Services: Auto-enable dependencies

[erikarvstedt]

Also, add missing dependencies to bitcoind, because bitcoind.enable is false outside of secure-node.nix.

Comparison: auto-enable vs. using assertions

Pros:

• Less code
• Adheres to the NixOS philosophy that services should automatically enable their dependencies
• Works like cryptoanarchy-deb-repo-builder, where, e.g., sudo apt install btcpayserver automatically installs all deps, including lnd.
• Enables auto-scenarios for all services, like run-tests.sh -s btcpayserver or run-tests.sh -s spark-wallet

Cons:

• The user might implicitly enable a lightning service and forget to back up its data. This is taken care of by the backup module.

Details

Note that we can't use the following elegant definition in btcpayserver:

services.${cfg.btcpayserver.lightningBackend}.enable = true;

This leads to infinite recursion due to the intricacies of module evaluation.
Instead, we use

services.clightning.enable = mkIf (cfg.btcpayserver.lightningBackend == "clightning") true;
services.lnd.enable = mkIf (cfg.btcpayserver.lightningBackend == "lnd") true;

[erikarvstedt]

Travis just had a spurious failure: The CI build just stopped while running the VM test, but the test itself didn't fail.
Solved by re-pushing the commit with a changed date.

[nixbitcoin]

If this is the NixOS way to do it, I think we should be consistent with NixOS user expectations. So concept ACK.

@jonasnick stated in #189 (comment) referring to modules implicitly enabling other modules, "I'd prefer to not do that to allow users to easily see and opt-in to what software they actually use on their node (for now at least... the number of modules is still manageable)." Has your opinion changed?

[nixbitcoin]

We should also probably remove the Only available if comments in examples/configuration.nix

[erikarvstedt]

Thanks, I've updated examples/configuration.nix.
I've left the webindex section unchanged. Fixing webindex to better adapt to the enabled services is on my to-do list.

[jonasnick]

It's fine either way. I'd personally still prefer being reminded of the dependencies but I can see that this is generally unnecessary. And if everyone has to manually enable the dependencies that's a lot of code duplication in a sense. Also, it's well documented now.

[nixbitcoin]

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