Allow to set watcher to follow symlinks

Signed-off-by: Knut Ahlers <knut@ahlers.me>

file/watcher.go

@@ -2,6 +2,7 @@ package file
 
 import (
 	"crypto/sha256"
+	"os"
 	"sync"
 	"time"
 
@@ -20,6 +21,7 @@ type (
 
 		c              chan WatcherEvent
 		checks         []WatcherCheck
+		followSymlinks bool
 		lock           sync.RWMutex
 		stateCache     map[string]any
 	}
@@ -30,6 +32,14 @@ type (
 	// WatcherEvent is the detected change to be signeld through the
 	// channel within the Watcher
 	WatcherEvent uint
+
+	// WatcherOpts holds configuration for newly created watcher
+	WatcherOpts struct {
+		// FollowSymlinks switches watchers based on file metadata into
+		// a mode where they follow a symlink and check the real target
+		// file instead of the symlink
+		FollowSymlinks bool
+	}
 )
 
 const (
@@ -40,6 +50,12 @@ const (
 	WatcherEventFileVanished
 )
 
+// DefaultWatcherOpts is used when creating the Watcher without
+// giving options
+var DefaultWatcherOpts = WatcherOpts{
+	FollowSymlinks: false,
+}
+
 // NewCryptographicWatcher is a wrapper around NewWatcher to configure
 // the Watcher with presence and sha256 hash checks.
 func NewCryptographicWatcher(filePath string, interval time.Duration) (*Watcher, error) {
@@ -53,15 +69,27 @@ func NewSimpleWatcher(filePath string, interval time.Duration) (*Watcher, error)
 }
 
 // NewWatcher creates a new Watcher configured with the given filePath,
-// interval and checks given. The checks are executed once during
-// initialization and will not cause an event to be sent. The created
-// Watcher will automatically start its periodic check and the C
-// channel should immediately be watched for changes. If the channel
-// is not listened on the check loop will be paused until events are
-// retrieved. If during the initial checks an error is detected the
-// loop is NOT started and the watcher needs to be initialized again.
+// default options, interval and checks given. The checks are executed
+// once during initialization and will not cause an event to be sent. The
+// created Watcher will automatically start its periodic check and the C
+// channel should immediately be watched for changes. If the channel is
+// not listened on the check loop will be paused until events are
+// retrieved. If during the initial checks an error is detected the loop
+// is NOT started and the watcher needs to be initialized again.
 func NewWatcher(filePath string, interval time.Duration, checks ...WatcherCheck) (*Watcher, error) {
-	w, err := newWatcher(filePath, interval, checks...)
+	return NewWatcherWithOpts(filePath, DefaultWatcherOpts, interval, checks...)
+}
+
+// NewWatcherWithOpts creates a new Watcher configured with the given
+// filePath, options, interval and checks given. The checks are executed
+// once during initialization and will not cause an event to be sent. The
+// created Watcher will automatically start its periodic check and the C
+// channel should immediately be watched for changes. If the channel is
+// not listened on the check loop will be paused until events are
+// retrieved. If during the initial checks an error is detected the loop
+// is NOT started and the watcher needs to be initialized again.
+func NewWatcherWithOpts(filePath string, opts WatcherOpts, interval time.Duration, checks ...WatcherCheck) (*Watcher, error) {
+	w, err := newWatcher(filePath, opts, interval, checks...)
 
 	if err == nil {
 		go w.loop()
@@ -70,7 +98,7 @@ func NewWatcher(filePath string, interval time.Duration, checks ...WatcherCheck)
 	return w, err
 }
 
