AU: Many minor cleanup changes

src/platform/installer/chromeos-postinst

 #!/bin/sh -e
 
 # Copyright (c) 2009 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.
 
 # This script is called after an AutoUpdate or USB install. The first argument
 # is the partition where the new rootfs is installed or empty. If non-empty
 # the rootfs should be updated w/ the new bootloader config. If empty, the
 # rootfs is mounted-read only and should not be updated.
 
 # Update /boot/extlinux.conf.
 INSTALL_ROOT=`dirname "$0"`
 INSTALL_DEV="$1"
+POSTCOMMIT="$2"
 
-# Only update extlinux.conf if $1 is non-empty
-if [ -n "$INSTALL_DEV" ]; then
-  # Set default label to chromeos-hd.
-  sed -i 's/^DEFAULT .*/DEFAULT chromeos-hd/' "$INSTALL_ROOT"/boot/extlinux.conf
-  sed -i "{ s:HDROOT:$INSTALL_DEV: }" "$INSTALL_ROOT"/boot/extlinux.conf
+if [ "$POSTCOMMIT" != "--postcommit" ]; then
+  # Pre-commit. Returning an error here will prevent ever booting into the
+  # installed system.
+
+  # If the mount-point is read-write, update the bootloader
+  # Only update extlinux.conf if $1 is non-empty
+  if [ -n "$INSTALL_DEV" ]; then
+    # Set default label to chromeos-hd.
+    sed -i 's/^DEFAULT .*/DEFAULT chromeos-hd/' \
+        "$INSTALL_ROOT"/boot/extlinux.conf || true
+    sed -i "{ s:HDROOT:$INSTALL_DEV: }" \
+        "$INSTALL_ROOT"/boot/extlinux.conf || true
+  fi
+
+else
+  # Post-commit. At this point an unexpected reboot may boot the installed
+  # system, but returning an error here will cause the updater to try to
+  # not boot the installed system, instead keeping the existing system.
+
 fi
 
+