[Bug 1561606] Re: Re-enable multi-layer squashfs support

[Richard Nelson]

Greetings,

After more testing of the previous patch submitted, I discovered that I
had the lower stack order inverted. Hence I have removed that patch and
I am submitting this revised patch. This new patch has been tested with
multiple additional squashfs files and is in working order for casper
live operations.

** Patch added: "Tested with 1.376 and Yakkety"
   https://bugs.launchpad.net/ubuntu/+source/casper/+bug/1561606/+attachment/4702920/+files/casper.patch

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Title:
  Re-enable multi-layer squashfs support

Status in casper package in Ubuntu:
  Confirmed

Bug description:
  AFAIK, Casper supports booting when there is more than one squashfs
  file in the casper directory. At least, this _was_ supported under
  Ubuntu trusty which used aufs as union file system. Then there came
  utopic, wily, vivid which switched to overlayfs. This file system
  didn't support layering more than one ro sources in its early versions
  so that this feature didn't work anymore. The code in the casper
  script looks like a work-around about this by stacked mounting
  multiple overlayfs. Unfortunately, I've never seen a version of
  overlayfs which supported another overlayfs as source.

  Since kernel 3.19, overlayfs officially supports multiple layers
  (http://www.phoronix.com/scan.php?page=news_item&px=MTg1Nzg), so it's
  time to rewrite this piece of code in the casper script.

  Fully-working patch attached.

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