validation: make CChainState::m_mempool optional

Since we now have multiple chainstate objects, only one of them is active at any given
time. An active chainstate has a mempool, but there's no point to others having one.

This change will simplify proposed assumeutxo semantics. See the discussion here:
https://github.com/bitcoin/bitcoin/pull/15606#pullrequestreview-692965905

Co-authored-by: Russell Yanofsky <russ@yanofsky.org>

1 files changed, 19 insertions, 7 deletions

diff --git a/src/validation.h b/src/validation.h
index 3d66e3161d..4823505cfc 100644
--- a/src/validation.h
+++ b/src/validation.h
@@ -587,8 +587,9 @@ protected:
      */
     mutable std::atomic<bool> m_cached_finished_ibd{false};
 
-    //! mempool that is kept in sync with the chain
-    CTxMemPool& m_mempool;
+    //! Optional mempool that is kept in sync with the chain.
+    //! Only the active chainstate has a mempool.
+    CTxMemPool* m_mempool;
 
     const CChainParams& m_params;
 
@@ -600,7 +601,10 @@ public:
     //! CChainState instances.
     BlockManager& m_blockman;
 
-    explicit CChainState(CTxMemPool& mempool, BlockManager& blockman, std::optional<uint256> from_snapshot_blockhash = std::nullopt);
+    explicit CChainState(
+        CTxMemPool* mempool,
+        BlockManager& blockman,
+        std::optional<uint256> from_snapshot_blockhash = std::nullopt);
 
     /**
      * Initialize the CoinsViews UTXO set database management data structures. The in-memory
@@ -729,7 +733,7 @@ public:
                       CCoinsViewCache& view, bool fJustCheck = false) EXCLUSIVE_LOCKS_REQUIRED(cs_main);
 
     // Apply the effects of a block disconnection on the UTXO set.
-    bool DisconnectTip(BlockValidationState& state, DisconnectedBlockTransactions* disconnectpool) EXCLUSIVE_LOCKS_REQUIRED(cs_main, m_mempool.cs);
+    bool DisconnectTip(BlockValidationState& state, DisconnectedBlockTransactions* disconnectpool) EXCLUSIVE_LOCKS_REQUIRED(cs_main, m_mempool->cs);
 
     // Manual block validity manipulation:
     /** Mark a block as precious and reorganize.
@@ -784,8 +788,8 @@ public:
     std::string ToString() EXCLUSIVE_LOCKS_REQUIRED(::cs_main);
 
 private:
-    bool ActivateBestChainStep(BlockValidationState& state, CBlockIndex* pindexMostWork, const std::shared_ptr<const CBlock>& pblock, bool& fInvalidFound, ConnectTrace& connectTrace) EXCLUSIVE_LOCKS_REQUIRED(cs_main, m_mempool.cs);
-    bool ConnectTip(BlockValidationState& state, CBlockIndex* pindexNew, const std::shared_ptr<const CBlock>& pblock, ConnectTrace& connectTrace, DisconnectedBlockTransactions& disconnectpool) EXCLUSIVE_LOCKS_REQUIRED(cs_main, m_mempool.cs);
+    bool ActivateBestChainStep(BlockValidationState& state, CBlockIndex* pindexMostWork, const std::shared_ptr<const CBlock>& pblock, bool& fInvalidFound, ConnectTrace& connectTrace) EXCLUSIVE_LOCKS_REQUIRED(cs_main, m_mempool->cs);
+    bool ConnectTip(BlockValidationState& state, CBlockIndex* pindexNew, const std::shared_ptr<const CBlock>& pblock, ConnectTrace& connectTrace, DisconnectedBlockTransactions& disconnectpool) EXCLUSIVE_LOCKS_REQUIRED(cs_main, m_mempool->cs);
 
     void InvalidBlockFound(CBlockIndex* pindex, const BlockValidationState& state) EXCLUSIVE_LOCKS_REQUIRED(cs_main);
     CBlockIndex* FindMostWorkChain() EXCLUSIVE_LOCKS_REQUIRED(cs_main);
@@ -798,6 +802,12 @@ private:
 
     bool LoadBlockIndexDB() EXCLUSIVE_LOCKS_REQUIRED(cs_main);
 
+    //! Indirection necessary to make lock annotations work with an optional mempool.
+    RecursiveMutex* MempoolMutex() const LOCK_RETURNED(m_mempool->cs)
+    {
+        return m_mempool ? &m_mempool->cs : nullptr;
+    }
+
     friend ChainstateManager;
 };
 
@@ -907,7 +917,9 @@ public:
     //                                  constructor
     //! @param[in] snapshot_blockhash   If given, signify that this chainstate
     //!                                 is based on a snapshot.
-    CChainState& InitializeChainstate(CTxMemPool& mempool, const std::optional<uint256>& snapshot_blockhash = std::nullopt)
+    CChainState& InitializeChainstate(
+        CTxMemPool* mempool,
+        const std::optional<uint256>& snapshot_blockhash = std::nullopt)
         EXCLUSIVE_LOCKS_REQUIRED(::cs_main);
 
     //! Get all chainstates currently being used.