I’m using Programming Phoenix by Chris McCord as a reference to build my personal blog. I’ve replicated the authentication implementation almost exactly, but now that I am working through the chapter about testing the authentication plug, I’m getting (Plug.Conn.AlreadySentError) the response was already sent. However, the site works fine when navigating through it regularly, just not in the tests. I’ve looked the problem up and ensured I don’t have plug :action anywhere and am calling halt() after I redirect when an action isn’t authorized. If anyone could help I’d really appreciate it 
Relevant code:
1) test login puts the user into the session (PhoenixBlog.AuthTest)
test/plugs/auth_test.exs:28
** (Plug.Conn.AlreadySentError) the response was already sent
stacktrace:
(plug) lib/plug/conn.ex:862: Plug.Conn.put_session/3
(phoenix_blog) web/plugs/auth.ex:25: PhoenixBlog.Plugs.Auth.login/2
test/plugs/auth_test.exs:31: (test)
2) test authenticate_user halts when no current_user exists (PhoenixBlog.AuthTest)
test/plugs/auth_test.exs:14
** (Plug.Conn.AlreadySentError) the response was already sent
stacktrace:
(plug) lib/plug/conn.ex:607: Plug.Conn.put_resp_header/3
(phoenix) lib/phoenix/controller.ex:303: Phoenix.Controller.redirect/2
(phoenix_blog) web/plugs/auth.ex:57: PhoenixBlog.Plugs.Auth.authenticate_user/2
test/plugs/auth_test.exs:15: (test)
Notice that only the tests which hit the current_user doesn’t exist clause in authenticate_user fail.
defmodule PhoenixBlog.AuthTest do
use PhoenixBlog.ConnCase
alias PhoenixBlog.Plugs.Auth
setup %{conn: conn} do
conn =
conn
|> bypass_through(Rumbl.Router, :browser)
|> get("/")
{:ok, %{conn: conn}}
end
test "authenticate_user halts when no current_user exists", %{conn: conn} do
conn = Auth.authenticate_user(conn, [])
assert conn.halted
end
test "authenticate_user continues when the current_user exists", %{conn: conn} do
conn =
conn
|> assign(:current_user, %PhoenixBlog.User{})
|> Auth.authenticate_user([])
refute conn.halted
end
test "login puts the user into the session", %{conn: conn} do
login_conn =
conn
|> Auth.login(%PhoenixBlog.User{id: 123})
|> send_resp(:ok, "")
next_conn = get(login_conn, "/")
assert get_session(next_conn, :user_id) == 123
end
end
Auth plug:
defmodule PhoenixBlog.Plugs.Auth do
import Plug.Conn
import Comeonin.Bcrypt, only: [checkpw: 2, dummy_checkpw: 0]
def init(opts) do
Keyword.fetch!(opts, :repo)
end
def call(conn, repo) do
user_id = get_session(conn, :user_id)
cond do
user = conn.assigns[:current_user] ->
conn
user = user_id && repo.get(PhoenixBlog.User, user_id) ->
assign(conn, :current_user, user)
true ->
assign(conn, :current_user, nil)
end
end
def login(conn, user) do
conn
|> assign(:current_user, user)
|> put_session(:user_id, user.id)
|> configure_session(renew: true)
end
def logout(conn) do
configure_session(conn, drop: true)
end
def login_by_username_and_pass(conn, username, given_pass, opts) do
repo = Keyword.fetch!(opts, :repo)
user = repo.get_by(PhoenixBlog.User, username: username)
cond do
user && checkpw(given_pass, user.password_hash) ->
{:ok, login(conn, user)}
user ->
{:error, :unauthorized, conn}
true ->
dummy_checkpw()
{:error, :not_found, conn}
end
end
import Phoenix.Controller
alias PhoenixBlog.Router.Helpers
def authenticate_user(conn, _opts) do
if conn.assigns.current_user do
conn
else
conn
|> put_flash(:error, "You must be logged in to access that page")
|> redirect(to: Helpers.page_path(conn, :index))
|> halt()
end
end
end
I also tried putting the 
Turns out I forgot to change 
