local: don't resolve relative roots to absolute paths - fixes #9510

cleanRootPath used filepath.Abs which prepends the current directory,
but the resulting absolute path does not always refer to the same
directory as the original relative path - for example when the current
directory is shadowed by a mount or has been removed. This made
"rclone copy --links . ../dst" fail where "cp -ra . ../dst" succeeds.

rclone now cleans the path lexically on non-Windows platforms instead,
leaving relative roots relative so the OS resolves them against the live
working directory. Windows still makes the path absolute as required for
UNC long-path conversion.

backend/local/local.go

@@ -1726,9 +1726,18 @@ func cleanRootPath(s string, noUNC bool, enc encoder.MultiEncoder) string {
 	if runtime.GOOS == "windows" {
 		s = vol + s
 	}
-	s2, err := filepath.Abs(s)
-	if err == nil {
-		s = s2
+	// UNC paths on Windows must be absolute, so make the path absolute
+	// there. On other platforms filepath.Abs would prepend the current
+	// directory, but the resulting absolute string is not guaranteed to
+	// refer to the same directory as the original relative path - for
+	// example when the current directory is shadowed by a mount or has been
+	// removed - so just clean it lexically instead.
+	if runtime.GOOS == "windows" {
+		if s2, err := filepath.Abs(s); err == nil {
+			s = s2
+		}
+	} else {
+		s = filepath.Clean(s)
 	}
 	if !noUNC {
 		// Convert to UNC. It does nothing on non windows platforms.

backend/local/tests_test.go

@@ -30,3 +30,29 @@ func TestCleanWindows(t *testing.T) {
 		}
 	}
 }
+
+// Relative roots must stay relative so the OS resolves them against the
+// live working directory rather than a canonicalised string that may no
+// longer refer to the same directory (#9510).
+var testsRelative = [][2]string{
+	{".", "."},
+	{"./", "."},
+	{"sub/dir", "sub/dir"},
+	{"sub/dir/", "sub/dir"},
+	{"./sub/dir", "sub/dir"},
+	{"sub/../dir", "dir"},
+	{"..", ".."},
+}
+
+func TestCleanRootPathRelative(t *testing.T) {
+	if runtime.GOOS == "windows" {
+		t.Skipf("non-windows only")
+	}
+	for _, test := range testsRelative {
+		got := cleanRootPath(test[0], true, encoder.OS)
+		expect := test[1]
+		if got != expect {
+			t.Fatalf("got %q, expected %q", got, expect)
+		}
+	}
+}