moonshot-ui needs to support libsecret1

> Debian and Ubuntu are removing libgnome-keyring in favor of the freedesktop.org secret service.
> For most OSes, libgnome-keyring has actually been proxying to the secret service for a while now (although Centos6 may be old enough that is not the case).

Indeed, in CentOS6 there is not libsecret available.

>
> The APIs are similar, but significantly different.

Yes.

>
> I've written a patch which I've checked in on the libsecret branch of the moonshot-ui repository to add this support.
> I'd appreciate review of that patch before releasing in Debian.
>
> The patch significantly refactors the keyring store code.
>
> * All the code that is independent of how we get to the linux keyring is
> factored into an abstract base class KeyringStoreBase (src/moonshot-
> keyring-store-base.vala). In any given build we'll only have one
> implementation of this class, but separation of concerns made it
> significantly easier to design the code and factor out common elements.
>
> * The serialization and deserialization code was inline with the load
> and store functions. Those functions were overly complex and would be
> hard to reuse in a future-proof manner. Refactored the deserialization
> and serialization code into the abstract base class. Seting with both
> libsecret and gnome-keyring suggests that I seem to have not
> accidentally changed the serialization format.
>
> * Create moonshot-keyring-store-secret|gnome.vala that both implement
> KeyringStore. At most one of these will be selected by configure.ac
> logic.
>
> * Enable keyring if either LIBSECRET_KEYRING or GNOME_KEYRING are
> defined.
>
> Again, this is available on the libsecret gitlab branch for review. I'd
> like to check this into Debian within a week or so to get in time for
> the buster freeze starting in January.

I agree with the proposed changes. I'm actually reviewing the code right
now and passing it through the CI.
I will provide feedback and additional commits to fix support for
CentOS6, Ubuntu 14.04 and Alpine that seem to not work out of the box.

Centos 7, Debian 8 and 9 and Ubuntu 16.04 and 18.04 seem ok.

Hi Sam,

Your code looks good to me. I've found only a couple of minor issues that I detail below:

* The code does not build in CentOS 6 due to a problem with how old vala handles Hash tables. This one is fixed on my branch (see below).
* The code does not build in Ubuntu 14.04 because for some reason it fails to find definitions for "Collections" and "Item". This is not a problem because we can build with GKR for this target.
* The code does not build in Alpine v3.8 because they do not ship the .vapi file for libsecret-1. This also easily fixed as just including it from another distribution in our vapi folder does the trick.

I've pushed a new branch called libsecret_alex including two additional commits on top of yours:
* The first one fixes building in CentOS6 by switching to HashTable.lookup() rather than HashTable[]. It also removes a spurious printf line.
* The second one just make indentation more consistent, by using 4 spaces as for the rest of the code.