Wednesday, 10 February 2016

Android 6 semi-adopted storage

How to split your SD card between adopted internal storage and portable external storage.

(see how-to instructions below)

The pain


Fed up of waiting on Motorola for the Marshmallow upgrade on my XT1072, and being short of internal storage, and being very fed up of having to move-to-sd apps after upgrade (along with occasional tricks such as deleting all data for drive, google+, chrome to scavenge extra memory when clear-cache wasn't enough) I decided to upgrade to CyanogenMod 13 to get Marshmallow that way.

I was really looking forward to being able to use my SD card as internal memory and have no more problems about storage.

What really happens is that with Marshmallow you cannot move-to-sd without adopting your SD card as internal. And then the only apps that will move-to-sd (now called Change) are those that could already move-to-sd.

And one of those apps that won't move-to-sd (even after the adoption of the SD card as internal storage) is Google Music.

Only now there is no SD card for Google Music to store the music on. So it stores it on the *internal* memory.

Whaaaat! My music collection is way bigger than my app collection, how does storing my music internally instead of some apps (all of which could move to SD anyway) help anything?

After messing abuot with Links2SD, Apps2SD, root shells and mount points, reading about volume manager and changing API's I decided that even loopback file fat32 systems in the adopted storage probably wouldn't work.

So I looked at re-partitioning an adopted storage card to shrink the adopted partition to make room for a fat32 partition.

In searching how to access the encrypted partition outside of Android (so that I could resize the file system within it) I came across these notes https://nelenkov.blogspot.co.uk/2015/06/decrypting-android-m-adopted-storage.html?view=flipcard I came across the sm command which can create a mixed or public or private volume.

Mixed turned out to be exactly what I was looking for!



How to split your card

WARNING: Before you do this, be sure to eject the card from the Settings/Storage & USB menu.

WARNING: Regardless of whether you split your card, or just adopt as encrypted, or even just re-format as FAT, it may not be good for the life of your card. It seems that some SD cards have long-life flash allocated to the first few blocks where the FAT is kept. Using a non-FAT file system, or having the FAT file system further down the device loses that benefit, possibly even altering the pre-shipped format could do that too. [http://events.linuxfoundation.org/sites/events/files/slides/fs-for-embedded-full_0.pdf]

I found how to partition my SD card to give 8G as internal storage to which all apps that can be moved will be moved and leave ~20GB as portable storage to hold music, etc.

First, you need adb working, and your SD card inserted and formatted as portable.

$ adb shell sm list-disks adoptable
disk:179,64

disk:179,64 is my SD card that can be made adoptable, I want 90% as external SD:

$ adb shell sm partition disk:179,64 mixed 90

Note: Your card may be listed with an underscore _ instead of an underscore, e.g. disk:179_64 in which case, that is what you type.

This erases the entire SD card, and then gives me 90% as portable storage and the rest as adopted internal storage.

The partition table looks like this:
Number  Start   End     Size    File system  Name            Flags
 1      1049kB  57.5GB  57.5GB  fat32        shared          msftdata
 2      57.5GB  57.5GB  16.8MB               android_meta
 3      57.5GB  63.9GB  6369MB               android_expand


The fat32 partition is not encrypted and can be mounted on a computer (provided it can handle the new GUID partition table format).

I advise a reboot after setting the new music storage location this as Google Music may get the wrong idea about much space is available.

Rename the adopted storage. If it had the same name as the portable storage partition then it may prevent one of the partitions from being available over USB MTP.

For reasons I don't understand, my disk label gets set as some junk similar to this: 82^GM-^KM-^?N-q^Xa^Oo and although I can change this by inserting it into a computer: mlabel -i /dev/sdf1 :: if I put it back into my phone, it looks right until I soft-eject and re-insert it (from the menu) - and then the weird label is back.

I wonder if this can be avoided by swipe-dismissing the notice that a new SD card is discovered (which shows after the mixed partition is complete) instead of selecting it.

