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16 Oct 2016 : Fixing snap apps with relocatable DATADIRS #

It's luminous, not florescent

Recently I've been exploring how to create snaps of some of my applications. Snap is the new 'universal packaging format' that Canonical is hoping will become the default way to deliver apps on Linux. The idea is to package up an app with all of its dependencies (everything needed apart from ubuntu-core), then have the app deployed in a read-only container. The snap creator gets to set what are essentially a set of permissions for their app, with the default preventing it from doing any damage (either to itself or others). However, it's quite possible to give enough permissions to allow a snap to do bad stuff, so we still have to trust the developers of the snap (or spend your life reading through source-code to check for yourself). If you want to know more about how snaps work, the material out there is surprisingly limited right now. Most of the good stuff - and happily it turns out to be excellent - can be found at snapcraft.io.

Predictably the first thing I tried out was creating a snap for functy, which means you can now install it just be typing 'snap install functy' on Yakkety Yak. If your application already uses one of the conventional build systems like cmake or autotools, creating a snap is pretty straightforward. If it's a command-line app, just specifying a few details in a yaml file may well be enough. Here's an example for a fictional utility called useless, which you can get hold of from GitLab if you're interested (the code isn't fictional, but the utility is!).

The snapcraft file for this looks like this.
 
name: useless
version: 0.0.1
summary: A poem transformation program
description:
  Has a very limited purpose. It's mostly an arbitrary example of code.
confinement: strict

apps:
  useless:
    command: useless
    plugs: []

parts:
  useless:
    plugin: autotools
    source: https://gitlab.com/flypig/useless.git
    build-packages:
      - pkg-config
      - libpcre2-dev
    stage-packages:
      - libpcre2-8-0
    after: []

This just specifies the build system (plugin), some general description, the repository for the code (source), a list of build and runtime dependencies (build-packages and stage-packages respectively) and the command to actually run the utility (command).

This really is all you need. To test it just copy the lot into a file called snapcraft.yaml, then enter this command while in the same directory.
 
snapcraft cleanbuild

And a snap is born.

This will create a file called useless_0.0.1_amd64.snap which you can install just fine. When you try to execute it things will go wrong though: you'll get some output like this.
 
flypig@Owen:~/Documents/useless/snap$ snap install --force-dangerous useless_0.0.1_amd64.snap

useless 0.0.1 installed
flypig@Owen:~/Documents/useless/snap$ useless
Opening poem file: /share/useless/dong.txt

Couldn't open file: /share/useless/dong.txt

The dong.txt file contains the Edward Lear poem "The Dong With a Luminous Nose". It's a great poem, and the utility needs it to execute properly. This file can be found in the assets folder, installed to the $(datadir)/@PACKAGE@ folder as specified in assets/Makefile.am:
 
uselessdir = $(datadir)/@PACKAGE@
useless_DATA = dong.txt COPYING
EXTRA_DIST = $(useless_DATA)

In practice the file will end up being installed somewhere like /usr/local/share/useless/dong.txt depending on your distribution. One of the nice things about using autotools is that neither the developer not the user needs to know exactly where in advance. Instead the developer can set a compile-time define that autotools will fill and embed in the app at compile time. Take a look inside src/Makefile.am:
 
bin_PROGRAMS = ../useless
___useless_SOURCES = useless.c

___useless_LDADD = -lm @USELESS_LIBS@

___useless_CPPFLAGS = -DUSELESSDIR=\"$(datadir)/@PACKAGE@\" -Wall @USELESS_CFLAGS@

Here we can see the important part which sets the USELESSDIR macro define. Prefixing this in front of a filename string literal will ensure our data gets loaded from the correct place, like this (from useless.c)
 
char * filename = USELESSDIR "/dong.txt";

If we were to package this up as a deb or rpm package this would work fine. The application and its data get stored in the same place and the useless app can find the data files it needs at runtime

Snappy does things differently. The files are managed in different ways at build-time and run-time, and the $(datadir) variable can't point to two different places depending on the context. As a result the wrong path gets baked into the executable and when you run the snap it complains just like we saw above. The snapcraft developers have a bug registered against the snapcraft package explaining this. Creating a generalised solution may not be straightforward, since many packages - just like functy - have been created on the assumption the build and run-time paths will be the same.

One solution is to allow the data directory location to be optionally specified at runtime as a command-line parameter. This is the approach I settled on for functy. If you want to snap an application that also has this problem, it may be worth considering something similar.

The first change needed is to add a suitable command line argument (if you're packaging someone else's application, check first in case there already is one; it could save you a lot of time!). The useless app didn't previously support any command line arguments, so I augmented it with some argp magic. Here's the diff for doing this. There's a fair bit of scaffolding required, but once in, adding or changing the command line arguments in the future becomes far easier.

