DirStore: add sub-store mounting (Mount/Unmount/Child)

All methods delegate to children when path contains '/'.
Children appear as directory entries in List().
Enables hierarchical namespace composition.

store/dirstore.go

@@ -9,7 +9,8 @@ import (
 
 // DirStore is a configurable filesystem-backed Store.
 // It supports single or multiple directories (union), read-only mode,
-// a synthetic index entry, subdirectory operations, and custom path resolution.
+// a synthetic index entry, subdirectory operations, custom path resolution,
+// and mounting child DirStores at names (sub-stores).
 type DirStore struct {
 	dirs      []string
 	perm      os.FileMode
@@ -21,6 +22,7 @@ type DirStore struct {
 	deleteFn  func([]string, string) error
 	renameFn  func([]string, string, string) error
 	openFn    func([]string, string) (StoreEntry, error)
+	children  map[string]*DirStore
 }
 
 // DirOption configures a DirStore.
@@ -75,6 +77,56 @@ func NewDirStore(dirs []string, perm os.FileMode, opts ...DirOption) *DirStore {
 	return d
 }
 
+// Mount attaches a child DirStore at the given name.
+func (d *DirStore) Mount(name string, child *DirStore) {
+	if d.children == nil {
+		d.children = make(map[string]*DirStore)
+	}
+	d.children[name] = child
+}
+
+// Unmount removes a mounted child.
+func (d *DirStore) Unmount(name string) {
+	delete(d.children, name)
+}
+
+// Child returns the child mounted at name, or nil.
+func (d *DirStore) Child(name string) *DirStore {
+	if d.children == nil {
+		return nil
+	}
+	return d.children[name]
+}
+
+// split checks if name routes to a child store. Returns (child, rest, true)
+// if a child handles this path, or (nil, "", false) otherwise.
+func (d *DirStore) split(name string) (*DirStore, string, bool) {
+	if d.children == nil {
+		return nil, "", false
+	}
+	// Check for exact child match (directory itself).
+	if child, ok := d.children[name]; ok {
+		return child, "", true
+	}
+	// Check for child prefix: "childname/rest"
+	if i := indexOf(name, '/'); i >= 0 {
+		prefix := name[:i]
+		if child, ok := d.children[prefix]; ok {
+			return child, name[i+1:], true
+		}
+	}
+	return nil, "", false
+}
+
+func indexOf(s string, b byte) int {
+	for i := range len(s) {
+		if s[i] == b {
+			return i
+		}
+	}
+	return -1
+}
+
 func (d *DirStore) resolve(name string) (string, error) {
 	if d.resolveFn != nil {
 		return d.resolveFn(d.dirs, name)
@@ -93,6 +145,14 @@ func (d *DirStore) writePath(name string) string {
 }
 
 func (d *DirStore) Stat(name string) (os.FileInfo, error) {
+	// Check children first.
+	if child, rest, ok := d.split(name); ok {
+		if rest == "" {
+			// The child directory itself.
+			return &SyntheticFileInfo{Name_: name, Mode_: 0555, IsDir_: true}, nil
+		}
+		return child.Stat(rest)
+	}
 	if name == "idx" && d.indexFn != nil {
 		return &SyntheticFileInfo{Name_: "idx", Mode_: 0444}, nil
 	}
@@ -105,7 +165,6 @@ func (d *DirStore) Stat(name string) (os.FileInfo, error) {
 		return nil, err
 	}
 	if d.resolveFn != nil {
-		// Virtual name may differ from physical filename.
 		return &SyntheticFileInfo{Name_: name, Mode_: info.Mode(), Size_: info.Size()}, nil
 	}
 	return info, nil
@@ -121,7 +180,10 @@ func (d *DirStore) List() ([]os.DirEntry, error) {
 			result := make([]os.DirEntry, 0, len(entries)+1)
 			result = append(result, FileEntry("idx", 0444))
 			result = append(result, entries...)
-			return result, nil
+			entries = result
+		}
+		for name := range d.children {
+			entries = append(entries, DirEntry(name, 0555))
 		}
 		return entries, nil
 	}
@@ -143,10 +205,22 @@ func (d *DirStore) List() ([]os.DirEntry, error) {
 			}
 		}
 	}
+	for name := range d.children {
+		if !seen[name] {
+			result = append(result, DirEntry(name, 0555))
+		}
+	}
 	return result, nil
 }
 
 func (d *DirStore) Open(name string) (StoreEntry, error) {
+	// Delegate to child stores.
+	if child, rest, ok := d.split(name); ok {
+		if rest == "" {
+			return nil, fmt.Errorf("%s: is a directory", name)
+		}
+		return child.Open(rest)
+	}
 	notBlocking := func(context.Context, string) ([]byte, string, error) {
 		return nil, "", fmt.Errorf("blocking read not supported")
 	}
@@ -188,6 +262,9 @@ func (d *DirStore) Open(name string) (StoreEntry, error) {
 }
 
 func (d *DirStore) Create(name string) error {
+	if child, rest, ok := d.split(name); ok && rest != "" {
+		return child.Create(rest)
+	}
 	if d.readOnly {
 		return fmt.Errorf("read-only store")
 	}
@@ -202,6 +279,9 @@ func (d *DirStore) Create(name string) error {
 }
 
 func (d *DirStore) Delete(name string) error {
+	if child, rest, ok := d.split(name); ok && rest != "" {
+		return child.Delete(rest)
+	}
 	if d.readOnly {
 		return fmt.Errorf("read-only store")
 	}
@@ -216,6 +296,9 @@ func (d *DirStore) Delete(name string) error {
 }
 
 func (d *DirStore) Rename(old, new string) error {
+	if child, rest, ok := d.split(old); ok && rest != "" {
+		return child.Rename(rest, new)
+	}
 	if d.readOnly {
 		return fmt.Errorf("read-only store")
 	}
@@ -229,15 +312,31 @@ func (d *DirStore) Rename(old, new string) error {
 	return os.Rename(p, filepath.Join(filepath.Dir(p), new))
 }
 
-// ListDir lists entries in a subdirectory relative to the first backing dir.
+// ListDir lists entries in a subdirectory.
 func (d *DirStore) ListDir(rel string) ([]os.DirEntry, error) {
+	// Check if rel routes to a child.
+	if child, rest, ok := d.split(rel); ok {
+		if rest == "" {
+			return child.List()
+		}
+		return child.ListDir(rest)
+	}
+	if len(d.dirs) == 0 {
+		return nil, fmt.Errorf("%s: not found", rel)
+	}
 	return os.ReadDir(filepath.Join(d.dirs[0], rel))
 }
 
 // MkdirAll creates a directory relative to the first backing dir.
 func (d *DirStore) MkdirAll(rel string) error {
+	if child, rest, ok := d.split(rel); ok && rest != "" {
+		return child.MkdirAll(rest)
+	}
 	if d.readOnly {
 		return fmt.Errorf("read-only store")
 	}
+	if len(d.dirs) == 0 {
+		return fmt.Errorf("no backing directory")
+	}
 	return os.MkdirAll(filepath.Join(d.dirs[0], rel), 0755)
 }