Use common error when removing an unwatched file

port of fsnotify/fsnotify#460

The errors returned by the various implementations of the watcher are all different
which makes handling them difficult. This PR follows the suggestion in:
fsnotify/fsnotify#455 (comment) by @mattn
to create a common error which is wrapped by the implementations.

fsnotify.go

@@ -65,5 +65,6 @@ func (e Event) String() string {
 
 // Common errors that can be reported by a watcher
 var (
-	ErrEventOverflow = errors.New("fsnotify queue overflow")
+	ErrNonExistentWatch = errors.New("fsnotify: can't remove non-existent watcher")
+	ErrEventOverflow    = errors.New("fsnotify: queue overflow")
 )

inotify.go

@@ -134,7 +134,7 @@ func (w *Watcher) Remove(name string) error {
 
 	// Remove it from inotify.
 	if !ok {
-		return fmt.Errorf("can't remove non-existent inotify watch for: %s", name)
+		return fmt.Errorf("%w: %s", ErrNonExistentWatch, name)
 	}
 
 	// We successfully removed the watch if InotifyRmWatch doesn't return an

inotify_test.go

@@ -8,6 +8,7 @@
 package fsnotify
 
 import (
+	"errors"
 	"fmt"
 	"os"
 	"path/filepath"
@@ -303,6 +304,8 @@ func TestInotifyRemoveTwice(t *testing.T) {
 	err = w.Remove(testFile)
 	if err == nil {
 		t.Fatalf("no error on removing invalid file")
+	} else if !errors.Is(err, ErrNonExistentWatch) {
+		t.Fatalf("unexpected error %v on removing invalid file", err)
 	}
 
 	w.mu.Lock()
@@ -384,7 +387,7 @@ func TestInotifyOverflow(t *testing.T) {
 	for dn := 0; dn < numDirs; dn++ {
 		testSubdir := fmt.Sprintf("%s/%d", testDir, dn)
 
-		err := os.Mkdir(testSubdir, 0777)
+		err := os.Mkdir(testSubdir, 0o777)
 		if err != nil {
 			t.Fatalf("Cannot create subdir: %v", err)
 		}

kqueue.go

@@ -74,7 +74,7 @@ func (w *Watcher) Close() error {
 	w.isClosed = true
 
 	// copy paths to remove while locked
-	var pathsToRemove = make([]string, 0, len(w.watches))
+	pathsToRemove := make([]string, 0, len(w.watches))
 	for name := range w.watches {
 		pathsToRemove = append(pathsToRemove, name)
 	}
@@ -107,7 +107,7 @@ func (w *Watcher) Remove(name string) error {
 	watchfd, ok := w.watches[name]
 	w.mu.Unlock()
 	if !ok {
-		return fmt.Errorf("can't remove non-existent kevent watch for: %s", name)
+		return fmt.Errorf("%w: %s", ErrNonExistentWatch, name)
 	}
 
 	const registerRemove = unix.EV_DELETE

windows.go

@@ -316,7 +316,7 @@ func (w *Watcher) remWatch(pathname string) error {
 	watch := w.watches.get(ino)
 	w.mu.Unlock()
 	if watch == nil {
-		return fmt.Errorf("can't remove non-existent watch for: %s", pathname)
+		return fmt.Errorf("%w: %s", ErrNonExistentWatch, pathname)
 	}
 	if pathname == dir {
 		w.sendEvent(watch.path, watch.mask&sysFSIGNORED)