Adventures in classic Python snaps, part II

[Sergio Schvezov]

On Fri, 17 Mar 2017 17:25:03 -0400, Barry Warsaw wrote:
> A few days I asked about classic snaps and problems I was having turning
> ubuntu-image into a classic snap (from devmode).  For the recently released
> u-i 1.0 I thought I'd solved the problem, and had an 
> autopkgtest to prove it.
> Since then I realized the problem was *not* solved and the autopkgtest had
> some dependency pollution that caused it to succeed when it should have
> failed.  I've fixed that, but now I'm finding that classic 
> Python snaps don't
> work -at least not in the way I'd expect.
>
> For reference, here's a baseline snapcraft.yaml that "works":
>
> -----snip snip-----
> name: ubuntu-image
> summary: Create Ubuntu images
> description: |
>   Use this tool to create Ubuntu images.
> version: 1.0+snap2
> grade: stable
> confinement: classic
>
> apps:
>   ubuntu-image:
>     command: usr/bin/ubuntu-image
>
> environment:
>   PATH: $SNAP/usr/bin:$PATH
>
> parts:
>   ubuntu-image:
>     plugin: python
>     source: .
>     python-packages:
>       - PyYAML
>       - attrs
>       - voluptuous
>     prime:
>       - usr
>       - lib
>       - sbin
>     stage-packages:
>       - e2fsprogs
>       - mtools
>       - python3-debian
>       - python3-parted
>       - util-linux
>       - ubuntu-image
>   snapd:
>     plugin: go
>     source: https://github.com/snapcore/snapd
>     source-type: git
>     go-importpath: github.com/snapcore/snapd
>     prime:
>       - bin/snap
> -----snip snip-----
>
> I was pointed to LP: #1670388 which helped me understand that I have to set
> $PATH (as seen in the `environment` section) so that the 'python3'  found in
> the script's shebang is the one inside the snap, not the one in the system.
> That's good enough to not *also* require $PYTHONPATH afaict.

We have a custom sitecustomize.py that takes care of the magic now. You don't need to set PATH though, alternatively in command you can do

```
command: /usr/bin/env $SNAP/usr/bin/python3 $SNAP/usr/bin/ubuntu-image
```

Next week I am going to be giving the python plugin some love.

> This only works for Zesty; it fails in Yakkety and Xenial, 
> presumably because
> of LP: #1665756.  The bug says this is fixed in snapd 2.23, 
> which is available
> atm only in zesty, yakkety-proposed, and xenial-proposed.

Yes, this is the case.

> There are a few other things to note.
>
> I don't think we need the snapd part.  Isn't it guaranteed that 
> if someone is
> running a classic snap, that /usr/bin/snap *must* be installed on their
> system?  It does seem to work when I remove that part, but I 
> want to make sure
> that autopktest package pollution isn't causing a false positive.

It can be in a different path though.

> There's one big cheat here.  I had to put ubuntu-image in stage-packages
> because afaict, for classic snaps, the Python snapcraft plugin does not
> arrange for the ubuntu_image Python package to live in stage, and thus it
> doesn't get into prime.  I don't think there's anything you could add to a
> `stage` section that would make that work.  After experimenting (the
> documentation is unclear) files listed in stage are relative to the part's
> installation directory:

I don't understand this fully, but I am way past EOD so I will look with fresh eyes on monday, but is this exclusive to classic confinement?

> modified   snapcraft.yaml
> @@ -21,6 +21,8 @@ parts:
>        - PyYAML
>        - attrs
>        - voluptuous
> +    stage:
> +      - stageme
>      prime:
>        - usr
>        - lib
>
> Staging ubuntu-image 
> [Errno 2] No such file or directory: 
> '/tmp/autopkgtest.LgVE5q/build.9QK/ubuntu-image-1.1+17.04ubuntu1/parts/ubuntu-image/install/stageme'
>
> root at autopkgtest:/tmp/autopkgtest.LgVE5q/build.9QK/ubuntu-image-1.1+17.04ubuntu1# 
> find parts/ubuntu-image/install -name ubuntu_image
> root at autopkgtest:/tmp/autopkgtest.LgVE5q/build.9QK/ubuntu-image-1.1+17.04ubuntu1# 
> ls parts/ubuntu-image/install/usr/lib/python3/dist-packages/
> chardet                 _ped.cpython-35m-x86_64-linux-gnu.so
> chardet-2.3.0.egg-info  pkg_resources
> deb822.py               pyparted-3.10.7.egg-info
> debian                  python_debian-0.1.30.egg-info
> debian_bundle           six-1.10.0.egg-info
> parted                  six.py
>
> You can see that the ubuntu_image package doesn't end up in the part's
> dist-packages, and there's nothing that can be staged.

It should end up in $SNAP/lib/site-packages if not using stage-packages.


> I haven't yet tried to play with scriptlets or other workaround.  Obviously,
> we don't want to snap up the ubuntu-image that's in the archive, but instead
> the one in the current source directory.
>
> This seems like a bug, but I wanted to get some feedback before 
> I filed a bug
> report.


-- 
Sent using Dekko from my Ubuntu device