lib/scanner, lib/fs: Don't create file infos with abs paths (fixes #4799) (#4800)

lib/fs/filesystem.go

@@ -198,3 +198,39 @@ func IsInternal(file string) bool {
 	}
 	return false
 }
+
+// Canonicalize checks that the file path is valid and returns it in the "canonical" form:
+// - /foo/bar -> foo/bar
+// - / -> "."
+func Canonicalize(file string) (string, error) {
+	pathSep := string(PathSeparator)
+
+	if strings.HasPrefix(file, pathSep+pathSep) {
+		// The relative path may pretend to be an absolute path within
+		// the root, but the double path separator on Windows implies
+		// something else and is out of spec.
+		return "", ErrNotRelative
+	}
+
+	// The relative path should be clean from internal dotdots and similar
+	// funkyness.
+	file = filepath.Clean(file)
+
+	// It is not acceptable to attempt to traverse upwards.
+	switch file {
+	case "..":
+		return "", ErrNotRelative
+	}
+	if strings.HasPrefix(file, ".."+pathSep) {
+		return "", ErrNotRelative
+	}
+
+	if strings.HasPrefix(file, pathSep) {
+		if file == pathSep {
+			return ".", nil
+		}
+		return file[1:], nil
+	}
+
+	return file, nil
+}