Pull request 2239: 7076 Empty FSWatcher

Updates #7076.

Squashed commit of the following:

commit 6d99de9bcd1a4882f96639cf7e54fe0f33cfbfd3
Merge: c545152fa 1c82be295
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Mon Jun 17 18:41:49 2024 +0300

    Merge branch 'master' into 7076-empty-fswatcher

commit c545152fa157e52f9ac5ebf2e58fdc1a254faf91
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Fri Jun 14 14:57:01 2024 +0300

    all: imp code

commit e033558d7027a40a6996c08b5125e45141192071
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Fri Jun 14 14:39:58 2024 +0300

    all: add & use empty fswatcher

internal/aghos/fswatcher.go

@@ -160,3 +160,34 @@ func (w *osWatcher) handleErrors() {
 		log.Error("%s: %s", osWatcherPref, err)
 	}
 }
+
+// EmptyFSWatcher is a no-op implementation of the [FSWatcher] interface.  It
+// may be used on systems not supporting filesystem events.
+type EmptyFSWatcher struct{}
+
+// type check
+var _ FSWatcher = EmptyFSWatcher{}
+
+// Start implements the [FSWatcher] interface for EmptyFSWatcher.  It always
+// returns nil error.
+func (EmptyFSWatcher) Start() (err error) {
+	return nil
+}
+
+// Close implements the [FSWatcher] interface for EmptyFSWatcher.  It always
+// returns nil error.
+func (EmptyFSWatcher) Close() (err error) {
+	return nil
+}
+
+// Events implements the [FSWatcher] interface for EmptyFSWatcher.  It always
+// returns nil channel.
+func (EmptyFSWatcher) Events() (e <-chan event) {
+	return nil
+}
+
+// Add implements the [FSWatcher] interface for EmptyFSWatcher.  It always
+// returns nil error.
+func (EmptyFSWatcher) Add(_ string) (err error) {
+	return nil
+}