compress: fix ChangeNotify

ChangeNotify has been broken on the compress backend for a long time!

Before this change it was wrapping the file names received rather than
unwrapping them to discover the original names.

It is likely ChangeNotify was working adequately though for users as
the VFS just uses the directories rather than the file names.

backend/compress/compress.go

@@ -257,6 +257,16 @@ func isMetadataFile(filename string) bool {
 	return strings.HasSuffix(filename, metaFileExt)
 }
 
+// Checks whether a file is a metadata file and returns the original
+// file name and a flag indicating whether it was a metadata file or
+// not.
+func unwrapMetadataFile(filename string) (string, bool) {
+	if !isMetadataFile(filename) {
+		return "", false
+	}
+	return filename[:len(filename)-len(metaFileExt)], true
+}
+
 // makeDataName generates the file name for a data file with specified compression mode
 func makeDataName(remote string, size int64, mode int) (newRemote string) {
 	if mode != Uncompressed {
@@ -980,6 +990,7 @@ func (f *Fs) ChangeNotify(ctx context.Context, notifyFunc func(string, fs.EntryT
 		fs.Logf(f, "path %q entryType %d", path, entryType)
 		var (
 			wrappedPath    string
+			isMetadataFile bool
 		)
 		switch entryType {
 		case fs.EntryDirectory:
@@ -987,7 +998,10 @@ func (f *Fs) ChangeNotify(ctx context.Context, notifyFunc func(string, fs.EntryT
 		case fs.EntryObject:
 			// Note: All we really need to do to monitor the object is to check whether the metadata changed,
 			// as the metadata contains the hash. This will work unless there's a hash collision and the sizes stay the same.
-			wrappedPath = makeMetadataName(path)
+			wrappedPath, isMetadataFile = unwrapMetadataFile(path)
+			if !isMetadataFile {
+				return
+			}
 		default:
 			fs.Errorf(path, "press ChangeNotify: ignoring unknown EntryType %d", entryType)
 			return