enter_chroot: introduce a sync process that synchronizes given files between chroot and host

enter_chroot.sh

 #!/bin/bash
 
 # Copyright (c) 2011 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.
 
 # Script to enter the chroot environment
 
 # --- BEGIN COMMON.SH BOILERPLATE ---
 # Load common CrOS utilities.  Inside the chroot this file is installed in
@@ skipping 111 matching lines @@
 INNER_CHROME_ROOT=$FLAGS_chrome_root_mount  # inside chroot
 CHROME_ROOT_CONFIG="/var/cache/chrome_root"  # inside chroot
 INNER_DEPOT_TOOLS_ROOT="/home/$USER/depot_tools"  # inside chroot
 FUSE_DEVICE="/dev/fuse"
 AUTOMOUNT_PREF="/apps/nautilus/preferences/media_automount"
 SAVED_AUTOMOUNT_PREF_FILE="/tmp/.automount_pref"
 
 sudo chmod 0777 "$FLAGS_chroot/var/lock"
 
 LOCKFILE="$FLAGS_chroot/var/lock/enter_chroot"
+SYNCERPIDFILE="${FLAGS_chroot}/var/tmp/enter_chroot_sync.pid"
 
 
 function ensure_mounted {
   # If necessary, mount $source in the host FS at $target inside the
   # chroot directory with $mount_args.
   local source="$1"
   local mount_args="$2"
   local target="$3"
 
   local mounted_path="$(readlink -f "${FLAGS_chroot}/$target")"
 
   if [ -z "$(mount | grep -F "on ${mounted_path} ")" ]; then
     # Attempt to make the mountpoint as the user.  This depends on the
     # fact that all mountpoints that should be owned by root are
     # already present.
     mkdir -p "${mounted_path}"
 
     # NB:  mount_args deliberately left unquoted
     debug mount ${mount_args} "${source}" "${mounted_path}"
     sudo -- mount ${mount_args} "${source}" "${mounted_path}" || \
       die "Could not mount ${source} on ${mounted_path}"
   fi
 }
 
+function env_sync_proc {
+  # This function runs and performs periodic updates to the chroot env, if
+  # necessary.
+
+  local poll_interval=10
+  local sync_files="etc/resolv.conf etc/hosts"
+
+  # Make sure the synced files are writable by normal user, so that we
+  # don't have to sudo inside the loop.
+  for file in ${sync_files}; do
+    sudo chown ${USER} ${FLAGS_chroot}/${file} 1>&2
+  done
+
+  while true; do
+    # Sync files
+    for file in ${sync_files}; do
+      if ! cmp /${file} ${FLAGS_chroot}/${file} &> /dev/null; then
+        cp -f /${file} ${FLAGS_chroot}/${file}
+      fi
+    done
+
+    sleep ${poll_interval}
+  done
+}
+
 function setup_env {
   # Validate sudo timestamp before entering the critical section so that we
   # don't stall for a password while we have the lockfile.
   # Don't use sudo -v since that has issues on machines w/ no password.
   sudo echo "" > /dev/null
 
+  # Syncer proc, but only the first time
+  if ! [ -f "${SYNCERPIDFILE}" ] || \
+     ! [ -d /proc/$(cat "${SYNCERPIDFILE}") ]; then
+    debug "Starting sync process"
+    env_sync_proc &
+    echo $! > "${SYNCERPIDFILE}"
+    disown $!
+  fi
+
   (
     flock 200
     echo $$ >> "$LOCKFILE"
 
     debug "Mounting chroot environment."
     ensure_mounted none "-t proc" /proc
     ensure_mounted none "-t sysfs" /sys
     ensure_mounted /dev "--bind" /dev
     ensure_mounted none "-t devpts" /dev/pts
     ensure_mounted "${FLAGS_trunk}" "--bind" "${CHROOT_TRUNK_DIR}"
@@ skipping 104 matching lines @@
     if [ $(which gconftool-2 2>/dev/null) ]; then
       SAVED_PREF=$(cat "${FLAGS_chroot}${SAVED_AUTOMOUNT_PREF_FILE}")
       gconftool-2 -s --type=boolean ${AUTOMOUNT_PREF} ${SAVED_PREF} || \
         warn "could not re-set your automount preference."
     fi
 
     if [ -s "$LOCKFILE" ]; then
       debug "At least one other pid is running in the chroot, so not"
       debug "tearing down env."
     else
+      debug "Stopping syncer process"
+      kill $(cat "${SYNCERPIDFILE}")
+
       MOUNTED_PATH=$(readlink -f "$FLAGS_chroot")
       debug "Unmounting chroot environment."
       # sort the list of mounts in reverse order, to ensure umount of
       # cascading mounts in proper order
       for i in \
         $(mount | grep -F "on $MOUNTED_PATH/" | sort -r | awk '{print $3}'); do
         safe_umount "$i"
       done
     fi
   ) 200>>"$LOCKFILE" || die "teardown_env failed"
@@ skipping 84 matching lines @@
 # Run command or interactive shell.  Also include the non-chrooted path to
 # the source trunk for scripts that may need to print it (e.g.
 # build_image.sh).
 sudo -- chroot "$FLAGS_chroot" sudo -i -u $USER $CHROOT_PASSTHRU \
   EXTERNAL_TRUNK_PATH="${FLAGS_trunk}" LANG=C SSH_AGENT_PID="${SSH_AGENT_PID}" \
   SSH_AUTH_SOCK="${SSH_AUTH_SOCK}" "$@"
 
 # Remove trap and explicitly unmount
 trap - EXIT
 teardown_env