Avoiding breakage when using content interface

snapcraft
1

My snap uses content[docker-executables] interface so that it can run docker/docker-compose commands without having to bundle those executables.

The snapcraft.yaml has the following relevant bits:

plugs:
  docker-executables:
    interface: content
    default-provider: docker
    target: $SNAP/docker-exe

environment:
  PATH: $PATH:$SNAP/docker-exe/usr/bin:$PATH:$SNAP/docker-exe/bin
  PYTHONPATH: $SNAP/docker-exe/lib/python2.7/site-packages

This was working fine until docker-snap was updated to 19.03 which upgraded the python version to 3.5. To fix the breakage I had to update PYTHONPATH to $SNAP/docker-exe/lib/python3.5/site-packages.

What is the recommended way of using content interface to avoid such breakages in future?

2

Snaps providing content via the content interface need to version the names of the interfaces, if I understand correctly. The only way to completely avoid the problem is to bundle them.

1 Like
3

@tianon, this is something you probably want to think about, other snaps use the docker content interface to call docker-compose which is Python based and thus could be broken by changes to the version of python in the snap, etc. Perhaps instead of putting the docker-compose file in that dir, you could put a little wrapper script which sets the proper env variables so that users of the content interface don’t need to mind this themselves? i.e. move the bin/docker-compose binary to bin/docker-compose-real and then bin/docker-compose becomes:

#!/bin/bash -e

# snippet from https://stackoverflow.com/a/246128/10102404
SCRIPT_DIR="$( cd "$( dirname "${BASH_SOURCE[0]}" )" >/dev/null && pwd )"

export PYTHONPATH="$SCRIPT_DIR/../lib/python3.5/site-packages"
exec $SCRIPT_DIR/bin/docker-compose-real "$@"

Otherwise, I think the typical convention of naming of these interfaces that is versioned would be something like (in the docker snap)

slots:
  docker-executables-16.04-python2:
    content: docker-executables
    interface: content
    read:
    - .

For changing the snap to use python3 or changing the base of the snap to 18.04 you could change the slot to docker-executables-16.04-python3 or docker-executables-18.04-python2 respectively. However then other snap applications like this user would be broken again if you switched the slot names so I would recommend just replacing the docker-compose binary as described above.

1 Like
4

Thanks @ijohnson. Your proposed solution of replacing the docker-compose binary would work great for us. @tianon: Thoughts?

5

@tianon: Any thoughts on the solution proposed by @ijohnson?