cros_sdk: Allow additional chroot mounts via .local_mounts file

This change was coopted from http://codereview.chromium.org/5331009/,
originally written by hungte@.  And the coopted commit message:

It would be helpful if we could share some directories inside/outside the
chroot (e.g. editor configuration or the default Downloads directory).  This
CL reads .local_mounts (just like .default_boards) from the "src/scripts"
folder, and mounts the directories whenever you do cros_sdk.

For safety concern, and to prevent the developer from accidentally deleting
their mounted files, the mounts are made read-only.

.local_mounts has a very simple syntax:
  mount_path
  or source_path(outside chroot) destination_path(inside chroot)
  or # comments.

Examples:
/usr/share/vim/google
/home/XXX/Downloads /outside

BUG=chromium-os:34561
TEST=Manually:
1. Create ~/trunk/src/scripts/.local_mounts with following content:
 # comment here
/usr/share/vim/google  # test
/home/XXX/Downloads /outside
2. cros_sdk
3. ls -l /usr/share/vim/google/ # ensure dir is mounted correctly
   ls -l /outside/  # ensure dir is mounted correctly
4. exit
5. mount | grep chroot  # ensure nothing is left

Change-Id: I6f3400a436a825e8cdfcb18b788afe96ebba6757
Reviewed-on: https://gerrit.chromium.org/gerrit/33585
Tested-by: Michael Krebs <mkrebs@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Commit-Ready: Michael Krebs <mkrebs@chromium.org>

.gitignore

@@ -1,6 +1,7 @@
 # User files
 /.chromeos_dev
 /.default_board
+/.local_mounts
 /shared_user_passwd.txt
 
 # Generated files

sdk_lib/enter_chroot.sh

@@ -123,11 +123,13 @@ queue_mount() {
     # Already mounted!
     ;;
   *)
-    MOUNT_QUEUE+=(
-      "mkdir -p '${mounted_path}'"
-      # The args are left unquoted on purpose.
-      "mount ${mount_args} '${source}' '${mounted_path}'"
-    )
+    MOUNT_QUEUE+=( "mkdir -p '${mounted_path}'" )
+    # The args are left unquoted on purpose.
+    if [[ -n ${source} ]]; then
+      MOUNT_QUEUE+=( "mount ${mount_args} '${source}' '${mounted_path}'" )
+    else
+      MOUNT_QUEUE+=( "mount ${mount_args} '${mounted_path}'" )
+    fi
     ;;
   esac
 }
@@ -390,6 +392,27 @@ setup_env() {
       queue_mount "$DEPOT_TOOLS" --bind "$INNER_DEPOT_TOOLS_ROOT"
     fi
 
+    # Mount additional directories as specified in .local_mounts file.
+    local local_mounts="${FLAGS_trunk}/src/scripts/.local_mounts"
+    if [[ -f ${local_mounts} ]]; then
+      info "Mounting local folders (read-only for safety concern)"
+      # format: mount_source
+      #      or mount_source mount_point
+      #      or # comments
+      local mount_source mount_point
+      while read mount_source mount_point; do
+        if [[ -z ${mount_source} ]]; then
+          continue
+        fi
+        # if only source is assigned, use source as mount point.
+        : ${mount_point:=${mount_source}}
+        debug "  mounting ${mount_source} on ${mount_point}"
+        queue_mount "${mount_source}" "--bind" "${mount_point}"
+        # --bind can't initially be read-only so we have to do it via remount.
+        queue_mount "" "-o remount,ro" "${mount_point}"
+      done < <(sed -e 's:#.*::' "${local_mounts}")
+    fi
+
     process_mounts
 
     CHROME_ROOT="$(readlink -f "$FLAGS_chrome_root" || :)"