Fix case where Plug.Conn.Adapter.upgrade/3 could return any error tuple

lib/plug/conn.ex

@@ -1470,8 +1470,8 @@ defmodule Plug.Conn do
   function; it is entirely possible (likely, even) that the server will only do the actual upgrade
   later in the connection lifecycle.
 
-  If the adapter does not support the requested protocol this function will raise an
-  `ArgumentError`. The underlying adapter may also signal errors in the provided arguments by
+  If the adapter does not support the requested protocol, or if it returns any other error
+  for the requested upgrade, this function will raise an `ArgumentError`. The underlying adapter may also signal errors in the provided arguments by
   raising; consult the corresponding adapter documentation for details.
   """
   @spec upgrade_adapter(t, atom, term) :: t
@@ -1485,6 +1485,10 @@ defmodule Plug.Conn do
 
       {:error, :not_supported} ->
         raise ArgumentError, "upgrade to #{protocol} not supported by #{inspect(adapter)}"
+
+      {:error, reason} ->
+        raise ArgumentError,
+              "upgrade to #{protocol} failed in #{inspect(adapter)}: #{inspect(reason)}"
     end
   end
 

test/plug/conn_test.exs

@@ -499,6 +499,21 @@ defmodule Plug.ConnTest do
     )
   end
 
+  defmodule UpgradeErrorAdapter do
+    def upgrade(_payload, _protocol, _args), do: {:error, :bad_args}
+  end
+
+  test "upgrade_adapter/3 raises an error on failed upgrades" do
+    assert_raise(
+      ArgumentError,
+      "upgrade to websocket failed in Plug.ConnTest.UpgradeErrorAdapter: :bad_args",
+      fn ->
+        %{conn(:get, "/foo") | adapter: {UpgradeErrorAdapter, nil}}
+        |> upgrade_adapter(:websocket, opt: :bad)
+      end
+    )
+  end
+
   test "upgrade_adapter/3 will raise if the response is sent before upgrading" do
     assert_raise(Plug.Conn.AlreadySentError, fn ->
       conn(:get, "/foo")