expose generate_id function and make it configurable in RequestId

lib/plug/request_id.ex

@@ -54,6 +54,9 @@ defmodule Plug.RequestId do
 
           plug Plug.RequestId, logger_metadata_key: :my_request_id
 
+    * `:generate_request_id` - The function used to generate the request ID, defaults to `generate_request_id/0`.
+
+          plug Plug.RequestId, generate_request_id: fn -> "myapp-" <> Plug.RequestId.generate_request_id() end
   """
 
   alias Plug.Conn
@@ -64,28 +67,31 @@ defmodule Plug.RequestId do
     {
       Keyword.get(opts, :http_header, "x-request-id"),
       Keyword.get(opts, :assign_as),
-      Keyword.get(opts, :logger_metadata_key, :request_id)
+      Keyword.get(opts, :logger_metadata_key, :request_id),
+      Keyword.get(opts, :generate_request_id, &__MODULE__.generate_request_id/0)
     }
   end
 
   @impl true
-  def call(conn, {header, assign_as, logger_metadata_key}) do
-    request_id = get_request_id(conn, header)
+  def call(conn, {header, assign_as, logger_metadata_key, generate_request_id}) do
+    request_id = get_request_id(conn, header, generate_request_id)
 
     Logger.metadata([{logger_metadata_key, 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
 
-  defp get_request_id(conn, header) do
+  defp get_request_id(conn, header, generate_request_id) do
     case Conn.get_req_header(conn, header) do
-      [] -> generate_request_id()
-      [val | _] -> if valid_request_id?(val), do: val, else: generate_request_id()
+      [val | _] when byte_size(val) in 20..200 -> val
+      _ -> generate_request_id.()
     end
   end
 
-  defp generate_request_id do
+  @doc "Generates Base64 encoded request ID."
+  @spec generate_request_id :: binary()
+  def generate_request_id do
     binary = <<
       System.system_time(:nanosecond)::64,
       :erlang.phash2({node(), self()}, 16_777_216)::24,
@@ -94,6 +100,4 @@ defmodule Plug.RequestId do
 
     Base.url_encode64(binary)
   end
-
-  defp valid_request_id?(s), do: byte_size(s) in 20..200
 end

test/plug/request_id_test.exs

@@ -7,6 +7,19 @@ defmodule Plug.RequestIdTest do
     Plug.RequestId.call(conn, Plug.RequestId.init(opts))
   end
 
+  test "generates new request id with generate_request_id" do
+    conn =
+      call(conn(:get, "/"),
+        generate_request_id: fn -> "myapp-" <> Plug.RequestId.generate_request_id() end
+      )
+
+    [res_request_id] = get_resp_header(conn, "x-request-id")
+    meta_request_id = Logger.metadata()[:request_id]
+    assert generated_request_id?(res_request_id)
+    assert res_request_id == meta_request_id
+    assert String.starts_with?(res_request_id, "myapp-")
+  end
+
   test "generates new request id if none exists" do
     conn = call(conn(:get, "/"), [])
     [res_request_id] = get_resp_header(conn, "x-request-id")