Content interface plugs autoconnect to the wrong provider

[Pat McGowan]

I have ubuntu-app-platform installed which exposes a slot called "platform"
I created a consumer that looks for slots provider1:shared1 and provider2:shared2

This is snap interfaces after installing my test snaps and without issuing any connect commands. Note that these (consumer, provider) are local snaps installed with --dangerous.

provider1:shared1 -
provider2:shared2
ubuntu-app-platform:platform consumer:shared1,consumer:shared2

$ snap version
snap 2.23.5
snapd 2.23.5
series 16
ubuntu 16.04
kernel 4.4.0-66-generic

Running consumer reports
cannot mount /snap/ubuntu-app-platform/34 at /snap/consumer/x1/shared1 with options bind,ro: No such file or directory

Only tested on a classic system, Xenial and Zesty

In another case, I had a snap that plugged ubuntu-app-platform and mir-libs, similar symptom but in fact the content from platform showed up in the mir-libs folder of the snap.

Manually connecting (and discard-ns) makes it work

[Pat McGowan (pat-mcgowan)]

Also happens with apps from the store without --dangerous

[Pat McGowan (pat-mcgowan)]

What also happens is the snap is plugged into two different snaps at the same time, not sure if thats expected.

This is after manually connecting mir-libs without first disconnecting from u-a-p

$ snap interfaces | grep mir-libs
mir-libs:mir-libs ubuntu-calculator-app,ubuntu-docviewer-app
ubuntu-app-platform:platform ubuntu-calculator-app,ubuntu-calculator-app:testability,ubuntu-docviewer-app:mir-libs,ubuntu-docviewer-app

[Pat McGowan (pat-mcgowan)]

This is what is the the assert file for ubuntu-app-platform

Shouldn't this be declaring autoconnect for the slot "platform" and not "content"?

type: snap-declaration
format: 1
authority-id: canonical
revision: 1
series: 16
snap-id: lVVvNqHcPT7JmLDWqwJ6q6ron9QBXcEz
publisher-id: canonical
slots:
  content:
    allow-auto-connection: true
snap-name: ubuntu-app-platform
...

[Jamie Strandboge (jdstrand)]

@Pat - 'Shouldn't this be declaring autoconnect for the slot "platform" and not "content"?'

No, 'content' is correct because that is the interface name. 'platform' is a reference used by the snaps and not for the snap declaration.

[Pat McGowan (pat-mcgowan)]

to repro currently install these packages from edge:
ubuntu-app-platform
mir-libs
ubuntu-docviewer-app

[Jamie Strandboge (jdstrand)]

There are several things going on:

1. your snap is using this for plugging the ubuntu-app-platform content interface:

plugs:
  shared1:
    content: shared1
    interface: content
    target: shared1
    default-provider: provider1:shared1
  shared2:
    content: shared2
    interface: content
    target: shared2
    default-provider: provider2:shared2

This yaml is using an invalid name for 'default-provider', is specifying a non-existent 'content' value and does not ship /run/consumer/current/shared1 or shared2. See https://github.com/snapcore/snapd/wiki/Interfaces#content for how these interface attributes are to be used.

Since the ubuntu-app-platform has this in its yaml:

slots:
  platform:
    content: ubuntu-app-platform1
    interface: content
    read:
    - .

your snapcraft.yaml should be something like for the ubuntu-app-platform snap:
plugs:
  uap1:
    content: ubuntu-app-platform1
    default-provider: ubuntu-app-platform
    target: shared1

where 'content' is the same as the 'content' value in the providing snap (ie, ubuntu-app-platform1), 'default-provider' is the name of the providing snap (ie, ubuntu-app-platform) and 'target' is the name of a directory in $SNAP for you snap (this directory *must* exist because it cannot be created for you).

2. 'sudo snap install ./consumer*snap --dangerous' results in autoconnecting ubuntu-app-platfrom despite all of the above issues with your snap.yaml. This is because of a too-lenient snap declaration.

3. Because of the too lenient snap declaration for ubuntu-app-platform, if you have a snap that plugs both ubuntu-app-platform and mir-libs, since mir-libs does not currently have a snap declaration, both plugs would be auto-connected to ubuntu-app-platform and mir-libs skipped.

Note that local unasserted snap installs of content snaps don't have a signed snap declaration on the system, so only the base declaration can be consulted. This accounts for differences in behavior with snaps installed with and without --dangerous.

I have adjusted the snap declaration for ubuntu-app-platform to have:

slots:
  content:
    allow-auto-connection:
      -
        plug-attributes:
          content: $SLOT(content)
        slot-attributes:
          content: ubuntu-app-platform1

This means that the auto-connection can only happen with the plugging snap that uses "content: ubuntu-app-platform1", which is precisely how it is supposed to work.

To fix your snap for ubuntu-app-platform and mir-libs, use this yaml:

plugs:
  uap1:
    interface: content
    content: ubuntu-app-platform1
    target: shared1
    default-provider: ubuntu-app-platform
  ml0:
    interface: content
    content: mir0
    target: shared2
    default-provider: mir-libs

then create the shared1 and shared2 directories beside your snapcraft.yaml file, then snapcraft will generate a snap that will work for you. Specifically, if you install ubuntu-app-platform first, upon install of your snap ubuntu-app-platform will be mounted on $SNAP/shared1 and nothing will be mounted on $SNAP/shared2 (since mir-libs doesn't yet have a snap declaration for auto-connection). When you 'sudo snap connect consumer:ml0 mir-libs:mir-libs',...

[Jamie Strandboge (jdstrand)]

FYI for future reference, I refined the snap declaration to be even more specific:

slots:
  content:
    allow-auto-connection:
      -
        plug-attributes:
          content: $SLOT(content)
        slot-attributes:
          content: ubuntu-app-platform1
        slot-publisher-id:
          - canonical
        slot-snap-id:
          - lVVvNqHcPT7JmLDWqwJ6q6ron9QBXcEz

In this manner, if this is the ubuntu-app-platform snap (snap id lVVvNqHcPT7JmLDWqwJ6q6ron9QBXcEz) from canonical, then a plugging snap that uses 'content: ubuntu-app-platform1' will be auto-connected.