[Bug 1932542] Re: Add support for zstd

[Dimitri John Ledkov]

(bionic-amd64)root at ottawa:/tmp# dpkg-query -W initramfs-tools liblz4-tool zstd
initramfs-tools	0.130ubuntu3.13
liblz4-tool	0.0~r131-2ubuntu3.1
zstd	1.3.3+dfsg-2ubuntu1.2

(bionic-amd64)root at ottawa:/tmp# mkinitramfs -c zstd -o zstd-initrd.img 4.15.0-150-generic
(bionic-amd64)root at ottawa:/tmp# mkinitramfs -c lz4 -o lz4-initrd.img 4.15.0-150-generic
(bionic-amd64)root at ottawa:/tmp# unmkinitramfs ./zstd-initrd.img zstd-unpacked
(bionic-amd64)root at ottawa:/tmp# unmkinitramfs ./lz4-initrd.img lz4-unpacked  
(bionic-amd64)root at ottawa:/tmp# ls zstd-unpacked/
bin  conf  etc  init  lib  lib64  run  sbin  scripts
(bionic-amd64)root at ottawa:/tmp# ls lz4-unpacked/
bin  conf  etc  init  lib  lib64  run  sbin  scripts

(bionic-amd64)root at ottawa:/tmp# file zstd-initrd.img 
zstd-initrd.img: Zstandard compressed data (v0.8+), Dictionary ID: None
(bionic-amd64)root at ottawa:/tmp# file lz4-initrd.img 
lz4-initrd.img: LZ4 compressed data (v0.1-v0.9)

All is good.

** Tags removed: verification-needed verification-needed-bionic
** Tags added: verification-done verification-done-bionic

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https://bugs.launchpad.net/bugs/1932542

Title:
  Add support for zstd

Status in initramfs-tools package in Ubuntu:
  Fix Released
Status in initramfs-tools source package in Bionic:
  Fix Committed
Status in initramfs-tools source package in Focal:
  Fix Released

Bug description:
  [Impact]

   * initramfs-tools in impish has changed default initrd compression to
  zstd. To allow compressing and uncompressing such initrds on Focal LTS
  release we should backport zstd support in the mkinitramfs &
  unmkinitramffs tooling. For example ubuntu-cdimage uses unmkinitramfs
  tool to unpack and inspect impish initrds on a focal host.

   * Cherrypicking the feature from 0.138 release, which is also present
  in Hirsute and Impish.

  [Test Plan]

   * Create initrd with zstd $ mkinitramfs -c zstd -o zstd-initrd.img

   * Uncompress it with $ unmkinitramfs ./zstd-initrd.img zstd-initrd-
  unpacked

   * For bionic also repeat the same with lz4.

  [Where problems could occur]

   * This adds support for zstd compressed initrd in both mkinitramfs
  and unmkinitramfs. However our GA kernels do not support zstd
  compressed initrds. And depends on zstd package is not added either.
  Meaning one still has to install zstd to gain the unpack support.
  Changing initramfs.conf to zstd may result in unbootable instance when
  used with GA kernel.

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