[WIP] [wallet] Remove Wallet dependencies from init.cpp

[jnewbery]

Work in progress. This will eventually build off #10767, but is now yet ready for review. Currently looking for feedback on concept and approach.

This continues the work of #7965. This PR, along with #10583, #10579, #10571, #8843 and one more PR to split createmultisig, would remove the remaining dependencies from libbitcoin_server.a on libbitcoin_wallet.a.

I've used the same boost::signal approach that is used in the validationinterface. bitcoind.cpp and qt/bitcoin.cpp register wallet initialization/destruction callbacks as slots in a WalletInitInterface, and init.cpp emits those signals during startup/shutdown. This could potentially be extended in future to allow multiple wallets or other modules to register.

To create the interface, I've just translated all the old init.cpp wallet function calls into signals. I've not done any thinking about whether it makes sense to change that interface by combining/splitting those calls. This is a purely internal interface, so there's no problem in changing it later.

[promag]

Will take a look.

[jnewbery]

Thanks @promag - this is very rough-and-ready at the moment. It builds and passes tests, but is not ready for code review. I'd be very happy to get feedback about general concept though.

[jnewbery]

I see that @theuni has opened #10756 to replace CNodeSignals with an interface class. Would that be a better approach here?

[ryanofsky]

I see that @theuni has opened #10756 to replace CNodeSignals with an interface class. Would that be a better approach here?

Yeah I was just about to make this suggestion. You could delete the WalletInitSignals struct and replace the WalletInitSignals global variable with a std::unique_ptr<WalletInitInterface> global. This would get rid of all the boost signals code and make the control flow simpler.

Also you should prefix global variables with g_. And I think this PR would be less error prone and simpler to review if you just left the init code in wallet.cpp for now and moved the relevant functions to walletinit.cpp in followup MOVEONLY PR.

[jnewbery]

I think this PR would be less error prone and simpler to review if you just left the init code in wallet.cpp for now and moved the relevant functions to walletinit.cpp in followup MOVEONLY PR.

Thanks @ryanofsky, you've convinced me - I'll split this PR into two. But I think I'll do it the other way round - first move the wallet initialization/destruction functions into their own walletinit translation unit, and then introduce the interface class in a second PR. The reason is that #10740 is built on walletinit.cpp so this PR and #10740 will no longer be interdependent.