Add module 'versioning'

[erikarvstedt]

This PR adds option nix-bitcoin.configVersion to inform users about backwards-incompatible changes and to provide migration hints.

Example

A notification about the recent lightning-loop update could have been implemented like this:

{
  version = "0.0.18";
  condition = config.services.lightning-loop.enable;
  message = ''
    The lightning-loop data dir location was changed.
    To move it to the new location, run the following shell command on your nix-bitcoin node:
    sudo mv ${config.services.lnd.dataDir}/.loop ${config.services.lightning-loop.dataDir}
  '';
}

To see it in action, checkout this example branch and run test/run-tests.sh eval

Example output:

This version of nix-bitcoin contains the following changes
that are incompatible with your config (version 0.0.17):

- The lightning-loop data dir location was changed.
To move it to the new location, run the following shell command on your nix-bitcoin node:
sudo mv /var/lib/lnd/.loop /var/lib/lightning-loop
(This change was introduced in version 0.0.18)

- dummy
(This change was introduced in version 0.1)

After addressing the above changes, set nix-bitcoin.configVersion = "0.1";
in your nix-bitcoin configuration.

Details

Note that nix-bitcoin.configVersion in examples/configuration.nix only needs to
be updated after adding new breaking changes.

[nixbitcoin]

Strong Concept ACK. I think this is pretty much the ideal fix for our release migration issues.

Could we also throw alerts on new features that may interest a user who's forked off the examples/configuration.nix?

[erikarvstedt]

home-manager implements news alerts like this. The date of the latest news that was read is stored inside $HOME.

In nix-bitcoin, the news items to be shown could be determined by configVersion, analogous to the breaking change messages.
We could certainly add this later on.

[jonasnick]

That's super helpful. ACK mod nit.

[jonasnick]

what does this "or true" do?

[erikarvstedt]

It returns true if the change has no condition attribute.

[jonasnick]

I see. Hadn't noticed that construction before.

[jonasnick]

Needs rebase

[jonasnick]

ACK d3ece59