The one part of this that isn't quite boilerplate is the following generate_data_path function.
 
char * generate_data_path (char const * leaf, arguments const * args) {
    char * result = NULL;
    int length;

    if (leaf) {
        length = snprintf(NULL, 0, "%s/%s", args->datadir, leaf);
        result = malloc(length + 2);
        snprintf(result, length + 2, "%s/%s", args->datadir, leaf);
    }

    return result;
}

This takes the leafname of the data file to load and patches together the full pathname using the path provided at the command line. It's simple stuff, the only catch is to remember to free the memory this function allocates after it's been called.

For functy I'm using GTK, so I use a combination of GOptions for command line parsing and GString for the string manipulation. The latter in particular makes for much cleaner and safe code, and helps simplify the memory management of this generate_data_path function.

Now we can execute the app and load in the dong.txt file from any location we choose.
 
useless --datadir=~/Documents/Development/Projects/useless/assets

There's one final step, which is to update the snapcraft file so that this gets added automatically when the snap-installed app is run. The only change now is set the executed command as follows.
 
    command: useless --datadir="${SNAP}/share/useless"

Here's the full, updated, snapcraft file.
 
name: useless
version: 0.0.1
summary: A poem transformation program
description:
  Has a very limited purpose. It's mostly an arbitrary example of code.
confinement: strict

apps:
  useless:
    command: useless --datadir="${SNAP}/share/useless"
    plugs: []

parts:
  useless:
    plugin: autotools
    source: https://gitlab.com/flypig/useless.git
    build-packages:
      - pkg-config
      - libpcre2-dev
    stage-packages:
      - libpcre2-8-0
    after: []

And that's it! Install the snap package, execute it (just by typing 'useless') and the utility will run and find it's needed dong.txt file.

There's definitely a sieve involved
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2 Oct 2016 : Server time-travel, upgrading four years in nine days #
Years of accumulation has left me with a haphazard collection of networked computers at home, from a (still fully working) 2002 Iyonix through to the ultrabook used as my daily development machine. There are six serious machines connected to the network, if you don't count the penumbra of occasional and IoT devices (smart TV, playstation, Raspberry Pis, retired laptops, A9).

All of this is ably managed by Constantia, my home server. As it explains on her webpage, Constantia's a small 9 Watt fanless server I bought in 2011, designed to be powered using solar panels in places like Africa with more sun than infrastructure (and not in a Transcendence kind of way).

Constantia's physical presence

Although Constantia's been doing a phenomenal job, until recently she was stuck running Ubuntu 12.04 LTS Precise Pangolin. Since there's no fancy graphics card and 12.04 was the last version of Ubuntu to support Unity 2D, I've been reticent to upgrade. A previous upgrade from 8.04 to 10.04 many years ago - when Constantia inhabited a different body - caused me a lot of display trouble, so she has form in this regard.

Precise is due to fall out of support next year, and I'd already started having to bolt on a myriad PPAs to keep all the services pumped up to the latest versions. So during my summer break I decided to allocate some time to performing the upgrade, giving me the scope to fix any problems that might arise in the process.

This journey, which started on 19th September, finished today, a full two weeks later.

As expected, the biggest issue was Unity, although the surprise was that it ran at all. Unity has a software graphics-rendering fallback using LLVMPipe, which was actually bearable to use, at least for the small amount of configuration needed to get a replacement desktop environment up and running. After some research comparing XFCE, LXDE and Gnome classic (the official fallback option) I decided to go for XFCE: lightweight but also mature and likely to be supported for the foreseeable future. Having been running it for a couple of weeks, I'm impressed by how polished it is, although it's not quite up there with Unity in terms of tight integration.

The XFCE desktop running on Constantia with beautiful NASA background

There were also problems with some of the cloud services I have installed. MediaWiki has evaporated, but I was hardly using it anyway. The PPA-cruft needed to support ownCloud, which I use a lot, has been building up all over the place. Happily these have now been stripped back to the standard repos, which makes me feel much more comfortable. Gitolite, bind, SVN and the rest all transferred with only minor incident.

The biggest and most exciting change is that I've switched my server backups from USB external storage to Amazon AWS S3 (client-side encrypted, of course). Following a couple of excellent tutorials on configuring deja-dup to use S3 by Juan Domenech, and on S3 IAM permissions by Max Goodman, got things up and running.

But even with these great tutorials, it was a bit of a nail-bighting experience. My first attempt to back things up took five days continuous uploading to reach less than 50% before I decided to reconfigure. I've now got it down to a full backup in four days. By the end of it, I feared I might have to re-mortgage to pay the Amazon fees.

So, how much does it cost to upload and store 46 GiB? As it turns out, not so much: $1.06. I'm willing to pay that each month for effective off-site backup.

The upgrade of Constantia also triggered some other life-refactoring, including the moving of my software from SourceForge to GitLab, but that's a story for another time.

After all this, the good news is that Constantia is now fully operational an up-to-date with Ubuntu 16.04 LTS Xenial Xerus. This should get her all the way through to 2021. Kudos to the folks at Aleutia for creating a machine up to the task, and to Ubuntu for the unexpectedly smooth upgrade process.

The bad news is that nextCloud is now waxing as ownCloud wanes. It doesn't seem to yet be the right time to switch, but that time is approaching rapidly. At that point, I'll need another holiday.
 
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