Concept ACK

Wow, this PR was a tough nut to crack! It took me quite a while to understand it thoroughly. That’s why I have attached my notes so that other reviewers might have an easier time reviewing this PR.

I agree with the changes introduced in this PR. Though I have some suggestions that might help improve this PR.

1. I suggest you split the PR into two commits: First, introduce the RestoreWallet function, and second, use it.
2. Due to the difference introduced in implementation, this snippet of code is getting duplicated. Though I can’t find a solution to avoid this, it also doesn’t feel intuitively correct. Maybe other reviewers might have some suggestions to improve upon it.

Notes:

• Introduced new pure virtual function restoreWallet in src/interfaces/wallet.h file. This function is then overridden in src/wallet/interfaces.h file.
• This new function allows the creation of a new wallet using backed-up wallet’s data. For this purpose, it uses the (newly introduced) function RestoreWallet.
• RestoreWallet is defined under src/wallet/wallet.h file. This function does preliminary checks for:
  1. If backup_file exist.
  2. If first, wallet_path (i.e., where wallet should be created) is empty, and, second, the directory can be created there.
     After these steps, the LoadWallet function is called, creating a wallet variable. Finally, if the wallet variable does not exist (if(!wallet)), the wallet_file and wallet_path are removed from the system. Finally, the wallet variable is returned if everything works out correctly.
• The LoadWalletHelper in src/wallet/rpc/util.cpp, which used to return {wallet, warning} has now been reduced to HandleWalletError. Reason:
  1. This function had two use cases: for restoreWallet() in src/wallet/rpc/backup.cpp file, and for loadWallet() in src/wallet/rpc/wallet.cpp file.
  2. Now, the restoreWallet() function was updated to use a more suitable RestoreWallet function, and loadWallet() is still using the LoadWallet function.
  3. This change leads to duplicating the following steps for both functions and removing them from the LoadWalletHelper function.

    DatabaseOptions options;
    DatabaseStatus status;
    options.require_existing = true;
    bilingual_str error;
    std::vector<bilingual_str> warnings;

• Finally, this PR introduced some minor changes to the test/functional/wallet_backup.py file:
  1. Changed the assert_raises_rpc_error code from -8 (RPC_INVALID_PARAMETER) to -4 (RPC_WALLET_ERROR)
  2. Updated code to make the error message more verbose, better explaining the cause of the error.

crACK 62fa61f

Great work, @w0xlt!