vfs: check destination on rename and delete if it exists first FIXME needs tests

Some backends (eg onedrive) will throw an error if we rename over an
existing file and some (drive) will create a duplicate.

This patch checks to see if the destination exists and removes it or
returns an error as appropriate.

Fixes #4293

vfs/dir.go

@@ -781,6 +781,9 @@ func (d *Dir) Rename(oldName, newName string, destDir *Dir) error {
 		fs.Errorf(oldPath, "Dir.Rename error: %v", err)
 		return err
 	}
+	// Check to see if destination exists
+	newNode, _ := destDir.stat(newName)
+	destinationExists := newNode != nil
 	switch x := oldNode.DirEntry().(type) {
 	case nil:
 		if oldFile, ok := oldNode.(*File); ok {
@@ -793,6 +796,19 @@ func (d *Dir) Rename(oldName, newName string, destDir *Dir) error {
 			return EPERM
 		}
 	case fs.Object:
+		if destinationExists {
+			// Can't overwrite dir with file
+			if newNode.IsDir() {
+				return EEXIST // EISDIR is POSIX
+			}
+			// Try to delete the file
+			fs.Debugf(newPath, "Dir.Rename removing existing file")
+			err = newNode.Remove()
+			if err != nil {
+				fs.Errorf(newPath, "Dir.Rename error: couldn't delete existing file: %v", err)
+				return err
+			}
+		}
 		if oldFile, ok := oldNode.(*File); ok {
 			if err = oldFile.rename(context.TODO(), destDir, newName); err != nil {
 				fs.Errorf(oldPath, "Dir.Rename error: %v", err)
@@ -804,6 +820,19 @@ func (d *Dir) Rename(oldName, newName string, destDir *Dir) error {
 			return err
 		}
 	case fs.Directory:
+		if destinationExists {
+			// Can't overwrite file with dir
+			if newNode.IsFile() {
+				return EEXIST // ENOTDIR is POSIX
+			}
+			// Try to delete the existing directory - will succeed only if empty
+			fs.Debugf(newPath, "Dir.Rename removing existing dir")
+			err = newNode.Remove()
+			if err != nil {
+				fs.Errorf(newPath, "Dir.Rename error: couldn't rmdir existing dir: %v", err)
+				return err
+			}
+		}
 		features := d.f.Features()
 		if features.DirMove == nil && features.Move == nil && features.Copy == nil {
 			err := errors.Errorf("Fs %q can't rename directories (no DirMove, Move or Copy)", d.f)

vfs/write.go

@@ -283,11 +283,9 @@ func (fh *WriteFileHandle) Stat() (os.FileInfo, error) {
 
 // Truncate file to given size
 func (fh *WriteFileHandle) Truncate(size int64) (err error) {
+	// defer log.Trace(fh.remote, "size=%d", size)("err=%v", &err)
 	fh.mu.Lock()
 	defer fh.mu.Unlock()
-	if fh.closed {
-		return ECLOSED
-	}
 	if size != fh.offset {
 		fs.Errorf(fh.remote, "WriteFileHandle: Truncate: Can't change size without --vfs-cache-mode >= writes")
 		return EPERM