feat: add native WebDAV submodule

This commit introduces a new, high-performance, and extensible WebDAV submodule, implemented natively without external dependencies.

The submodule includes:
- A core WebDAV handler that supports essential methods: PROPFIND, MKCOL, GET, PUT, DELETE, COPY, MOVE, LOCK, and UNLOCK.
- An extensible design using a `FileSystem` interface to decouple the protocol logic from the storage backend.
- Two `FileSystem` implementations:
  - `MemFS`: An in-memory, tree-based filesystem for testing and ephemeral storage. It correctly handles path segments like `.` and `..`.
  - `OSFS`: A secure, OS-based filesystem that interacts with the local disk. It includes robust path traversal protection that correctly handles symbolic links.
- A `LockSystem` interface with an in-memory implementation (`MemLock`) to support resource locking (DAV Class 2). It includes a graceful shutdown mechanism to prevent goroutine leaks.
- RFC 4918 compliance for core operations, including correct status codes for `COPY`/`MOVE` and preventing `DELETE` on non-empty collections.
- Comprehensive unit tests covering all major functionalities.
- A working example application demonstrating how to mount and use the submodule with a local directory.

The Touka framework's core has been updated to recognize WebDAV-specific HTTP methods.

This implementation addresses numerous points from detailed code reviews, including security vulnerabilities, memory leaks, RFC compliance issues, and path handling bugs.

webdav/webdav.go

@@ -284,7 +284,39 @@ func (h *Handler) handleGetHead(c *touka.Context) {
 
 func (h *Handler) handleDelete(c *touka.Context) {
 	path, _ := c.Get("webdav_path")
-	if err := h.FileSystem.RemoveAll(c.Context(), path.(string)); err != nil {
+	pathStr := path.(string)
+
+	info, err := h.FileSystem.Stat(c.Context(), pathStr)
+	if err != nil {
+		if os.IsNotExist(err) {
+			c.Status(http.StatusNotFound)
+		} else {
+			c.Status(http.StatusInternalServerError)
+		}
+		return
+	}
+
+	if info.IsDir() {
+		file, err := h.FileSystem.OpenFile(c.Context(), pathStr, os.O_RDONLY, 0)
+		if err != nil {
+			c.Status(http.StatusInternalServerError)
+			return
+		}
+		defer file.Close()
+
+		// Check if the directory has any children. Readdir(1) is enough.
+		children, err := file.Readdir(1)
+		if err != nil && err != io.EOF {
+			c.Status(http.StatusInternalServerError)
+			return
+		}
+		if len(children) > 0 {
+			c.Status(http.StatusConflict) // 409 Conflict for non-empty collection
+			return
+		}
+	}
+
+	if err := h.FileSystem.RemoveAll(c.Context(), pathStr); err != nil {
 		if os.IsNotExist(err) {
 			c.Status(http.StatusNotFound)
 		} else {
@@ -347,11 +379,13 @@ func (h *Handler) handleCopy(c *touka.Context) {
         overwrite = "T" // Default is to overwrite
     }
 
-    if overwrite == "F" {
-        if _, err := h.FileSystem.Stat(c.Context(), destPath); err == nil {
-            c.Status(http.StatusPreconditionFailed)
-            return
-        }
+    // Check for existence before the operation to determine status code later.
+	_, err = h.FileSystem.Stat(c.Context(), destPath)
+	existed := err == nil
+
+    if overwrite == "F" && existed {
+        c.Status(http.StatusPreconditionFailed)
+        return
     }
 
     if err := h.copy(c.Context(), srcPath.(string), destPath); err != nil {
@@ -359,7 +393,11 @@ func (h *Handler) handleCopy(c *touka.Context) {
         return
     }
 
-    c.Status(http.StatusCreated)
+	if existed {
+		c.Status(http.StatusNoContent)
+	} else {
+		c.Status(http.StatusCreated)
+	}
 }
 
 func (h *Handler) handleMove(c *touka.Context) {
@@ -382,11 +420,13 @@ func (h *Handler) handleMove(c *touka.Context) {
         overwrite = "T" // Default is to overwrite
     }
 
-    if overwrite == "F" {
-        if _, err := h.FileSystem.Stat(c.Context(), destPath); err == nil {
-            c.Status(http.StatusPreconditionFailed)
-            return
-        }
+    // Check for existence before the operation to determine status code later.
+	_, err = h.FileSystem.Stat(c.Context(), destPath)
+	existed := err == nil
+
+    if overwrite == "F" && existed {
+        c.Status(http.StatusPreconditionFailed)
+        return
     }
 
     if err := h.FileSystem.Rename(c.Context(), srcPath.(string), destPath); err != nil {
@@ -394,7 +434,11 @@ func (h *Handler) handleMove(c *touka.Context) {
         return
     }
 
-    c.Status(http.StatusCreated)
+	if existed {
+		c.Status(http.StatusNoContent)
+	} else {
+		c.Status(http.StatusCreated)
+	}
 }
 
 func (h *Handler) copy(ctx context.Context, src, dest string) error {