Prevent exits from leaking in ExUnit.CaptureServer

When a process that traps exits used ExUnit's log capture functionality,
it would receive stray exits from the Tasks started inside it that could
lead to issues if not handled. We fix this by disabling exit trapping
while awaiting the tasks in the CaptureServer's log handler.

Author: SteffenDE

lib/ex_unit/lib/ex_unit/capture_server.ex

@@ -231,6 +231,8 @@ defmodule ExUnit.CaptureServer do
   ## :logger handler callback.
 
   def log(event, _config) do
+    trap_exits = Process.flag(:trap_exit, false)
+
     :ets.tab2list(@ets)
     |> Enum.filter(fn {_ref, _string_io, level, _formatter_mod, _formatter_config} ->
       :logger.compare_levels(event.level, level) in [:gt, :eq]
@@ -255,6 +257,8 @@ defmodule ExUnit.CaptureServer do
     end)
     |> Task.await_many(:infinity)
 
+    Process.flag(:trap_exit, trap_exits)
+
     :ok
   end
 end

lib/ex_unit/test/ex_unit/capture_log_test.exs

@@ -95,6 +95,16 @@ defmodule ExUnit.CaptureLogTest do
     end)
   end
 
+  test "exits don't leak" do
+    Process.flag(:trap_exit, true)
+
+    capture_log(fn ->
+      Logger.error("oh no!")
+    end)
+
+    refute_receive {:EXIT, _, _}
+  end
+
   describe "with_log/2" do
     test "returns the result and the log" do
       {result, log} =