wallet: importprivkey fails to import private key #21275

As reported by @chaihahaha in https://github.com/bitcoin-core/gui/issues/218, importprivkey is failing to import a private key into a blank wallet.

To Reproduce (steps with bitcoin-qt):

• Have a private key, I obtained one with dumpprivkey
• Create a new Encrypted wallet with Make Blank Wallet option selected. This option means that the wallet starts out with no private keys and you can add them in later with importprivkey or sethdseed
• Go over to console and unlock wallet with walletpassphrase "123" 60
• In console, run: importprivkey "priv_key" "label" false
• Run getnewaddress -> get error: Error: This wallet has no available keys (code -4)

Note: I was also able to reproduce using bitcoind and bitcoin-cli. Additionally, the debug log says I already have this key in pubkey and already have the script. This is similar to #16711. You can still populate the blank wallet with a private key using sethdseed.

Running getwalletinfo shows that, in fact, the private key was not added:

{
  "walletname": "new-wallet",
  "walletversion": 169900,
  "format": "bdb",
  "balance": 0.00000000,
  "unconfirmed_balance": 0.00000000,
  "immature_balance": 0.00000000,
  "txcount": 0,
  "keypoololdest": 1614066198,
  "keypoolsize": 0,
  "keypoolsize_hd_internal": 0,
  "paytxfee": 0.00000000,
  "private_keys_enabled": true,
  "avoid_reuse": false,
  "scanning": false,
  "descriptors": false
}

System: macOS 11.1, reproduced on master

This is expected behavior. importprivkey does not import the key into the keypool which is what getnewaddress uses to fetch new addresses.

The help for importprivkey states that it adds a private key to your wallet. I take this to mean that it's populating my wallet with a private key that I can then use to generate a new address. Maybe the help message can be updated as to not cause this confusion.