auth: Support passing a 0-arity function (#417)

lib/req.ex

@@ -223,6 +223,8 @@ defmodule Req do
 
         * `string` - sets to this value.
 
+        * `&fun/0` - a function that returns one of the above (such as a `{:bearer, token}`).
+
   Request body encoding options ([`encode_body`](`Req.Steps.encode_body/1`)):
 
     * `:form` - if set, encodes the request body as `application/x-www-form-urlencoded`

lib/req/request.ex

@@ -1256,6 +1256,10 @@ defmodule Req.Request do
       {:bearer, redact(bearer)}
     end
 
+    defp redact_option(:auth, fun) when is_function(fun, 0) do
+      fun
+    end
+
     defp redact_option(:auth, {:basic, userinfo}) do
       {:basic, redact(userinfo)}
     end

lib/req/steps.ex

@@ -184,24 +184,32 @@ defmodule Req.Steps do
 
         * `{:netrc, path}` - load credentials from `path`
 
+        * `fn -> {:bearer, "eyJ0eXAi..." } end` - a 0-arity function that returns one of the aforementioned types.
+
   ## Examples
 
       iex> Req.get!("https://httpbin.org/basic-auth/foo/bar", auth: {:basic, "foo:foo"}).status
       401
       iex> Req.get!("https://httpbin.org/basic-auth/foo/bar", auth: {:basic, "foo:bar"}).status
       200
+      iex> Req.get!("https://httpbin.org/basic-auth/foo/bar", auth: fn -> {:basic, "foo:bar"} end).status
+      200
 
       iex> Req.get!("https://httpbin.org/bearer", auth: {:bearer, ""}).status
       401
       iex> Req.get!("https://httpbin.org/bearer", auth: {:bearer, "foo"}).status
       200
+      iex> Req.get!("https://httpbin.org/bearer", auth: fn -> {:bearer, "foo"} end).status
+      200
 
       iex> System.put_env("NETRC", "./test/my_netrc")
       iex> Req.get!("https://httpbin.org/basic-auth/foo/bar", auth: :netrc).status
       200
 
       iex> Req.get!("https://httpbin.org/basic-auth/foo/bar", auth: {:netrc, "./test/my_netrc"}).status
       200
+      iex> Req.get!("https://httpbin.org/basic-auth/foo/bar", auth: fn -> {:netrc, "./test/my_netrc"} end).status
+      200
   """
   @doc step: :request
   def auth(request) do
@@ -224,6 +232,16 @@ defmodule Req.Steps do
     Req.Request.put_header(request, "authorization", "Bearer " <> token)
   end
 
+  defp auth(request, fun) when is_function(fun, 0) do
+    value = fun.()
+
+    if is_function(value, 0) do
+      raise ArgumentError, "setting `auth: fn -> ... end` should not return another function"
+    end
+
+    auth(request, value)
+  end
+
   defp auth(request, :netrc) do
     path = System.get_env("NETRC") || Path.join(System.user_home!(), ".netrc")
     authenticate_with_netrc(request, path)

test/req_test.exs

@@ -56,6 +56,8 @@ defmodule ReqTest do
 
     assert inspect(Req.new(auth: {:basic, "foo:bar"})) =~ ~s|auth: {:basic, "foo****"}|
 
+    assert inspect(Req.new(auth: fn -> {:basic, "foo:bar"} end)) =~ ~s|auth: #Function|
+
     if Req.MixProject.legacy_headers_as_lists?() do
       assert inspect(Req.new(headers: [authorization: "bearer foobar"])) =~
                ~s|{"authorization", "bearer ***"}|