wip: union: implement the stat function

Signed-off-by: Anagh Kumar Baranwal <6824881+darthShadow@users.noreply.github.com>

Author: darthShadow

backend/union/union.go

@@ -800,35 +800,48 @@ func (f *Fs) ListR(ctx context.Context, dir string, callback fs.ListRCallback) (
 //
 // It should return fs.ErrorNotImplemented if it can't return a DirEntry
 func (f *Fs) Stat(ctx context.Context, dir string, leaf string) (fs.DirEntry, error) {
-	//entriesList := make([][]upstream.Entry, len(f.upstreams))
-	//errs := Errors(make([]error, len(f.upstreams)))
-	//multithread(len(f.upstreams), func(i int) {
-	//	u := f.upstreams[i]
-	//	entries, err := u.List(ctx, dir)
-	//	if err != nil {
-	//		errs[i] = fmt.Errorf("%s: %w", u.Name(), err)
-	//		return
-	//	}
-	//	uEntries := make([]upstream.Entry, len(entries))
-	//	for j, e := range entries {
-	//		uEntries[j], _ = u.WrapEntry(e)
-	//	}
-	//	entriesList[i] = uEntries
-	//})
-	//if len(errs) == len(errs.FilterNil()) {
-	//	errs = errs.Map(func(e error) error {
-	//		if errors.Is(e, fs.ErrorDirNotFound) {
-	//			return nil
-	//		}
-	//		return e
-	//	})
-	//	if len(errs) == 0 {
-	//		return nil, fs.ErrorDirNotFound
-	//	}
-	//	return nil, errs.Err()
-	//}
-	//return f.mergeDirEntries(entriesList)
-	return nil, fs.ErrorNotImplemented
+	entriesList := make([][]upstream.Entry, len(f.upstreams))
+	errs := Errors(make([]error, len(f.upstreams)))
+
+	multithread(len(f.upstreams), func(i int) {
+		u := f.upstreams[i]
+		stater, ok := u.Fs.(fs.Stater)
+		if !ok {
+			errs[i] = fs.ErrorNotImplemented
+			return
+		}
+		entry, err := stater.Stat(ctx, dir, leaf)
+		if err != nil {
+			errs[i] = fmt.Errorf("%s: %w", u.Name(), err)
+			return
+		}
+		uEntries := make([]upstream.Entry, 1)
+		uEntries[0], _ = u.WrapEntry(entry)
+		entriesList[i] = uEntries
+	})
+
+	if len(errs) == len(errs.FilterNil()) {
+		errs = errs.Map(func(e error) error {
+			if errors.Is(e, fs.ErrorDirNotFound) {
+				return nil
+			}
+			return e
+		})
+		if len(errs) == 0 {
+			return nil, fs.ErrorDirNotFound
+		}
+		return nil, errs.Err()
+	}
+
+	entries, err := f.mergeDirEntries(entriesList)
+	if err != nil {
+		return nil, err
+	}
+	if len(entries) != 1 {
+		return nil, fmt.Errorf("internal error: expected 1 entry but got %d", len(entries))
+	}
+
+	return entries[0], nil
 }
 
 // NewObject creates a new remote union file object