Recovery tarballs for snap builds

We need to specify this in a great deal more detail; it's not actionable until we do. Things I can think of right now include:

 * Is the "branch" part truly necessary, as opposed to simply creating a tarball that somebody can then later unpack and drop into a branch if necessary? The latter is a lot easier - we don't have many of the mechanisms we'd need to create a branch at this point, and although they could no doubt be built, there are many other non-obvious things here such as how to specify the branch ownership, whether it should be Bazaar or Git, etc.
 * snapcraft.yaml often specifies remote branches / artifacts / etc. to fetch. How would we suppress this behaviour for a subsequent rebuild? Would we need to produce an edited version of snapcraft.yaml?
 * The output of "snapcraft pull" isn't likely to be anyone's preferred form for modification. Have there been any experiments done on the snapcraft side to work out what the workflow would actually look like for developers? I think this sort of thing needs to come before implementation in launchpad-buildd.
 * Exactly what files and directories need to be saved from the "snapcraft pull" step in order that snapcraft won't try to re-fetch them in a later step? What must we, or indeed developers working with the output, avoid doing so that snapcraft won't decide that it needs to pull again?

I misused the term branch here. On the Launchpad side, only a tarball should be created and stored. I'll rename the bug accordingly. Actually creating a VCS branch from that is optionally done by a person when and if that is necessary.

This mechanism is indeed meant for recovery purposes only. Not something to be maintained frequently but if there's no better option than to apply an urgent patch on the existing sources. Pulling in a new version of the same snap might contain any number of changes. By the same token, patching and re-building the same snap from source may pull in different dependencies or rely on external sources no longer available.

The result of "snapcraft pull" is actually to be taken as-is here. The entire folder should be copied. After running it, all artifacts will be contained in the project tree. When building from this copy, Snapcraft can detect that all dependencies are already satisfied and will build from it without reaching out to the network to retrieve them. No modifications to the snapcraft.yaml are therefore necessary.

OK. Is there any preference for what this tarball should be called? Should the tarball always be created, or is there some reasonable condition we can use to decide whether to create it?

The condition would be that it's a stable snap (you wouldn't apply security patches to unstable releases). However it should probably be enabled explicitly to limit the amount of data for snaps that don't make use of it.

r13842 in db-stable (http://bazaar.launchpad.net/~launchpad-pqm/launchpad/db-stable/revision/13842) is part of this bug's fix.

2018-04-22 07:34:41,648 INFO 2209-83-2 applied just now in 0.2 seconds

Fixed in stable r18623 <http://bazaar.launchpad.net/~launchpad-pqm/launchpad/stable/revision/18623>.

launchpad-buildd (161) xenial; urgency=medium

  * Pass build URL to snapcraft using SNAPCRAFT_IMAGE_INFO.
  * Add an option to generate source tarballs for snaps after pulling
    external dependencies (LP: #1763639).

 -- Colin Watson <email address hidden> Mon, 23 Apr 2018 09:44:49 +0100