bisync/dropbox: fail closed on listing collapse and cursor races

Refuse --delta-list without --check-access. Abort if a listing goes empty
or shrinks past --max-delete versus the prior snapshot (full relist and
local walk included). Persist cursor before listing so a crash cannot
pair a new listing with a stale cursor. Reconstruct delta remotes from
path_lower plus leaf casing; ListR probes a non-recursive list when the
recursive result is empty.

backend/dropbox/dropbox.go

@@ -1382,6 +1382,26 @@ func (f *Fs) ListR(ctx context.Context, dir string, callback fs.ListRCallback) (
 		}
 	}
 
+	if len(items) == 0 {
+		arg := files.NewListFolderArg(f.opt.Enc.FromStandardPath(root))
+		arg.Recursive = false
+		arg.Limit = 1
+		if root == "/" {
+			arg.Path = ""
+		}
+		var probe *files.ListFolderResult
+		err = f.pacer.Call(func() (bool, error) {
+			probe, err = f.srv.ListFolderContext(ctx, arg)
+			return shouldRetry(ctx, err)
+		})
+		if err != nil {
+			return fmt.Errorf("dropbox ListR: empty recursive listing and probe failed: %w", err)
+		}
+		if probe != nil && len(probe.Entries) > 0 {
+			return errors.New("dropbox ListR: recursive listing empty but directory has children; refusing incomplete listing")
+		}
+	}
+
 	skipPrefixes := make([]string, 0)
 	for _, it := range items {
 		if it.folder == nil || it.folder.SharingInfo == nil || it.folder.SharingInfo.SharedFolderId == "" {
@@ -2630,30 +2650,45 @@ func (f *Fs) ListFolderDeltas(ctx context.Context, cursor string) (newCursor str
 
 func (f *Fs) listingChangeFromEntry(entry files.IsMetadata) (ListingChange, bool) {
 	var (
-		ch        ListingChange
-		entryPath string
+		ch    ListingChange
+		md    *files.Metadata
+		lower string
 	)
 	switch info := entry.(type) {
 	case *files.FolderMetadata:
 		ch.IsDir = true
-		entryPath = info.PathDisplay
+		md = &info.Metadata
+		lower = info.PathLower
 	case *files.FileMetadata:
-		entryPath = info.PathDisplay
+		md = &info.Metadata
+		lower = info.PathLower
 		ch.Size = int64(info.Size)
 		ch.ModTime = time.Time(info.ClientModified)
 		ch.Hash = info.ContentHash
 	case *files.DeletedMetadata:
 		ch.Deleted = true
-		ch.IsDir = true
-		entryPath = info.PathDisplay
+		md = &info.Metadata
+		lower = info.PathLower
 	default:
 		return ListingChange{}, false
 	}
-	entryPath = trimPrefixFold(entryPath, f.slashRootSlash)
-	if entryPath == "" {
+	if lower == "" {
 		return ListingChange{}, false
 	}
-	ch.Remote = f.opt.Enc.ToStandardPath(entryPath)
+	leaf := listRLeaf(md)
+	rel := trimPrefixFold(lower, f.slashRootSlash)
+	if rel == "" {
+		return ListingChange{}, false
+	}
+	parent := path.Dir(rel)
+	if parent == "." || parent == "/" {
+		parent = ""
+	}
+	if parent == "" {
+		ch.Remote = f.opt.Enc.ToStandardName(leaf)
+	} else {
+		ch.Remote = path.Join(f.opt.Enc.ToStandardPath(parent), f.opt.Enc.ToStandardName(leaf))
+	}
 	return ch, true
 }
 

backend/dropbox/dropbox_internal_test.go

@@ -545,6 +545,7 @@ func TestListingChangeFromEntry(t *testing.T) {
 		Metadata: files.Metadata{
 			Name:        "a.txt",
 			PathDisplay: "/Dropbox/sub/a.txt",
+			PathLower:   "/dropbox/sub/a.txt",
 		},
 		Size:           42,
 		ContentHash:    "abc",
@@ -561,6 +562,7 @@ func TestListingChangeFromEntry(t *testing.T) {
 		Metadata: files.Metadata{
 			Name:        "sub",
 			PathDisplay: "/Dropbox/sub",
+			PathLower:   "/dropbox/sub",
 		},
 	}
 	ch, ok = f.listingChangeFromEntry(del)