Single-stage installer

Hot on the heels of Joey's tale of getting rid of base-config (the second stage of the installer) in Debian, we've now pretty much got rid of it in Ubuntu Dapper too. The upshot of this is that rather than asking a bunch of questions, installing the base system, and rebooting to install everything else, we now just install everything in one go and reboot into a completed system.

This does mean that, if your system doesn't boot, you don't get to find out about it for a bit longer. However, it has lots of advantages in terms of speed (the much-maligned archive-copier mostly goes away), reducing code duplication (base-config had a bunch of infrastructure of its own which was done better in the core installer anyway), comprehensibility, and killing off some annoying bugs like #13561 (duplicate mirror questions in netboot installs), #15213 (second stage hangs if you skip archive-copier in the first stage), and #19571 (kernel messages scribble over base-config's UI).

To go with Joey's Debian timeline, the Ubuntu history looks a bit like this:

• 2004 (Jul): First base-config modifications for Ubuntu; we need to install the default desktop rather than dropping into tasksel.
• 2004 (Aug): Mark phones me up and asks if I can make the installer not need the CD in the second stage by copying all the packages across beforehand. Although it's a bit awkward, I can see the UI advantages in that, so I write archive-copier at the Canonical conference in Oxford.
• 2004 (Sep): Mark asks me if we can ask the timezone, user/password, and apt configuration questions before the first reboot. With less than a month to go until our first release, I have a heart-attack at how much needs to be done, and it eventually gets deferred to Hoary.
• 2005 (Jan): Matt fixes up debconf's passthrough frontend for use on the live CD, and we realise that this is an obvious way to run bits of base-config before the first reboot. It's rather messy and takes until March or so before it really works right, but we get there in the end.
• 2005 (Apr): I get "put a progress bar in front of the dpkg output in the second stage" as a goal for Breezy. Naïvely, I think it's a simple matter of programming, since I'd already done something similar for debootstrap and base-installer the previous year.
• 2005 (May): I hack progress bar support into debconf. Nothing actually uses it for anything yet, except as a convenient passthrough stub.
• 2005 (Jul/Aug): I actually try to implement the second-stage progress bar and realise that it's about an order of magnitude harder than I thought, requiring a whole load of extra infrastructure in apt. Fortunately Michael Vogt saves the day here by writing lots of working code, and the progress bar works by early August.
• 2005 (Sep-Dec): Upstream d-i development ramps back up again, with tzsetup, clock-setup, apt-setup, and user-setup all being cranked out in short order and the corresponding pieces removed from base-config. I merge these as they mature, and manage to get agreement on including the Ubuntu debconf template changes in upstream apt-setup, which helps the diff size a lot.
• 2005 (Nov/Dec): Joey and I chat one evening about the Ubuntu second-stage progress bar work, and we end up designing and writing debconf-apt-progress based on its ideas, after which Joey knocks up pkgsel in no time flat.
• 2006 (Jan): The rest of the pieces land in Ubuntu, and we drop base-config out of the installer. To my surprise, nearly everything still just works.

Although it caused some friction, I'm glad that we did the first cuts of many of these things outside Debian and got to try things out before landing version-2-quality code in Debian. The end result is much nicer than the intermediate ones ever were.

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