add support for bidirectional mount event propagation in /media

[Zygmunt Krynicki]

[Impact]

The /media directory is special in that mount events propagate outward from the mount namespace used by snap applications into the main mount
namespace.

[Test Case]

The test case can be found here:

https://github.com/snapcore/snap-confine/blob/master/spread-tests/main/media-sharing/task.yaml

The test case is ran automatically for each pull request and for each final release. It can be reproduced manually by executing the shell commands listed in the prepare/execute/restore phases manually.
The commands there assume that snapd and snap-confine are installed.
No other additional setup is necessary.

[Regression Potential]

This change involved relatively complex changes in the core logic of snap-confine and while extensive testing was performed since in both core and classic environments there's always possibility of some edge case.

For inspecting the layout of the mount namespace with this feature enabled please look at

https://github.com/snapcore/snap-confine/tree/master/spread-tests/main/mount-ns-layout

[Other Info]

* snap-confine is technically an integral part of snapd which has an SRU exception and is allowed to introduce new features and take advantage of accelerated procedure. For more information see https://wiki.ubuntu.com/SnapdUpdates

== # Pre-SRU bug description follows # ==

Snaps such as udisks can mount anything in /media. While this works the mount is only visible to the snap that performed the operation (as it is stuck in the mount namespace).

The mount namespace should be setup up in a way that makes /media shared with the outside namespace.

[Zygmunt Krynicki (zyga)]

This has been fixed with the following pull request https://github.com/snapcore/snap-confine/pull/168

[Andy Whitcroft (apw)]

Hello Zygmunt, or anyone else affected,

Accepted snap-confine into yakkety-proposed. The package will build now and be available at https://launchpad.net/ubuntu/+source/snap-confine/1.0.44-0ubuntu1~16.10 in a few hours, and then in the -proposed repository.

Please help us by testing this new package. See https://wiki.ubuntu.com/Testing/EnableProposed for documentation how to enable and use -proposed. Your feedback will aid us getting this update out to other Ubuntu users.

If this package fixes the bug for you, please add a comment to this bug, mentioning the version of the package you tested, and change the tag from verification-needed to verification-done. If it does not fix the bug for you, please add a comment stating that, and change the tag to verification-failed. In either case, details of your testing will help us make a better decision.

Further information regarding the verification process can be found at https://wiki.ubuntu.com/QATeam/PerformingSRUVerification . Thank you in advance!

[Andy Whitcroft (apw)]

Hello Zygmunt, or anyone else affected,

Accepted snapcraft into xenial-proposed. The package will build now and be available at https://launchpad.net/ubuntu/+source/snapcraft/1.0.44-0ubuntu1~16.04 in a few hours, and then in the -proposed repository.

Please help us by testing this new package. See https://wiki.ubuntu.com/Testing/EnableProposed for documentation how to enable and use -proposed. Your feedback will aid us getting this update out to other Ubuntu users.

If this package fixes the bug for you, please add a comment to this bug, mentioning the version of the package you tested, and change the tag from verification-needed to verification-done. If it does not fix the bug for you, please add a comment stating that, and change the tag to verification-failed. In either case, details of your testing will help us make a better decision.

Further information regarding the verification process can be found at https://wiki.ubuntu.com/QATeam/PerformingSRUVerification . Thank you in advance!

[Andy Whitcroft (apw)]

Hello Zygmunt, or anyone else affected,

Accepted snap-confine into xenial-proposed. The package will build now and be available at https://launchpad.net/ubuntu/+source/snap-confine/1.0.44-0ubuntu1~16.04 in a few hours, and then in the -proposed repository.

Please help us by testing this new package. See https://wiki.ubuntu.com/Testing/EnableProposed for documentation how to enable and use -proposed. Your feedback will aid us getting this update out to other Ubuntu users.

If this package fixes the bug for you, please add a comment to this bug, mentioning the version of the package you tested, and change the tag from verification-needed to verification-done. If it does not fix the bug for you, please add a comment stating that, and change the tag to verification-failed. In either case, details of your testing will help us make a better decision.

Further information regarding the verification process can be found at https://wiki.ubuntu.com/QATeam/PerformingSRUVerification . Thank you in advance!

[Federico Gimenez (fgimenez)]

Verified executing manually the steps in the automated test https://github.com/snapcore/snap-confine/blob/master/spread-tests/main/media-sharing/task.yaml