Refactor base paths with centralized path management (#901)

* Initial plan

* refactor: centralize path management and improve memory storage configuration

* fix: update memory storage path in config.example.yaml for clarity

* Initial plan

* Address PR #901 review comments: security fixes and documentation improvements

Co-authored-by: foreleven <4785594+foreleven@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: foreleven <4785594+foreleven@users.noreply.github.com>

backend/src/config/paths.py

@@ -0,0 +1,157 @@
+import os
+import re
+from pathlib import Path
+
+# Virtual path prefix seen by agents inside the sandbox
+VIRTUAL_PATH_PREFIX = "/mnt/user-data"
+
+_SAFE_THREAD_ID_RE = re.compile(r"^[A-Za-z0-9_\-]+$")
+
+
+class Paths:
+    """
+    Centralized path configuration for DeerFlow application data.
+
+    Directory layout (host side):
+        {base_dir}/
+        ├── memory.json
+        └── threads/
+            └── {thread_id}/
+                └── user-data/         <-- mounted as /mnt/user-data/ inside sandbox
+                    ├── workspace/     <-- /mnt/user-data/workspace/
+                    ├── uploads/       <-- /mnt/user-data/uploads/
+                    └── outputs/       <-- /mnt/user-data/outputs/
+
+    BaseDir resolution (in priority order):
+        1. Constructor argument `base_dir`
+        2. DEER_FLOW_HOME environment variable
+        3. Local dev fallback: cwd/.deer-flow  (when cwd is the backend/ dir)
+        4. Default: $HOME/.deer-flow
+    """
+
+    def __init__(self, base_dir: str | Path | None = None) -> None:
+        self._base_dir = Path(base_dir).resolve() if base_dir is not None else None
+
+    @property
+    def base_dir(self) -> Path:
+        """Root directory for all application data."""
+        if self._base_dir is not None:
+            return self._base_dir
+
+        if env_home := os.getenv("DEER_FLOW_HOME"):
+            return Path(env_home).resolve()
+
+        cwd = Path.cwd()
+        if cwd.name == "backend" or (cwd / "pyproject.toml").exists():
+            return cwd / ".deer-flow"
+
+        return Path.home() / ".deer-flow"
+
+    @property
+    def memory_file(self) -> Path:
+        """Path to the persisted memory file: `{base_dir}/memory.json`."""
+        return self.base_dir / "memory.json"
+
+    def thread_dir(self, thread_id: str) -> Path:
+        """
+        Host path for a thread's data: `{base_dir}/threads/{thread_id}/`
+
+        This directory contains a `user-data/` subdirectory that is mounted
+        as `/mnt/user-data/` inside the sandbox.
+
+        Raises:
+            ValueError: If `thread_id` contains unsafe characters (path separators
+                        or `..`) that could cause directory traversal.
+        """
+        if not _SAFE_THREAD_ID_RE.match(thread_id):
+            raise ValueError(
+                f"Invalid thread_id {thread_id!r}: only alphanumeric characters, "
+                "hyphens, and underscores are allowed."
+            )
+        return self.base_dir / "threads" / thread_id
+
+    def sandbox_work_dir(self, thread_id: str) -> Path:
+        """
+        Host path for the agent's workspace directory.
+        Host: `{base_dir}/threads/{thread_id}/user-data/workspace/`
+        Sandbox: `/mnt/user-data/workspace/`
+        """
+        return self.thread_dir(thread_id) / "user-data" / "workspace"
+
+    def sandbox_uploads_dir(self, thread_id: str) -> Path:
+        """
+        Host path for user-uploaded files.
+        Host: `{base_dir}/threads/{thread_id}/user-data/uploads/`
+        Sandbox: `/mnt/user-data/uploads/`
+        """
+        return self.thread_dir(thread_id) / "user-data" / "uploads"
+
+    def sandbox_outputs_dir(self, thread_id: str) -> Path:
+        """
+        Host path for agent-generated artifacts.
+        Host: `{base_dir}/threads/{thread_id}/user-data/outputs/`
+        Sandbox: `/mnt/user-data/outputs/`
+        """
+        return self.thread_dir(thread_id) / "user-data" / "outputs"
+
+    def sandbox_user_data_dir(self, thread_id: str) -> Path:
+        """
+        Host path for the user-data root.
+        Host: `{base_dir}/threads/{thread_id}/user-data/`
+        Sandbox: `/mnt/user-data/`
+        """
+        return self.thread_dir(thread_id) / "user-data"
+
+    def ensure_thread_dirs(self, thread_id: str) -> None:
+        """Create all standard sandbox directories for a thread."""
+        self.sandbox_work_dir(thread_id).mkdir(parents=True, exist_ok=True)
+        self.sandbox_uploads_dir(thread_id).mkdir(parents=True, exist_ok=True)
+        self.sandbox_outputs_dir(thread_id).mkdir(parents=True, exist_ok=True)
+
+    def resolve_virtual_path(self, thread_id: str, virtual_path: str) -> Path:
+        """Resolve a sandbox virtual path to the actual host filesystem path.
+
+        Args:
+            thread_id: The thread ID.
+            virtual_path: Virtual path as seen inside the sandbox, e.g.
+                          ``/mnt/user-data/outputs/report.pdf``.
+                          Leading slashes are stripped before matching.
+
+        Returns:
+            The resolved absolute host filesystem path.
+
+        Raises:
+            ValueError: If the path does not start with the expected virtual
+                        prefix or a path-traversal attempt is detected.
+        """
+        stripped = virtual_path.lstrip("/")
+        prefix = VIRTUAL_PATH_PREFIX.lstrip("/")
+
+        # Require an exact segment-boundary match to avoid prefix confusion
+        # (e.g. reject paths like "mnt/user-dataX/...").
+        if stripped != prefix and not stripped.startswith(prefix + "/"):
+            raise ValueError(f"Path must start with /{prefix}")
+
+        relative = stripped[len(prefix) :].lstrip("/")
+        base = self.sandbox_user_data_dir(thread_id).resolve()
+        actual = (base / relative).resolve()
+
+        try:
+            actual.relative_to(base)
+        except ValueError:
+            raise ValueError("Access denied: path traversal detected")
+
+        return actual
+
+
+# ── Singleton ────────────────────────────────────────────────────────────
+
+_paths: Paths | None = None
+
+
+def get_paths() -> Paths:
+    """Return the global Paths singleton (lazy-initialized)."""
+    global _paths
+    if _paths is None:
+        _paths = Paths()
+    return _paths