Can a snap package add files to a pre-existing|installed snaps files?

snapcraft
1

Suppose there is a snap package named ‘snap1’ that installs files that looks like

/snap/snap1/current/bin/run.sh
/snap/snap1/current/plugins/a
/snap/snap1/current/plugins/b
/snap/snap1/current/plugins/c

is it possible for a second snap package named ‘snap2’ to add a file to ‘snap1’ files? for example to add a file named ‘new_plugin_from_snap2’ into /snap/snap1/10/plugins? so if the user installed snap1 and snap2 they would have the following files

/snap/snap1/current/bin/run.sh
/snap/snap1/current/plugins/a
/snap/snap1/current/plugins/b
/snap/snap1/current/plugins/c
/snap/snap1/current/plugins/new_plugin_from_snap2

I am trying to port debian packages to the snap format and looking to mirror the above behavior of those debian packages in the snap world. I understand this sort of behavior is very “un snap like”

2

Yes it is possible! I will reply with details on how to do it tomorrow.

3

Not sure if this is what @zyga-snapd was going to show, but you can accomplish this with 2 technologies in snapd, the content interface and layouts. If snap1 is consuming the files but the files are located in snap2, you would first plug a content interface inside snap1 like this in the snapcraft.yaml for snap1:

plugs:
  plugin:
    content: plugin
    interface: content
    target: $SNAP_DATA/plugins

and then snap2 would expose a content interface slot like this in the snapcraft.yaml for snap2 (assuming the plugin files in snap2 are located in $SNAP/plugins):

slots:
  plugin:
    content: plugin
    interface: content
    read: [$SNAP/plugins]

Then when both snaps are installed you can connect the interfaces (put the plug in the slot) like so:

$ snap connect snap1:plugin snap2:plugin

And the files will show up in $SNAP_DATA/plugins for snap1. Then you can use layouts to map $SNAP_DATA/plugins to $SNAP/plugins/new_plugin_from_snap2 by adding the following to the snippet in the snapcraft.yaml for snap1:

layout:
  $SNAP/plugins/new_plugin_from_snap2:
    bind: $SNAP_DATA/plugins
4

I meant to show the source property that allows you to connect any number of plugins to one directory. Sorry for not having the time to do so yet.

5

Is there any documentation on the source property?

I tried to follow ijohnson example with no success so far.

6

I had a look at https://docs.snapcraft.io/content-interface but didn’t find this documented. I will look at fixing that.