filer.remote.sync: an unread offset never resolves to the mount's creation time

With the offset read now bounded, a timed-out read fell through to the
directory's creation time and replayed every event already synced. The
error goes back to the startup retry loop, as with -timeAgo; a missing
offset still reads as zero without an error.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

weed/command/filer_remote_sync_dir.go

@@ -1120,8 +1120,11 @@ func resolveLastSyncOffset(mountedDir string, timeAgo time.Duration, lookupMount
 		}
 
 		lastOffsetTsNs, err := readOffset()
+		if err != nil {
+			return time.Time{}, fmt.Errorf("read the sync offset of %s: %w", mountedDir, err)
+		}
 		if mountedDirEntry != nil {
-			if err == nil && mountedDirEntry.Attributes.Crtime < lastOffsetTsNs/1000000 {
+			if mountedDirEntry.Attributes.Crtime < lastOffsetTsNs/1000000 {
 				lastOffsetTs = time.Unix(0, lastOffsetTsNs)
 				glog.V(0).Infof("resume from %v", lastOffsetTs)
 			} else {

weed/command/filer_remote_sync_dir_test.go

@@ -665,6 +665,12 @@ func TestResolveLastSyncOffsetReturnsTheErrorToTheRetryLoop(t *testing.T) {
 		t.Fatalf("got %v, want the lookup error: without a saved offset the caller's retry loop decides, not an exit", err)
 	}
 
+	// the mount answers but the saved offset is unreadable: never its creation time
+	mount := &filer_pb.Entry{Name: "x", Attributes: &filer_pb.FuseAttributes{Crtime: 1}}
+	if _, err := resolveLastSyncOffset("/buckets/x", 0, func() (*filer_pb.Entry, error) { return mount, nil }, readFails); err == nil || !strings.Contains(err.Error(), "filer unavailable") {
+		t.Fatalf("got %v, want the offset read error: the creation time would replay everything already synced", err)
+	}
+
 	// -timeAgo set, the saved offset unreadable: never now minus timeAgo
 	if _, err := resolveLastSyncOffset("/buckets/x", time.Hour, func() (*filer_pb.Entry, error) { return nil, nil }, readFails); err == nil || !strings.Contains(err.Error(), "filer unavailable") {
 		t.Fatalf("got %v, want the offset read error", err)