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>

7 files changed, 64 insertions, 40 deletions

diff --git a/src/init.cpp b/src/init.cpp
index ae96f510bc..d85dc2380e 100644
--- a/src/init.cpp
+++ b/src/init.cpp
@@ -1349,7 +1349,7 @@ bool AppInitMain(NodeContext& node, interfaces::BlockAndHeaderTipInfo* tip_info)
             const int64_t load_block_index_start_time = GetTimeMillis();
             try {
                 LOCK(cs_main);
-                chainman.InitializeChainstate(*Assert(node.mempool));
+                chainman.InitializeChainstate(Assert(node.mempool.get()));
                 chainman.m_total_coinstip_cache = nCoinCacheUsage;
                 chainman.m_total_coinsdb_cache = nCoinDBCache;
 
diff --git a/src/test/util/setup_common.cpp b/src/test/util/setup_common.cpp
index 748272bb1d..d9d236be1d 100644
--- a/src/test/util/setup_common.cpp
+++ b/src/test/util/setup_common.cpp
@@ -180,7 +180,7 @@ TestingSetup::TestingSetup(const std::string& chainName, const std::vector<const
     // instead of unit tests, but for now we need these here.
     RegisterAllCoreRPCCommands(tableRPC);
 
-    m_node.chainman->InitializeChainstate(*m_node.mempool);
+    m_node.chainman->InitializeChainstate(m_node.mempool.get());
     m_node.chainman->ActiveChainstate().InitCoinsDB(
         /* cache_size_bytes */ 1 << 23, /* in_memory */ true, /* should_wipe */ false);
     assert(!m_node.chainman->ActiveChainstate().CanFlushToDisk());
diff --git a/src/test/validation_chainstate_tests.cpp b/src/test/validation_chainstate_tests.cpp
index 92d8cf2e7d..2893b412fb 100644
--- a/src/test/validation_chainstate_tests.cpp
+++ b/src/test/validation_chainstate_tests.cpp
@@ -35,7 +35,7 @@ BOOST_AUTO_TEST_CASE(validation_chainstate_resize_caches)
         return outp;
     };
 
-    CChainState& c1 = WITH_LOCK(cs_main, return manager.InitializeChainstate(mempool));
+    CChainState& c1 = WITH_LOCK(cs_main, return manager.InitializeChainstate(&mempool));
     c1.InitCoinsDB(
         /* cache_size_bytes */ 1 << 23, /* in_memory */ true, /* should_wipe */ false);
     WITH_LOCK(::cs_main, c1.InitCoinsCache(1 << 23));
diff --git a/src/test/validation_chainstatemanager_tests.cpp b/src/test/validation_chainstatemanager_tests.cpp
index 7c1db9d4b9..0bd378631b 100644
--- a/src/test/validation_chainstatemanager_tests.cpp
+++ b/src/test/validation_chainstatemanager_tests.cpp
@@ -36,7 +36,7 @@ BOOST_AUTO_TEST_CASE(chainstatemanager)
 
     // Create a legacy (IBD) chainstate.
     //
-    CChainState& c1 = WITH_LOCK(::cs_main, return manager.InitializeChainstate(mempool));
+    CChainState& c1 = WITH_LOCK(::cs_main, return manager.InitializeChainstate(&mempool));
     chainstates.push_back(&c1);
     c1.InitCoinsDB(
         /* cache_size_bytes */ 1 << 23, /* in_memory */ true, /* should_wipe */ false);
@@ -66,7 +66,7 @@ BOOST_AUTO_TEST_CASE(chainstatemanager)
     //
     const uint256 snapshot_blockhash = GetRandHash();
     CChainState& c2 = WITH_LOCK(::cs_main, return manager.InitializeChainstate(
-        mempool, snapshot_blockhash));
+        &mempool, snapshot_blockhash));
     chainstates.push_back(&c2);
 
     BOOST_CHECK_EQUAL(manager.SnapshotBlockhash().value(), snapshot_blockhash);
@@ -129,7 +129,7 @@ BOOST_AUTO_TEST_CASE(chainstatemanager_rebalance_caches)
 
     // Create a legacy (IBD) chainstate.
     //
-    CChainState& c1 = WITH_LOCK(cs_main, return manager.InitializeChainstate(mempool));
+    CChainState& c1 = WITH_LOCK(cs_main, return manager.InitializeChainstate(&mempool));
     chainstates.push_back(&c1);
     c1.InitCoinsDB(
         /* cache_size_bytes */ 1 << 23, /* in_memory */ true, /* should_wipe */ false);
@@ -147,7 +147,7 @@ BOOST_AUTO_TEST_CASE(chainstatemanager_rebalance_caches)
 
