Add a safe_unmount function and use it in enter_chroot.

src/scripts/common.sh

 # Copyright (c) 2010 The Chromium OS Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # Common constants for build scripts
 # This must evaluate properly for both /bin/bash and /bin/sh
 
 # All scripts should die on error unless commands are specifically excepted
 # by prefixing with '!' or surrounded by 'set +e' / 'set -e'.
 # TODO: Re-enable this once shflags is less prone to dying.
@@ skipping 319 matching lines @@
 function sudo_clobber() {
   sudo tee "$1" > /dev/null
 }
 
 # Writes stdin to the given file name as root using sudo in append mode.
 #
 # $1 - The output file name.
 function sudo_append() {
   sudo tee -a "$1" > /dev/null
 }
+
+# Unmounts a directory, if the unmount fails, warn, and then lazily unmount.
+#
+# $1 - The path to unmount.
+function safe_umount {
+  path=${1:?}
+  shift
+
+  if ! sudo umount -d "${path}"; then
+    warn "Failed to unmount ${path}"
+    warn "Doing a lazy unmount"
+
+    sudo umount -d -l "${path}" || die "Failed to lazily unmount ${path}"
+  fi
+}