[Merge] ~jchittum/livecd-rootfs:sru-vagrant-slow-boot-workaround into livecd-rootfs:ubuntu/focal

[John Chittum]

John Chittum has proposed merging ~jchittum/livecd-rootfs:sru-vagrant-slow-boot-workaround into livecd-rootfs:ubuntu/focal.

Commit message:
ubuntu-cpc:Vagrant Serial to Null LP: (#1874453)
    
Original fix proposed by Stanislav German-Evtushenko (giner)
    
CPC Ubuntu cloud images default to enabling a serial console connection
via the kernel commandline option `console=ttyS0`. Many clouds support
the serial connection, and utilize it for debugging purposes. Virtualbox
supports the serial connection as well. In Bionic and earlier images,
Vagrant boxes created a serial log file in the directory of the
Vagrantfile by default. However this is not standard behaviour for
Vagrant images, and so it was removed in Eoan onwards.
    
Starting in Eoan, there were reports of image booting slowdown (1874453
is a single example). After testing, it was determined that the serial
connection starting, without a device attached, was the cause of the
slow down. However, we did not want to revert to the old functionality
of creating a file. Much thanks to <giner> for providing the Ruby syntax
for sending to File::NULL.
  
This option will not create a local file, however, the default
Vagrantfile configuration is overwritable via a users Vagrantfile. The
original syntax for creating a file local to the users Vagrantfile has
been included as an example.



Requested reviews:
  Ubuntu Core Development Team (ubuntu-core-dev)

For more details, see:
https://code.launchpad.net/~jchittum/livecd-rootfs/+git/livecd-rootfs/+merge/391169

SRU Information:

[IMPACT]

* Users are unable to boot a vagrant image, as the boot time without a serial console connection exceeds the vagrant timeout
* bug is focal as well as latest devel devel release.
* Upload fixes the bug by creating a serial console connection to a null file

[TEST CASE]

* run livecd-rootfs from focal branch with vagrant target
* download and add the create vagrant box (vagrant box add <PATH TO BOX> --name focal-test-image
* vagrant init
* update Vagrant file to use focal-test-image as the image
* observe that image successfully boots (should boot in < 60s)
* vagrant halt
* update vagrant file to have a new serial connection and send to a log (this shows that the default NULL connection can be overridden by a local user. an example is in box Vagrantfile, findable in ~/.vagrant.d/boxes/focal-test-image/0/virtualbox/Vagrantfile)
* vagrant up and observe that a serial connection has been established, and a log is written

[Regression Potential]

* possibility of incorrect configuration, leading to either the serial console not engaging properly, or a file being written instead of sent to a NULL file.
-- 
Your team Ubuntu Core Development Team is requested to review the proposed merge of ~jchittum/livecd-rootfs:sru-vagrant-slow-boot-workaround into livecd-rootfs:ubuntu/focal.
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