Merge #21201: rpc: Disallow sendtoaddress and sendmany when private keys disabled

6bfbc97d71 test: disallow sendtoaddress/sendmany when private keys disabled (Jon Atack)
0997019e76 Disallow sendtoaddress and sendmany when private keys disabled (Andrew Chow)

Pull request description:

  Since `sendtoaddress` and `sendmany` (which use the `SendMoney` function) create and commit a transaction, they should not do anything when the wallet does not have private keys. Otherwise a valid transaction cannot be made.

  Fixes #21104

ACKs for top commit:
  jonatack:
    ACK 6bfbc97d71
  meshcollider:
    utACK 6bfbc97d71
  kristapsk:
    ACK 6bfbc97d71. "Error: Private keys are disabled for this wallet" is definitely a better error message than "Insufficient funds" here. Hopefully change of error code from -6 to -4 doesn't break any software using Bitcoin JSON-RPC API.

Tree-SHA512: f277d6b5252e43942d568614032596f2c0827f00cd0cb71e44ffcb9822bfb15a71730a3e3688f31e59ba4eb7d275250c4e65ad4b6b3e96be6314c56a672432fb

test/functional/wallet_watchonly.py

@@ -2,7 +2,7 @@
 # Copyright (c) 2018-2019 The Bitcoin Core developers
 # Distributed under the MIT software license, see the accompanying
 # file COPYING or http://www.opensource.org/licenses/mit-license.php.
-"""Test createwallet arguments.
+"""Test createwallet watchonly arguments.
 """
 
 from test_framework.test_framework import BitcoinTestFramework
@@ -49,6 +49,11 @@ class CreateWalletWatchonlyTest(BitcoinTestFramework):
         assert_equal(len(wo_wallet.listtransactions()), 1)
         assert_equal(wo_wallet.getbalance(include_watchonly=False), 0)
 
+        self.log.info('Test sending from a watch-only wallet raises RPC error')
+        msg = "Error: Private keys are disabled for this wallet"
+        assert_raises_rpc_error(-4, msg, wo_wallet.sendtoaddress, a1, 0.1)
+        assert_raises_rpc_error(-4, msg, wo_wallet.sendmany, amounts={a1: 0.1})
+
         self.log.info('Testing listreceivedbyaddress watch-only defaults')
         result = wo_wallet.listreceivedbyaddress()
         assert_equal(len(result), 1)