Improve reliability of EventedFileUpdateCheckerTest fork test

`Listen.to` starts a bunch of background threads that need to perform
some work before they are able to receive events, but it doesn't block
until they are ready, which expose us to a race condition.

With `wait_for_state(:processing_events)` we can ensure that it's ready on
Linux, however on macOS, the Darwin backend has a second background thread
we can't wait on.

As a workaround we wait a bit after the fork to allow that thread to
reach it's listning state.

activesupport/lib/active_support/evented_file_update_checker.rb

@@ -45,6 +45,10 @@ def initialize(files, dirs = {}, &block)
       ObjectSpace.define_finalizer(self, @core.finalizer)
     end
 
+    def inspect
+      "#<ActiveSupport::EventedFileUpdateChecker:#{object_id} @files=#{@core.files.to_a.inspect}"
+    end
+
     def updated?
       if @core.restart?
         @core.thread_safely(&:restart)
@@ -68,7 +72,7 @@ def execute_if_updated
     end
 
     class Core
-      attr_reader :updated
+      attr_reader :updated, :files
 
       def initialize(files, dirs)
         @files = files.map { |file| Pathname(file).expand_path }.to_set
@@ -86,7 +90,11 @@ def initialize(files, dirs)
         @mutex = Mutex.new
 
         start
-        @after_fork = ActiveSupport::ForkTracker.after_fork { start }
+        # inotify / FSEvents file descriptors are inherited on fork, so
+        # we need to reopen them otherwise only the parent or the child
+        # will be notified.
+        # FIXME: this callback is keeping a reference on the instance
+        @after_fork = ActiveSupport::ForkTracker.after_fork { restart }
       end
 
       def finalizer
@@ -107,6 +115,11 @@ def start
         @dtw, @missing = [*@dtw, *@missing].partition(&:exist?)
         @listener = @dtw.any? ? Listen.to(*@dtw, &method(:changed)) : nil
         @listener&.start
+
+        # Wait for the listener to be ready to avoid race conditions
+        # Unfortunately this isn't quite enough on macOS because the Darwin backend
+        # has an extra private thread we can't wait on.
+        @listener&.wait_for_state(:processing_events)
       end
 
       def stop

activesupport/test/evented_file_update_checker_test.rb

@@ -26,7 +26,7 @@ def teardown
   end
 
   def wait
-    sleep 1
+    sleep 0.5
   end
 
   def mkdir(dirs)
@@ -60,6 +60,10 @@ def rm_f(files)
     pid = fork do
       assert_not_predicate checker, :updated?
 
+      # The listen gem start multiple background threads that need to reach a ready state.
+      # Unfortunately, it doesn't look like there is a clean way to block until they are ready.
+      wait
+
       # Fork is booted, ready for file to be touched
       # notify parent process.
       boot_writer.write("booted")
@@ -70,7 +74,7 @@ def rm_f(files)
 
       assert_predicate checker, :updated?
     rescue Exception => ex
-      result_writer.write(ex.class.name)
+      result_writer.write("#{ex.class.name}: #{ex.message}")
       raise
     ensure
       result_writer.close