lib/model, lib/osutil: Verify target directory before pulling / requesting

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3798

lib/osutil/isdir.go

@@ -0,0 +1,49 @@
+// Copyright (C) 2016 The Syncthing Authors.
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this file,
+// You can obtain one at http://mozilla.org/MPL/2.0/.
+
+package osutil
+
+import (
+	"os"
+	"path/filepath"
+	"strings"
+)
+
+// IsDir returns true if base and every path component of name up to and
+// including filepath.Join(base, name) is a directory (and not a symlink or
+// similar). Base and name must both be clean and name must be relative to
+// base.
+func IsDir(base, name string) bool {
+	path := base
+	info, err := Lstat(path)
+	if err != nil {
+		return false
+	}
+	if !info.IsDir() {
+		return false
+	}
+
+	if name == "." {
+		// The result of calling IsDir("some/where", filepath.Dir("foo"))
+		return true
+	}
+
+	parts := strings.Split(name, string(os.PathSeparator))
+	for _, part := range parts {
+		path = filepath.Join(path, part)
+		info, err := Lstat(path)
+		if err != nil {
+			return false
+		}
+		if info.Mode()&os.ModeSymlink != 0 {
+			return false
+		}
+		if !info.IsDir() {
+			return false
+		}
+	}
+	return true
+}