get(opts, :http_header, "x-request-id"),
+ Keyword.get(opts, :assign_as)
+ }
  end
 
  @impl true
- def call(conn, req_id_header) do
+ def call(conn, {header, assign_as}) do
  conn
- |> get_request_id(req_id_header)
- |> set_request_id(req_id_header)
+ |> get_request_id(header)
+ |> set_request_id(header, assign_as)
  end
 
  defp get_request_id(conn, header) do
@@ -59,8 +68,11 @@ defmodule Plug.RequestId do
  end
  end
 
- defp set_request_id({conn, request_id}, header) do
+ defp set_request_id({conn, request_id}, header, assign_as) do
  Logger.metadata(request_id: request_id)
+
+ conn = if assign_as, do: Conn.assign(conn, assign_as, request_id), else: conn
+
  Conn.put_resp_header(conn, header, request_id)
  end
 

test/plug/request_id_test.exs

@@ -65,6 +65,22 @@ defmodule Plug.RequestIdTest do
  assert res_request_id == meta_request_id
  end
 
+ test "assigns the request id to conn.assigns when given an assignment key" do
+ request_id = "existingidthatislongenough"
+
+ conn =
+ conn(:get, "/")
+ |> put_req_header("x-request-id", request_id)
+ |> call(assign_as: :plug_request_id)
+
+ [res_request_id] = get_resp_header(conn, "x-request-id")
+ meta_request_id = Logger.metadata()[:request_id]
+ assigned_request_id = conn.assigns.plug_request_id
+ assert assigned_request_id == request_id
+ assert res_request_id == assigned_request_id
+ assert res_request_id == meta_request_id
+ end
+
  defp generated_request_id?(request_id) do
  Regex.match?(~r/\A[A-Za-z0-9-_]+\z/, request_id)
  end