     // Create a snapshot-based chainstate.
     //
-    CChainState& c2 = WITH_LOCK(cs_main, return manager.InitializeChainstate(mempool, GetRandHash()));
+    CChainState& c2 = WITH_LOCK(cs_main, return manager.InitializeChainstate(&mempool, GetRandHash()));
     chainstates.push_back(&c2);
     c2.InitCoinsDB(
         /* cache_size_bytes */ 1 << 23, /* in_memory */ true, /* should_wipe */ false);
diff --git a/src/test/validation_flush_tests.cpp b/src/test/validation_flush_tests.cpp
index a3b344d2c9..2572e2025b 100644
--- a/src/test/validation_flush_tests.cpp
+++ b/src/test/validation_flush_tests.cpp
@@ -20,7 +20,7 @@ BOOST_AUTO_TEST_CASE(getcoinscachesizestate)
 {
     CTxMemPool mempool;
     BlockManager blockman{};
-    CChainState chainstate{mempool, blockman};
+    CChainState chainstate{&mempool, blockman};
     chainstate.InitCoinsDB(/*cache_size_bytes*/ 1 << 10, /*in_memory*/ true, /*should_wipe*/ false);
     WITH_LOCK(::cs_main, chainstate.InitCoinsCache(1 << 10));
     CTxMemPool tx_pool{};
diff --git a/src/validation.cpp b/src/validation.cpp
index 65d2dfa3b7..1658b2cb76 100644
--- a/src/validation.cpp
+++ b/src/validation.cpp
@@ -329,7 +329,8 @@ static bool IsCurrentForFeeEstimation(CChainState& active_chainstate) EXCLUSIVE_
     return true;
 }
 
-/* Make mempool consistent after a reorg, by re-adding or recursively erasing
+/**
+ * Make mempool consistent after a reorg, by re-adding or recursively erasing
  * disconnected block transactions from the mempool, and also removing any
  * other transactions from the mempool that are no longer valid given the new
  * tip/height.
@@ -1208,7 +1209,7 @@ void CoinsViews::InitCache()
     m_cacheview = std::make_unique<CCoinsViewCache>(&m_catcherview);
 }
 
-CChainState::CChainState(CTxMemPool& mempool, BlockManager& blockman, std::optional<uint256> from_snapshot_blockhash)
+CChainState::CChainState(CTxMemPool* mempool, BlockManager& blockman, std::optional<uint256> from_snapshot_blockhash)
     : m_mempool(mempool),
       m_params(::Params()),
       m_blockman(blockman),
@@ -2053,7 +2054,7 @@ bool CChainState::FlushStateToDisk(
         bool fFlushForPrune = false;
         bool fDoFullFlush = false;
 
-        CoinsCacheSizeState cache_state = GetCoinsCacheSizeState(&m_mempool);
+        CoinsCacheSizeState cache_state = GetCoinsCacheSizeState(m_mempool);
         LOCK(cs_LastBlockFile);
         if (fPruneMode && (fCheckForPruning || nManualPruneHeight > 0) && !fReindex) {
             // make sure we don't prune above the blockfilterindexes bestblocks
@@ -2205,11 +2206,13 @@ static void AppendWarning(bilingual_str& res, const bilingual_str& warn)
 }
 
 /** Check warning conditions and do some notifications on new chain tip set. */
-static void UpdateTip(CTxMemPool& mempool, const CBlockIndex* pindexNew, const CChainParams& chainParams, CChainState& active_chainstate)
+static void UpdateTip(CTxMemPool* mempool, const CBlockIndex* pindexNew, const CChainParams& chainParams, CChainState& active_chainstate)
     EXCLUSIVE_LOCKS_REQUIRED(::cs_main)
 {
     // New best block
-    mempool.AddTransactionsUpdated(1);
+    if (mempool) {
+        mempool->AddTransactionsUpdated(1);
+    }
 
     {
         LOCK(g_best_block_mutex);
@@ -2254,7 +2257,7 @@ static void UpdateTip(CTxMemPool& mempool, const CBlockIndex* pindexNew, const C
 bool CChainState::DisconnectTip(BlockValidationState& state, DisconnectedBlockTransactions* disconnectpool)
 {
     AssertLockHeld(cs_main);
-    AssertLockHeld(m_mempool.cs);
+    if (m_mempool) AssertLockHeld(m_mempool->cs);
 
     CBlockIndex *pindexDelete = m_chain.Tip();
     assert(pindexDelete);
@@ -2280,7 +2283,7 @@ bool CChainState::DisconnectTip(BlockValidationState& state, DisconnectedBlockTr
         return false;
     }
 
-    if (disconnectpool) {
+    if (disconnectpool && m_mempool) {
         // Save transactions to re-add to mempool at end of reorg
         for (auto it = block.vtx.rbegin(); it != block.vtx.rend(); ++it) {
             disconnectpool->addTransaction(*it);
@@ -2288,7 +2291,7 @@ bool CChainState::DisconnectTip(BlockValidationState& state, DisconnectedBlockTr
         while (disconnectpool->DynamicMemoryUsage() > MAX_DISCONNECTED_TX_POOL_SIZE * 1000) {
             // Drop the earliest entry, and remove its children from the mempool.
             auto it = disconnectpool->queuedTx.get<insertion_order>().begin();
-            m_mempool.removeRecursive(**it, MemPoolRemovalReason::REORG);
+            m_mempool->removeRecursive(**it, MemPoolRemovalReason::REORG);
             disconnectpool->removeEntry(it);
         }
     }
@@ -2357,7 +2360,7 @@ public:
 bool CChainState::ConnectTip(BlockValidationState& state, CBlockIndex* pindexNew, const std::shared_ptr<const CBlock>& pblock, ConnectTrace& connectTrace, DisconnectedBlockTransactions& disconnectpool)
 {
     AssertLockHeld(cs_main);
-    AssertLockHeld(m_mempool.cs);
+    if (m_mempool) AssertLockHeld(m_mempool->cs);
 
