Implement Implement Xgit.Util.TrailingHashReadDevice.valid?/1. (#99)

* Implement Implement Xgit.Util.TrailingHashReadDevice.valid?/1.

* Add tests and bug fixes.

lib/xgit/util/trailing_hash_read_device.ex

@@ -50,6 +50,17 @@ defmodule Xgit.Util.TrailingHashReadDevice do
   def open_string(s) when is_binary(s) and byte_size(s) >= 20,
     do: GenServer.start_link(__MODULE__, {:string, s})
 
+  @doc ~S"""
+  Returns `true` if this is process is an `TrailingHashReadDevice` instance.
+
+  Note the difference between this function and `valid_hash?/1`.
+  """
+  @spec valid?(v :: any) :: boolean
+  def valid?(v) when is_pid(v),
+    do: GenServer.call(v, :valid_trailing_hash_read_device?) == :valid_trailing_hash_read_device
+
+  def valid?(_), do: false
+
   @doc ~S"""
   Returns `true` if the hash at the end of the file matches the hash
   generated while reading the file.
@@ -96,6 +107,9 @@ defmodule Xgit.Util.TrailingHashReadDevice do
   end
 
   @impl true
+  def handle_call(:valid_trailing_hash_read_device?, _from_, state),
+    do: {:reply, :valid_trailing_hash_read_device, state}
+
   def handle_call(:valid_hash?, _from, %{remaining_bytes: 0, crypto: :done} = state),
     do: {:reply, :already_called, state}
 

test/xgit/util/trailing_hash_read_device_test.exs

@@ -21,6 +21,7 @@ defmodule Xgit.Util.TrailingHashReadDeviceTest do
   describe "open_file/1" do
     test "simple file" do
       assert {:ok, device} = open_file_with_trailing_hash("hello")
+      assert THR.valid?(device)
 
       assert "hello" = IO.binread(device, 5)
       assert :eof = IO.binread(device, 5)
@@ -107,6 +108,8 @@ defmodule Xgit.Util.TrailingHashReadDeviceTest do
 
       assert capture_log(fn ->
                send(device, :random_unknown_message)
+               Process.sleep(10)
+               # Give time for message to land.
              end) =~ "TrailingHashReadDevice received unexpected message :random_unknown_message"
     end
 
@@ -122,6 +125,7 @@ defmodule Xgit.Util.TrailingHashReadDeviceTest do
   describe "open_string/1" do
     test "simple file" do
       assert {:ok, device} = open_string_with_trailing_hash("hello")
+      assert THR.valid?(device)
 
       assert "hello" = IO.binread(device, 5)
       assert :eof = IO.binread(device, 5)
@@ -145,6 +149,17 @@ defmodule Xgit.Util.TrailingHashReadDeviceTest do
     end
   end
 
+  describe "valid?/1" do
+    test "other process" do
+      {:ok, pid} = GenServer.start_link(NotValid, nil)
+      refute THR.valid?(pid)
+    end
+
+    test "not a PID" do
+      refute THR.valid?("blah")
+    end
+  end
+
   defp open_file_with_trailing_hash(s) do
     Temp.track!()
     path = Temp.path!()