Have testBlockValidity hold cs_main instead of caller

The goal of interfaces is to eventually run in their own process,
so we can't use EXCLUSIVE_LOCKS_REQUIRED in their declaration.

However TestBlockValidaty will crash (in its call to ConnectBlock)
if the tip changes from under the proposed block.

Have the testBlockValidity implementation  hold the lock instead,
and non-fatally check for this condition.

Author: Sjors

Diff for: src/interfaces/mining.h

@@ -67,7 +67,7 @@ class Mining
      * @param[in] block the block to validate
      * @param[in] check_merkle_root call CheckMerkleRoot()
      * @param[out] state details of why a block failed to validate
-     * @returns false if any of the checks fail
+     * @returns false if it does not build on the current tip, or any of the checks fail
      */
     virtual bool testBlockValidity(const CBlock& block, bool check_merkle_root, BlockValidationState& state) = 0;
 

Diff for: src/node/interfaces.cpp

@@ -872,8 +872,15 @@ class MinerImpl : public Mining
 
     bool testBlockValidity(const CBlock& block, bool check_merkle_root, BlockValidationState& state) override
     {
-        LOCK(::cs_main);
-        return TestBlockValidity(state, chainman().GetParams(), chainman().ActiveChainstate(), block, chainman().ActiveChain().Tip(), /*fCheckPOW=*/false, check_merkle_root);
+        LOCK(cs_main);
+        CBlockIndex* tip{chainman().ActiveChain().Tip()};
+        // Fail if the tip updated before the lock was taken
+        if (block.hashPrevBlock != tip->GetBlockHash()) {
+            state.Error("Block does not connect to current chain tip.");
+            return false;
+        }
+
+        return TestBlockValidity(state, chainman().GetParams(), chainman().ActiveChainstate(), block, tip, /*fCheckPOW=*/false, check_merkle_root);
     }
 
     std::unique_ptr<CBlockTemplate> createNewBlock(const CScript& script_pub_key, bool use_mempool) override

Diff for: src/rpc/mining.cpp

@@ -371,8 +371,6 @@ static RPCHelpMan generateblock()
 
     ChainstateManager& chainman = EnsureChainman(node);
     {
-        LOCK(cs_main);
-
         std::unique_ptr<CBlockTemplate> blocktemplate{miner.createNewBlock(coinbase_script, /*use_mempool=*/false)};
         if (!blocktemplate) {
             throw JSONRPCError(RPC_INTERNAL_ERROR, "Couldn't create new block");
@@ -387,8 +385,6 @@ static RPCHelpMan generateblock()
     RegenerateCommitments(block, chainman);
 
     {
-        LOCK(cs_main);
-
         BlockValidationState state;
         if (!miner.testBlockValidity(block, /*check_merkle_root=*/false, state)) {
             throw JSONRPCError(RPC_VERIFY_ERROR, strprintf("testBlockValidity failed: %s", state.ToString()));