The Snapcraft Parts Ecosystem


Today I am going to be discussing parts. This is one of the pillars of snapcraft (together with plugins and the lifecycle).

For those not familiar, this is snapcraft’s general purpose landing page, http://snapcraft.io/ but if you are a developer and have already been introduced to this new world of snaps, you probably want to just go and hop on to http://snapcraft.io/create/

If you go over this snapcraft tour you will notice the many uses of parts and start to wonder how to get started or think that maybe you are duplicating work done by others, or even better, maybe an upstream. This is where we start to think about the idea of sharing parts and this is exactly what we are going to go over in this post.

To be able to reproduce what follows, you’d need to have snapcraft 2.12 installed.

An overview to using remote parts

So imagine I am someone wanting to use libcurl. Normally I would write the part definition from scratch and be on with my own business but surely I might be missing out on something about optimal switches used to configure the package or even build it. I would also need to research on how to use the specific plugin required. So instead, I’ll see if someone already has done the work for me, hence I will,

$ snapcraft update
Updating parts list... |
$ snapcraft search curl
PART NAME  DESCRIPTION
curl       A tool and a library (usable from many languages) for client side URL tra...

Great, there’s a match, but is this what I want?

$ snapcraft define curl
Maintainer: 'Sergio Schvezov <sergio.schvezov@ubuntu.com>'
Description: 'A tool and a library (usable from many languages) for client side URL transfers, supporting FTP, FTPS, HTTP, HTTPS, TELNET, DICT, FILE and LDAP.'

curl:
  configflags:
  - --enable-static
  - --enable-shared
  - --disable-manual
  plugin: autotools
  snap:
  - -bin
  - -lib/*.a
  - -lib/pkgconfig
  - -lib/*.la
  - -include
  - -share
  source: http://curl.haxx.se/download/curl-7.44.0.tar.bz2
  source-type: tar

Yup, it’s what I want.

An example

There are two ways to use these parts in your snapcraft.yaml, say this is your parts section

parts:
    client:
       plugin: autotools
       source: .

My client part which is using sources that sit alongside this snapcraft.yaml, will hypothetically fail to build as it depends on the curl library I don’t yet have. There are some options here to get this going, one using after in the part definition implicitly, another involving composing and last but not least just copy pasting what snapcraft define curl returned for the part.

Implicitly

The implicit path is really straightforward. It only involves making the part look like:

parts:
    client:
       plugin: autotools
       source: .
       after: [curl]

This will use the cached definition of the part and may potentially be updated by running snapcraft update.

Composing

What if we like the part, but want to try out a new configure flag or source release? Well we can override pieces of the part; so for the case of wanting to change the source:

parts:
    client:
        plugin: autotools
        source: .
        after: [curl]
    curl:
        source: http://curl.haxx.se/download/curl-7.45.0.tar.bz2

And we will get to build curl but using a newer version of curl. The trick is that the part definition here is missing the plugin entry, thereby instructing snapcraft to look for the full part definition from the cache.

Copy/Pasting

This path is a path one would take if they want full control over the part. It is as simple as copying in the part definition we got from running snapcraft define curl into your own. For the sake of completeness here’s how it would look like:

parts:
    client:
        plugin: autotools
        source: .
        after: [curl]
    curl:
        configflags:
            - --enable-static
            - --enable-shared
            - --disable-manual
        plugin: autotools
        snap:
            - -bin
            - -lib/*.a
            - -lib/pkgconfig
            - -lib/*.la
            - -include
            - -share
        source: http://curl.haxx.se/download/curl-7.44.0.tar.bz2
        source-type: tar

Sharing your part

Now what if you have a part and want to share it with the rest of the world? It is rather simple really, just head over to https://wiki.ubuntu.com/snapcraft/parts and add it.

In the case of curl, I would write a yaml document that looks like:

origin: https://github.com/sergiusens/curl.git
maintainer: Sergio Schvezov <sergio.schvezov@ubuntu.com>
description:
  A tool and a library (usable from many languages) for
  client side URL transfers, supporting FTP, FTPS, HTTP,
  HTTPS, TELNET, DICT, FILE and LDAP.
project-part: curl

What does this mean? Well, the part itself is not defined on the wiki, just a pointer to it with some meta data, the part is really defined inside a snapcraft.yaml living in the origin we just told it to use.

The extent of the keywords is explained in the documentation, that is an upstream link to it.

The core idea is that a maintainer decides he wants to share a part. Such a maintainer would add a description that provides an idea of what that part (or collection of parts) is doing. Then, last but not least, the maintainer declares which parts to expose to the world as maybe not all of them should. The main part is exposed in project-part and will carry a top level name, the maintainer can expose more parts from snapcraft.yaml using the general parts keyword. These parts will be namespaced with the project-part.

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