If you want apps to be installed on the adopted SD partition by default, then you need to choose the Migrate Data option from the menu:



For CM13 this seems to work as an alternative to my original suggestion below which required the phone to be rooted, but for stock it seems ineffective.


# You won't need this if you chose migrate data above
$ adb root pm set-install-location 2

to have apps installed on the storage by default where possible. It is very effective. (Location 1 means internal, and 0 means auto-choose, but I don't know on what criteria).

However that command requires you to have rooted your phone. I wish this could be set another way.

(If you want to root your XT1072, follow the top-post instructions here: http://forum.xda-developers.com/moto-g-lte/general/root-marshmallow-stock-rom-t3317859)

This lets you use USB devices as adopted/portable (if you are rooted):
$ adb shell sm set-force-adoptable true

Monday, 1 February 2016

Using flock in bash without invoking a subshell

flock -c can call external commands but not bash functions. Consequently users mess about with file descriptors, and often making a mess of it.

Inspired by Ivan's post http://blog.famzah.net/2013/07/31/using-flock-in-bash-without-invoking-a-subshell I've written a flock wrapper for bash, that uses flock underneath but allows flock -c to work for bash functions.

It can be called just like the regular flock command, with the benefit that the -c invocation is supported for bash functions; so you can use it like this:

flock -o /tmp/process my_thing "$@"

and I strongly recommend the -o option so that the file descriptor used for the lock is not passed to any sub-processes, which could be problematic if long live sub-processes (e.g. re-spawned daemons) keep it open.

It will pass through and invoke the regular flock if the command isn't a bash function, or if you aren't trying to execute a function.

Sadly, it doesn't recognize bash built-in's.

But the good news is, you can use flock -o ...file... on a shell function inside your shell script without having to worry about file descriptors.

# Helper function (in order to preserve $@ in the caller)
# If this isn't used to call a shell function then return 0
# otherwise return $? as the argumment number which represents
# the command/function to be called
_is_standard_flock() {
  local args=$#
  # find the first argument that doesn't begin with a -
  while test $# != 0
  do case "$1" in
     -*) shift ; continue ;;
     *) break ;;
     esac
  done

  # if it is numeric and there are not additional arguments 
  test $# = 1 -a -n "$1" -a -z "${1//[0-9]}" && return 0
  shift
  # (skipping -c if present)
  # or the following argument is also not a shell function then use the original flock
  if test "$1" = "-c"
  then declare -F "$2" >/dev/null || return 0
  else declare -F "$1" >/dev/null || return 0
  fi

  # we can't have shifted many args if this is a legitimate use of flock
  # so we will be in range of the exit code
  return $(( args - $# + 1))
}

# Help function to determine if -o or --close was given in the flock arguments
_wants_close() {
  test "${*/#--close/}" != "$*" && return # will also match bogus arguments like --closed
  # remove any -- options
  set -- "${@/#--*/}"
  # look for options with o
  test "${*/#-*o/}" != "$*" && return
  return 1
}

flock() {
  if _is_standard_flock "$@"
  then : # do outside the if-clause so bash can optimise exec where possible
  else # save the exit code (offset) as $1
       set -- $? "$@"
       # ${!$1} is the lock file 
       # ${@:$(($1 + 1))} might be -c
       test "${@:$(($1 + 1)):1}" = "-c" && set -- "${@:1:$1}" "${@:$(($1 + 2))}"
       if _wants_close "${*:2:$(( $1 - 1))}"
       then { set -- "$1" "$_" "${@:2}" ; command flock "${@:3:$(( $1 - 2))}" $2 && eval '"${@:$(( $1 + 2))}"' "$2>&-" ; set -- $? $2 ; command flock -u $2 ; return $1 ; } {_}<"${!1}"
       else { set -- "$1" "$_" "${@:2}" ; command flock "${@:3:$(( $1 - 2))}" $2 &&       "${@:$(( $1 + 2))}"          ; set -- $? $2 ; command flock -u $2 ; return $1 ; } {_}<"${!1}"
       fi
  fi
  command flock "$@"
}

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