Comment 28 for bug 575469

Yes it's quite a few changes to introduce pretty late in the cycle, the main reason though is getting recovery mode to actually act like one.
What we currently have basically shows you friendly-recovery at the end of the boot sequence when most of your services are already running and file systems are already mounted. I think we definitely need to fix that to make our recovery boot useful.

The renaming from single to recovery is needed to avoid additional changes into upstart, I think it's a good idea to split actual "single user mode" and "recovery mode". This essentially gives us:
 - when booting with "single" only => start some services + sysvinit scripts for runlevel S and give the user a root shell
 - when booting with "recovery" only => start friendly-recovery with a read-only filesystem, on resume, just continue a regular boot sequence
 - when booting with "single" and "recovery" => start friendly-recovery, on resume continue in single mode and give the user a root shell

I think this makes sense and the changes to grub and upstart are really limited and I think are pretty safe.
The changes to friendly-recovery itself are quite substantial to implement the read-only mode and switch to read/write but it's very localized change that affects only friendly-recovery and that I think can't get us a worse experience than what we had in the past.