Provide a GUI elevation dialog for GNU/Linux (#440)

Fixes: https://github.com/resin-io/etcher/issues/439
Fixes: https://github.com/resin-io/etcher/issues/412
Signed-off-by: Juan Cruz Viotti <jviottidc@gmail.com>

Etcher.desktop

@@ -1,6 +1,6 @@
 [Desktop Entry]
 Name=Etcher
-Exec=etcher.wrapper
+Exec=elevate.wrapper
 Comment=Burn images to SD cards & USB drives, safe & easy
 Icon=icon
 Type=Application

scripts/build/linux.sh

@@ -134,7 +134,8 @@ function app_dir_create {
   cp ./Etcher.desktop $output_directory
   cp ./assets/icon.png $output_directory
   cp -rf $source_directory/* $output_directory/usr/bin
-  cp ./scripts/desktopintegration $output_directory/usr/bin/etcher.wrapper
+  cp ./scripts/desktopintegration $output_directory/usr/bin/elevate.wrapper
+  cp ./scripts/elevate-linux.sh $output_directory/usr/bin/elevate
 }
 
 function installer {

scripts/elevate-linux.sh

@@ -0,0 +1,74 @@
+#!/bin/bash
+
+###
+# Copyright 2016 Resin.io
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#   http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+###
+
+set -e
+
+binary=usr/bin/etcher
+
+error() {
+  if [ -x /usr/bin/zenity ] ; then
+    LD_LIBRARY_PATH="" zenity --error --text "${1}" 2>/dev/null
+  elif [ -x /usr/bin/kdialog ] ; then
+    LD_LIBRARY_PATH="" kdialog --msgbox "${1}" 2>/dev/null
+  elif [ -x /usr/bin/Xdialog ] ; then
+    LD_LIBRARY_PATH="" Xdialog --msgbox "${1}" 2>/dev/null
+  else
+    echo "${1}"
+  fi
+  exit 1
+}
+
+# Check if we're running as root
+if [ "$EUID" -eq 0 ]; then
+  ./$binary
+else
+
+  # Determine a unique mountpoint based on the current mount point.
+  mountpoint=$(mount | grep $(basename $APPIMAGE) | awk '{ print $3 }')-elevated
+
+  # We remount the AppImage to be able to workaround FUSE a
+  # security measure of not allowing root to run binaries
+  # from FUSE mounted filesystems.
+  #
+  # - The `ro` option is required, since otherwise `mount` will
+  # refuse to mount the same AppImage in two different locations.
+  #
+  # - We don't want to go through the desktop integration helper
+  # again, so we call the `etcher` binary directly.
+  #
+  # - We need to wait for a little bit for `umount` to be
+  # successfull, otherwise it complains with an `EBUSY` error.
+  runcommand="mkdir -p $mountpoint && mount -o loop -o ro $APPIMAGE $mountpoint && $mountpoint/$binary; sleep 1; umount $mountpoint"
+
+  # We prefer gksudo since it gives a nicer looking dialog
+  if command -v gksudo 2>/dev/null; then
+    gksudo --preserve-env --description "Etcher" -- sh -c "$runcommand"
+  elif command -v kdesudo 2>/dev/null; then
+    kdesudo -d --comment "Etcher" -- sh -c "$runcommand"
+  elif command -v pkexec 2>/dev/null; then
+
+    # We need to inherit DISPLAY and XAUTHORITY, otherwise
+    # pkexec will not know how to run X11 applications.
+    # See http://askubuntu.com/a/332847
+    pkexec env DISPLAY=$DISPLAY XAUTHORITY=$XAUTHORITY sh -c "$runcommand"
+
+  else
+    error "Please install gksudo, kdesudo, or pkexec to run this application."
+  fi
+fi
+