Content sharing from snap common is broken

[Tim Kuhlman]

Running on yakety with snapd at version 2.18+ppa123.a5877498-1 from the snappy edge ppa the content interface does not work for content in $SNAP_COMMON

I'm working with the promreg charm which is the producer, it has this config:
slots:
  content:
    content: promreg
    read: [$SNAP_COMMON/promreg]

The consumer is the prometheus charm which has this config.
plugs:
  content:
    content: promreg
    target: etc/prometheus/promreg

The directory '/var/snap/promreg/common/promreg' does exist and has data in it. The destination directory in the prometheus snap didn't originally exist but I tried creating it after reading
http://askubuntu.com/questions/841004/cannot-get-basic-content-interface-example-working-with-snapcraft

After getting a shell with the command `sudo snap run --shell prometheus` I see that the destination dir is empty if the destination dir was pre-existing and it doesn't exist at all if dir wasn't pre-existing. There is no error message to indicate what the problem is.

[Pat McGowan (pat-mcgowan)]

See updated documentation at https://github.com/snapcore/snapd/wiki/Content-Interface. Appears to be fixed in 2.19.1+

[Jacek Nykis (jacekn)]

Thank's to zyga for interactive troubleshooting session! Here is additional info and workaround:

1. Sharing content from $SNAP_COMMON is supported in snapd 2.20.1 and later, it's available in -proposed
2. Docs here mention "writable" content sharing: https://github.com/snapcore/snapd/wiki/Content-Interface but it has "writable-data" and "write" in a few places. I wrongly assumed was to allow one snap to write into another one. That section actually refers to "/var/snap/" data
3. Once snaps are connected this commands makes content visible:

/usr/lib/snapd/snap-discard-ns name-of-consumer-snap

I was told that 3 will be fixed. 2 is just wiki edit to clarify info a bit more

[Zygmunt Krynicki (zyga)]

The one thing in content sharing that was never implemented is live modification of preseved mount namespaces. If that means nothing you don't dispair. I'm working on fixing this now.

[Zygmunt Krynicki (zyga)]

This is now tracked as https://github.com/snapcore/snapd/pull/2624

There's a chance this will not land as it seems to cause an oops in the kernel.

[Zygmunt Krynicki (zyga)]

Ooops, sorry, I commented on the wrong bug. Please disregard comment #4

[Zygmunt Krynicki (zyga)]

I'm closing this bug. The issue that affects content sharing not being reflected in existing apps is tracked elsewhere.

[Jacek Nykis (jacekn)]

@zyga

Can you please comment on this bug indicating where it is tracked (I disregarded comment #4 lik you asked)?

[Zygmunt Krynicki (zyga)]

This is now fixed in master. Snapd can create directories on the fly, even in read-only $SNAP locations. It is also extensively tested. Please try snapd 2.31 release candidate 2 once it gets released in a day or two.

[Zygmunt Krynicki (zyga)]

This has been fixed and released for some time now. Marking as such.