chunker: fix NewFs when root points to composite multi-chunk file without metadata

Before this change, calling NewFs on a composite multi-chunk file with
--chunker-meta-format "none"
would fail due to f.base pointing to the wrong Fs. This change fixes the issue,
which was discovered on the bisync integration tests.

backend/chunker/chunker.go

@@ -101,8 +101,10 @@ var (
 //
 // And still chunker's primary function is to chunk large files
 // rather than serve as a generic metadata container.
-const maxMetadataSize = 1023
-const maxMetadataSizeWritten = 255
+const (
+	maxMetadataSize        = 1023
+	maxMetadataSizeWritten = 255
+)
 
 // Current/highest supported metadata format.
 const metadataVersion = 2
@@ -317,11 +319,13 @@ func NewFs(ctx context.Context, name, rpath string, m configmap.Mapper) (fs.Fs,
 	// i.e. `rpath` does not exist in the wrapped remote, but chunker
 	// detects a composite file because it finds the first chunk!
 	// (yet can't satisfy fstest.CheckListing, will ignore)
-	if err == nil && !f.useMeta && strings.Contains(rpath, "/") {
+	if err == nil && !f.useMeta {
 		firstChunkPath := f.makeChunkName(remotePath, 0, "", "")
-		_, testErr := cache.Get(ctx, baseName+firstChunkPath)
+		newBase, testErr := cache.Get(ctx, baseName+firstChunkPath)
 		if testErr == fs.ErrorIsFile {
+			f.base = newBase
 			err = testErr
+			cache.PinUntilFinalized(f.base, f)
 		}
 	}
 
@@ -1134,8 +1138,8 @@ func (o *Object) readXactID(ctx context.Context) (xactID string, err error) {
 // put implements Put, PutStream, PutUnchecked, Update
 func (f *Fs) put(
 	ctx context.Context, in io.Reader, src fs.ObjectInfo, remote string, options []fs.OpenOption,
-	basePut putFn, action string, target fs.Object) (obj fs.Object, err error) {
-
+	basePut putFn, action string, target fs.Object,
+) (obj fs.Object, err error) {
 	// Perform consistency checks
 	if err := f.forbidChunk(src, remote); err != nil {
 		return nil, fmt.Errorf("%s refused: %w", action, err)