rpc: Extend scope of validation mutex in generateblock

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)

The diff can be reviewed with the git options
--ignore-all-space --function-context

Github-Pull: 31563
Rebased-From: fa62c8b

Author: MarcoFalke

src/rpc/mining.cpp

@@ -1,5 +1,5 @@
 // Copyright (c) 2010 Satoshi Nakamoto
-// Copyright (c) 2009-2022 The Bitcoin Core developers
+// Copyright (c) 2009-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.
 
@@ -370,20 +370,21 @@ static RPCHelpMan generateblock()
 
     ChainstateManager& chainman = EnsureChainman(node);
     {
-        std::unique_ptr<CBlockTemplate> blocktemplate{miner.createNewBlock(coinbase_script, {.use_mempool = false})};
-        if (!blocktemplate) {
-            throw JSONRPCError(RPC_INTERNAL_ERROR, "Couldn't create new block");
+        LOCK(chainman.GetMutex());
+        {
+            std::unique_ptr<CBlockTemplate> blocktemplate{miner.createNewBlock(coinbase_script, {.use_mempool = false})};
+            if (!blocktemplate) {
+                throw JSONRPCError(RPC_INTERNAL_ERROR, "Couldn't create new block");
+            }
+            block = blocktemplate->block;
         }
-        block = blocktemplate->block;
-    }
 
-    CHECK_NONFATAL(block.vtx.size() == 1);
+        CHECK_NONFATAL(block.vtx.size() == 1);
 
-    // Add transactions
-    block.vtx.insert(block.vtx.end(), txs.begin(), txs.end());
-    RegenerateCommitments(block, chainman);
+        // Add transactions
+        block.vtx.insert(block.vtx.end(), txs.begin(), txs.end());
+        RegenerateCommitments(block, chainman);
 
-    {
         BlockValidationState state;
         if (!miner.testBlockValidity(block, /*check_merkle_root=*/false, state)) {
             throw JSONRPCError(RPC_VERIFY_ERROR, strprintf("testBlockValidity failed: %s", state.ToString()));