Print process mailbox on failing assert_receive

Author: edgurgel

lib/ex_unit/lib/ex_unit/assertions.ex

@@ -281,13 +281,30 @@ defmodule ExUnit.Assertions do
           unquote(expected) = received -> received
         after
           unquote(timeout) ->
-            flunk unquote(message)
+            flunk(unquote(message) <> mailbox(self))
         end
       end
 
     {:receive, [{:export_head, true}|meta], args}
   end
 
+  @max_mailbox_length 10
+
+  @doc false
+  def mailbox(pid) do
+    {:messages, messages} = Process.info(pid, :messages)
+    length = length(messages)
+    mailbox = Enum.take(messages, @max_mailbox_length)
+      |> Enum.map_join("\n", &inspect/1)
+    ". Process mailbox (#{length}):" <> mailbox_message(length, mailbox)
+  end
+
+  defp mailbox_message(0, _mailbox), do: " []"
+  defp mailbox_message(length, mailbox) when length > 10 do
+    "\n" <> mailbox <> "\nShowing only #{@max_mailbox_length} first messages."
+  end
+  defp mailbox_message(_length, mailbox), do: "\n" <> mailbox
+
   @doc """
   Asserts the `exception` is raised during `function` execution with
   the `expected_message`. Returns the rescued exception, fails otherwise.

lib/ex_unit/test/ex_unit/assertions_test.exs

@@ -86,12 +86,32 @@ defmodule ExUnit.AssertionsTest do
     :hello = assert_received :hello
   end
 
-  test "assert received when different" do
+  test "assert received when empty mailbox" do
     try do
       "This should never be tested" = assert_received :hello
     rescue
       error in [ExUnit.AssertionError] ->
-        "No message matching :hello" = error.message
+        "No message matching :hello. Process mailbox (0): []" = error.message
+    end
+  end
+
+  test "assert received when different message" do
+    send self, {:message, :not_expected, :at_all}
+    try do
+      "This should never be tested" = assert_receive :hello, 5_000
+    rescue
+      error in [ExUnit.AssertionError] ->
+        "No message matching :hello. Process mailbox (1):\n{:message, :not_expected, :at_all}" = error.message
+    end
+  end
+
+  test "assert received when different message having more than 10 on mailbox" do
+    for i <- 1..11, do: send(self, {:message, i})
+    try do
+      "This should never be tested" = assert_receive :hello, 5_000
+    rescue
+      error in [ExUnit.AssertionError] ->
+        "No message matching :hello. Process mailbox (11):\n{:message, 1}\n{:message, 2}\n{:message, 3}\n{:message, 4}\n{:message, 5}\n{:message, 6}\n{:message, 7}\n{:message, 8}\n{:message, 9}\n{:message, 10}\nShowing only 10 first messages." = error.message
     end
   end