Add follow_redirect/2 to Phoenix.ConnTest (#5797)

This PR adds follow_redirect/2 to Phoenix.ConnTest.

When testing complex redirect flows e.g a callback from an external source, then I found it as usefull as Phoenix.LiveViewTest.follow_redirect.

lib/phoenix/test/conn_test.ex

@@ -243,6 +243,29 @@ defmodule Phoenix.ConnTest do
     |> endpoint.call(endpoint.init(action))
   end
 
+  @doc """
+  Follows a redirected conn.
+
+  ## Examples
+
+
+      assert follow_redirect(conn, ~"/").resp_body =~ "foo"
+  """
+  def follow_redirect(%Conn{status: status} = conn, expected_to) when status in 300..308 do
+    to = Plug.Conn.get_resp_header(conn, "location")
+    if "#{expected_to}" == "#{to}" do
+      dispatch(conn, conn.private.phoenix_endpoint, :get, expected_to)
+    else
+      raise ArgumentError, "expected connection to redirect to #{inspect(expected_to)}, but got #{inspect(to)}"
+    end
+  end
+  def follow_redirect(%Conn{status: status}, _expected_to) when status not in 300..308 do
+    raise "expected redirection with status 302, got: #{status}"
+  end
+  def follow_redirect(%Conn{} = _conn, _exptected_to) do
+    raise "expected connection to have redirected but no response was set/sent"
+  end
+
   defp from_set_to_sent(%Conn{state: :set} = conn), do: Conn.send_resp(conn)
   defp from_set_to_sent(conn), do: conn
 

test/phoenix/test/conn_test.exs

@@ -156,6 +156,47 @@ defmodule Phoenix.Test.ConnTest do
     refute conn.private.phoenix_recycled
   end
 
+  test "follow_redirect/2" do
+    Enum.each 300..308, fn(status) ->
+      conn =
+        build_conn(:get, "/")
+        |> Endpoint.call(Endpoint.init([]))
+        |> put_resp_header("location", "/new_location")
+        |> send_resp(status, "foo")
+
+      assert follow_redirect(conn, "/new_location").resp_body == "foo"
+    end
+  end
+
+  test "follow_redirect/2 without header" do
+    assert_raise ArgumentError,
+                 "expected connection to redirect to \"/new_location\", but got []", fn ->
+      build_conn(:get, "/")
+      |> send_resp(302, "ok")
+      |> follow_redirect("/new_location")
+    end
+  end
+
+  test "follow_redirect/2 without redirection" do
+    assert_raise RuntimeError,
+                 "expected redirection with status 302, got: 200", fn ->
+      build_conn(:get, "/")
+      |> put_resp_header("location", "new location")
+      |> send_resp(200, "ok")
+      |> follow_redirect("/new_location")
+    end
+  end
+
+  test "follow_redirect/2 with wrong path" do
+    assert_raise ArgumentError,
+                 "expected connection to redirect to \"/wrong_location\", but got [\"/new_location\"]", fn ->
+        build_conn(:get, "/")
+        |> put_resp_header("location", "/new_location")
+        |> send_resp(302, "ok")
+        |> follow_redirect("/wrong_location")
+    end
+  end
+
   describe "recycle/1" do
     test "relevant request headers are persisted" do
       conn =