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

CHANGELOG.md

@@ -49,6 +49,11 @@ NOTE: Add new changes BELOW THIS COMMENT.
 
 - Node 18 support, Node 20 will be required in future releases.
 
+### Fixed
+
+- Tracking `/etc/hosts` file changes causing panics within particular
+  filesystems on start ([#7076]).
+
 [#7053]: https://github.com/AdguardTeam/AdGuardHome/issues/7053
 
 [install-script]: https://github.com/AdguardTeam/AdGuardHome/?tab=readme-ov-file#automated-install-linux-and-mac

internal/aghnet/hostscontainer.go

@@ -161,7 +161,8 @@ func (hc *HostsContainer) handleEvents() {
 
 	defer close(hc.updates)
 
-	ok, eventsCh := true, hc.watcher.Events()
+	eventsCh := hc.watcher.Events()
+	ok := eventsCh != nil
 	for ok {
 		select {
 		case _, ok = <-eventsCh:

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
+}

internal/home/home.go

@@ -232,7 +232,9 @@ func configureOS(conf *configuration) (err error) {
 func setupHostsContainer() (err error) {
 	hostsWatcher, err := aghos.NewOSWritesWatcher()
 	if err != nil {
-		return fmt.Errorf("initing hosts watcher: %w", err)
+		log.Info("WARNING: initializing filesystem watcher: %s; not watching for changes", err)
+
+		hostsWatcher = aghos.EmptyFSWatcher{}
 	}
 
 	paths, err := hostsfile.DefaultHostsPaths()