allow stopping a notifier from within an event callback

This was allowed pre-0.10.0 but got broken by the synchronization
of #stop with #run quitting.

lib/rb-inotify/notifier.rb

@@ -224,10 +224,13 @@ def watch(path, *flags, &callback)
     # @see #process
     def run
       @running.synchronize do
+        Thread.current[:INOTIFY_RUN_THREAD] = true
         @stop = false
 
         process until @stop
       end
+    ensure
+      Thread.current[:INOTIFY_RUN_THREAD] = false
     end
 
     # Stop watching for filesystem events.
@@ -237,8 +240,10 @@ def stop
       @stop = true
       @pipe.last.write "."
 
-      @running.synchronize do
-        # no-op: we just needed to wait until the lock was available
+      unless Thread.current[:INOTIFY_RUN_THREAD]
+        @running.synchronize do
+          # no-op: we just needed to wait until the lock was available
+        end
       end
     end
 

spec/notifier_spec.rb

@@ -101,6 +101,16 @@
 
         expect(events.map(&:name)).to match_array(%w(one.txt two.txt))
       end
+
+      it "can be stopped from within a callback" do
+        barriers = Array.new(3) { Concurrent::Event.new }
+        barrier_queue = barriers.dup
+        events = recording(dir, :create) { @notifier.stop }
+
+        run_thread = Thread.new { @notifier.run }
+        dir.join("one.txt").write("hello world")
+        run_thread.join
+      end
     end
 
     describe :fd do