Content Interface

Overview

The content interface is a way for two or more snaps to share something. Sharing happens at the filesystem level so anything that can be expressed as a file can be shared. This includes executables, libraries, data files but also sockets. Let's look at how this works in a few simple examples. At a very basic level, one directory, file or socket can be made to appear in a place where another snap can access it.

Examples

All of the examples below involve two snaps. One snap is offering some content (using a content slot) and the other snap is consuming that content (using a content plug). Let's call those snaps producer and consumer.

Sharing read-only snap content

Sharing an executable

producer/snapcraft.yaml

slots:
  content:
    content: executables
    read: 
      - $SNAP/bin

consumer/snapcraft.yaml_

plugs:
  content:
    content: executables
    target: $SNAP/extra-bin

When the two interfaces are connected the consumer snap can invoke executables from $SNAP/extra-bin. The directory can be added to PATH in the wrapper script if desired. The directory can be inspected by any applications that wish to check if the extra executables are available (they can then fail gracefully).

Sharing a C-level library

TBD

Sharing writable data

Sharing writable files

TBD

Sharing UNIX sockets

TBD

Technical background

The content interface is implemented with an interplay of two systems: apparmor and bind mounts. The apparmor sandbox by default allows writes to $SNAP_DATA and reads from $SNAP (TODO: Link to environment overview). We can take advantage of this by putting content from other location and make it appear to show up in $SNAP or $SNAP_DATA. We can now take data from one snap's $SNAP (e.g. from /snap/my-snap/1234/content and bind mount it to an empty directory in /snap/my-other-snap/4321/incoming-content). The same can be done for particular files if desired but