-func newWatcher(filePath string, interval time.Duration, checks ...WatcherCheck) (*Watcher, error) {
+func newWatcher(filePath string, opts WatcherOpts, interval time.Duration, checks ...WatcherCheck) (*Watcher, error) {
 	notify := make(chan WatcherEvent, 1)
 
 	w := &Watcher{
@@ -80,6 +108,7 @@ func newWatcher(filePath string, interval time.Duration, checks ...WatcherCheck)
 
 		c:              notify,
 		checks:         checks,
+		followSymlinks: opts.FollowSymlinks,
 		stateCache:     make(map[string]any),
 	}
 	// Initially run checks once
@@ -141,3 +170,11 @@ func (w *Watcher) runStateChecks() (WatcherEvent, error) {
 
 	return WatcherEventNoChange, nil
 }
+
+func (w *Watcher) stat(filePath string) (os.FileInfo, error) {
+	if w.followSymlinks {
+		return os.Stat(filePath)
+	}
+
+	return os.Lstat(filePath)
+}

file/watcher_checks.go

@@ -29,7 +29,7 @@ func WatcherCheckHash(hcf func() hash.Hash) WatcherCheck {
 			lastHash = v
 		}
 
-		if _, err := os.Lstat(w.FilePath); errors.Is(err, fs.ErrNotExist) {
+		if _, err := w.stat(w.FilePath); errors.Is(err, fs.ErrNotExist) {
 			return WatcherEventInvalid, nil
 		}
 
@@ -65,7 +65,7 @@ func WatcherCheckMtime(w *Watcher) (WatcherEvent, error) {
 		lastChange = v
 	}
 
-	s, err := os.Lstat(w.FilePath)
+	s, err := w.stat(w.FilePath)
 	switch {
 	case err == nil:
 		// handle size change
@@ -94,7 +94,7 @@ func WatcherCheckPresence(w *Watcher) (WatcherEvent, error) {
 		wasPresent = v
 	}
 
-	_, err := os.Lstat(w.FilePath)
+	_, err := w.stat(w.FilePath)
 	if err != nil && !errors.Is(err, fs.ErrNotExist) {
 		// Some weird error occurred
 		return WatcherEventInvalid, errors.Wrap(err, "getting file stat")
@@ -124,7 +124,7 @@ func WatcherCheckSize(w *Watcher) (WatcherEvent, error) {
 		knownSize = v
 	}
 
-	s, err := os.Lstat(w.FilePath)
+	s, err := w.stat(w.FilePath)
 	switch {
 	case err == nil:
 		// handle size change

file/watcher_test.go

@@ -22,7 +22,7 @@ func TestWatcherCheckHash(t *testing.T) {
 
 	testFile := path.Join(testDir, "test.txt")
 
-	w, err := newWatcher(testFile, time.Second, WatcherCheckHash(sha256.New))
+	w, err := newWatcher(testFile, DefaultWatcherOpts, time.Second, WatcherCheckHash(sha256.New))
 	require.NoError(t, err, "initial check should not error on non existing file")
 
 	evt, err := w.runStateChecks()
@@ -73,7 +73,7 @@ func TestWatcherCheckMtime(t *testing.T) {
 
 	testFile := path.Join(testDir, "test.txt")
 
-	w, err := newWatcher(testFile, time.Second, WatcherCheckMtime)
+	w, err := newWatcher(testFile, DefaultWatcherOpts, time.Second, WatcherCheckMtime)
 	require.NoError(t, err, "initial check should not error on non existing file")
 
 	evt, err := w.runStateChecks()
@@ -128,7 +128,7 @@ func TestWatcherCheckPresence(t *testing.T) {
 
 	testFile := path.Join(testDir, "test.txt")
 
-	w, err := newWatcher(testFile, time.Second, WatcherCheckPresence)
+	w, err := newWatcher(testFile, DefaultWatcherOpts, time.Second, WatcherCheckPresence)
 	require.NoError(t, err, "initial check should not error on non existing file")
 
 	evt, err := w.runStateChecks()
@@ -165,7 +165,7 @@ func TestWatcherCheckSize(t *testing.T) {
 
 	testFile := path.Join(testDir, "test.txt")
 
-	w, err := newWatcher(testFile, time.Second, WatcherCheckSize)
+	w, err := newWatcher(testFile, DefaultWatcherOpts, time.Second, WatcherCheckSize)
 	require.NoError(t, err, "initial check should not error on non existing file")
 
 	evt, err := w.runStateChecks()