Frankenwheezy! Keeping wheezy alive on a container host running libc6 2.24

It’s Alive!

The day before yesterday (at Infoxchange, a non-profit whose mission is “Technology for Social Justice”, where I do a few days/week of volunteer systems & dev work), I had to build a docker container based on an ancient wheezy image. It built fine, and I got on with working with it.

Yesterday, I tried to get it built on my docker machine here at home so I could keep working on it, but the damn thing just wouldn’t build. At first I thought it was something to do with networking, because running curl in the Dockerfile was the point where it was crashing – but it turned out that many programs would segfault – e.g. it couldn’t run bash, but sh (dash) was OK.

I also tried running a squeeze image, and that had the same problem. A jessie image worked fine (but the important legacy app we need wheezy for doesn’t yet run in jessie).

After a fair bit of investigation, it turned out that the only significant difference between my workstation at IX and my docker machine at home was that I’d upgraded my home machines to libc6 2.24-2 a few days ago, whereas my IX workstation (also running sid) was still on libc6 2.23.

Anyway, the point of all this is that if anyone else needs to run a wheezy on a docker host running libc6 2.24 (which will be quite common soon enough), you have to upgrade libc6 and related packages (and any -dev packages, including libc6-dev, you might need in your container that are dependant on the specific version of libc6).

In my case, I was using docker but I expect that other container systems will have the same problem and the same solution: install libc6 from jessie into wheezy. Also, I haven’t actually tested installing jessie’s libc6 on squeeze – if it works, I expect it’ll require a lot of extra stuff to be installed too.

I built a new frankenwheezy image that had libc6 2.19-18+deb8u4 from jessie.

To build it, I had to use a system which hadn’t already been upgraded to libc6 2.24. I had already upgraded libc6 on all the machines on my home network. Fortunately, I still had my old VM that I created when I first started experimenting with docker – crazily, it was a VM with two ZFS ZVOLs, a small /dev/vda OS/boot disk, and a larger /dev/vdb mounted as /var/lib/docker. The crazy part is that /dev/vdb was formatted as btrfs (mostly because it seemed a much better choice than aufs). Disk performance wasn’t great, but it was OK…and it worked. Docker has native support for ZFS, so that’s what I’m using on my real hardware.

I started with the base wheezy image we’re using and created a Dockerfile etc to update it. First, I added deb lines to the /etc/apt/sources.list for my local jessie and jessie-updates mirror, then I added the following line to /etc/apt/apt.conf:

APT::Default-Release "wheezy";

Without that, any other apt-get installs in the Dockerfile will install from jesssie rather than wheezy, which will almost certainly break the legacy app. I forgot to do it the first time, and had to waste another 10 minutes or so building the app’s container again.

I then installed the following:

apt-get -t jessie install libc6 locales libc6-dev krb5-multidev comerr-dev zlib1g-dev libssl-dev libpq-dev

To minimise the risk of incompatible updates, it’s best to install the bare minimum of jessie packages required to get your app running. The only reason I needed to install all of those -dev packages was because we needed libpq-dev, which pulled in all the rest. If your app doesn’t need to talk to postgresql, you can skip them. In fact, I probably should try to build it again without them – I added them after the first build failed but before I remembered to set Apt::Default::Release (OTOH, it’s working OK now and we’re probably better off with libssl-dev from jessie anyway).

Once it built successfully, I exported the image to a tar file, copied it back to my real Docker machine (co-incidentally, the same machine with the docker VM installed) and imported it into docker there and tested it to make sure it didn’t have the same segfault issues that the original wheezy image did. No problem, it worked perfectly.

That worked, so I edited the FROM line in the Dockerfile for our wheezy app to use frankenwheezy and ran make build. It built, passed tests, deployed and is running. Now I can continue working on the feature I’m adding to it, but I expect there’ll be a few more yaks to shave before I’m finished.

When I finish what I’m currently working on, I’ll take a look at what needs to be done to get this app running on jessie. It’s on the TODO list at work, but everyone else is too busy – a perfect job for an unpaid volunteer. Wheezy’s getting too old to keep using, and this frankenwheezy needs to float away on an iceberg.