Merge bitcoin/bitcoin#31563: rpc: Extend scope of validation mutex in generateblock

fa63b8232f test: generateblocks called by multiple threads (MarcoFalke)
fa62c8b1f0 rpc: Extend scope of validation mutex in generateblock (MarcoFalke)

Pull request description:

  The mutex (required by TestBlockValidity) must be held after creating the block, until TestBlockValidity is called. Otherwise, it is possible that the chain advances in the meantime and leads to a crash in TestBlockValidity: `Assertion failed: pindexPrev && pindexPrev == chainstate.m_chain.Tip() (validation.cpp: TestBlockValidity: 4338)`

  Fixes #31562

ACKs for top commit:
  davidgumberg:
    reACK fa63b8232f
  achow101:
    ACK fa63b8232f
  ismaelsadeeq:
    re-ACK fa63b8232f
  mzumsande:
    utACK fa63b8232f

Tree-SHA512: 3dfda1192af52546ab11fbffe44af8713073763863f4a63fbcdbdf95b1c6cbeb003dc4b8b29e7ec67362238ad15e07d8f6855832a0c68dc5370254f8cbf9445c

test/functional/rpc_generate.py

@@ -1,9 +1,11 @@
 #!/usr/bin/env python3
-# Copyright (c) 2020-2022 The Bitcoin Core developers
+# Copyright (c) 2020-present 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 generate* RPCs."""
 
+from concurrent.futures import ThreadPoolExecutor
+
 from test_framework.test_framework import BitcoinTestFramework
 from test_framework.wallet import MiniWallet
 from test_framework.util import (
@@ -83,6 +85,13 @@ class RPCGenerateTest(BitcoinTestFramework):
         txid = block['tx'][1]
         assert_equal(node.getrawtransaction(txid=txid, verbose=False, blockhash=hash), rawtx)
 
+        # Ensure that generateblock can be called concurrently by many threads.
+        self.log.info('Generate blocks in parallel')
+        generate_50_blocks = lambda n: [n.generateblock(output=address, transactions=[]) for _ in range(50)]
+        rpcs = [node.cli for _ in range(6)]
+        with ThreadPoolExecutor(max_workers=len(rpcs)) as threads:
+            list(threads.map(generate_50_blocks, rpcs))
+
         self.log.info('Fail to generate block with out of order txs')
         txid1 = miniwallet.send_self_transfer(from_node=node)['txid']
         utxo1 = miniwallet.get_utxo(txid=txid1)