     assert(pindexNew->pprev == m_chain.Tip());
     // Read block from disk.
@@ -2401,8 +2404,10 @@ bool CChainState::ConnectTip(BlockValidationState& state, CBlockIndex* pindexNew
     int64_t nTime5 = GetTimeMicros(); nTimeChainState += nTime5 - nTime4;
     LogPrint(BCLog::BENCH, "  - Writing chainstate: %.2fms [%.2fs (%.2fms/blk)]\n", (nTime5 - nTime4) * MILLI, nTimeChainState * MICRO, nTimeChainState * MILLI / nBlocksTotal);
     // Remove conflicting transactions from the mempool.;
-    m_mempool.removeForBlock(blockConnecting.vtx, pindexNew->nHeight);
-    disconnectpool.removeForBlock(blockConnecting.vtx);
+    if (m_mempool) {
+        m_mempool->removeForBlock(blockConnecting.vtx, pindexNew->nHeight);
+        disconnectpool.removeForBlock(blockConnecting.vtx);
+    }
     // Update m_chain & related variables.
     m_chain.SetTip(pindexNew);
     UpdateTip(m_mempool, pindexNew, m_params, *this);
@@ -2495,7 +2500,7 @@ void CChainState::PruneBlockIndexCandidates() {
 bool CChainState::ActivateBestChainStep(BlockValidationState& state, CBlockIndex* pindexMostWork, const std::shared_ptr<const CBlock>& pblock, bool& fInvalidFound, ConnectTrace& connectTrace)
 {
     AssertLockHeld(cs_main);
-    AssertLockHeld(m_mempool.cs);
+    if (m_mempool) AssertLockHeld(m_mempool->cs);
 
     const CBlockIndex* pindexOldTip = m_chain.Tip();
     const CBlockIndex* pindexFork = m_chain.FindFork(pindexMostWork);
@@ -2507,7 +2512,7 @@ bool CChainState::ActivateBestChainStep(BlockValidationState& state, CBlockIndex
         if (!DisconnectTip(state, &disconnectpool)) {
             // This is likely a fatal error, but keep the mempool consistent,
             // just in case. Only remove from the mempool in this case.
-            UpdateMempoolForReorg(*this, m_mempool, disconnectpool, false);
+            if (m_mempool) UpdateMempoolForReorg(*this, *m_mempool, disconnectpool, false);
 
             // If we're unable to disconnect a block during normal operation,
             // then that is a failure of our local system -- we should abort
@@ -2551,7 +2556,7 @@ bool CChainState::ActivateBestChainStep(BlockValidationState& state, CBlockIndex
                     // A system error occurred (disk space, database error, ...).
                     // Make the mempool consistent with the current tip, just in case
                     // any observers try to use it before shutdown.
-                    UpdateMempoolForReorg(*this, m_mempool, disconnectpool, false);
+                    if (m_mempool) UpdateMempoolForReorg(*this, *m_mempool, disconnectpool, false);
                     return false;
                 }
             } else {
@@ -2565,12 +2570,12 @@ bool CChainState::ActivateBestChainStep(BlockValidationState& state, CBlockIndex
         }
     }
 
-    if (fBlocksDisconnected) {
+    if (fBlocksDisconnected && m_mempool) {
         // If any blocks were disconnected, disconnectpool may be non empty.  Add
         // any disconnected transactions back to the mempool.
-        UpdateMempoolForReorg(*this, m_mempool, disconnectpool, true);
+        UpdateMempoolForReorg(*this, *m_mempool, disconnectpool, true);
     }
-    m_mempool.check(*this);
+    if (m_mempool) m_mempool->check(*this);
 
     CheckForkWarningConditions();
 
@@ -2642,7 +2647,8 @@ bool CChainState::ActivateBestChain(BlockValidationState& state, std::shared_ptr
 
         {
             LOCK(cs_main);
-            LOCK(m_mempool.cs); // Lock transaction pool for at least as long as it takes for connectTrace to be consumed
+            // Lock transaction pool for at least as long as it takes for connectTrace to be consumed
+            LOCK(MempoolMutex());
             CBlockIndex* starting_tip = m_chain.Tip();
             bool blocks_connected = false;
             do {
@@ -2792,7 +2798,9 @@ bool CChainState::InvalidateBlock(BlockValidationState& state, CBlockIndex* pind
         LimitValidationInterfaceQueue();
 
         LOCK(cs_main);
-        LOCK(m_mempool.cs); // Lock for as long as disconnectpool is in scope to make sure UpdateMempoolForReorg is called after DisconnectTip without unlocking in between
+        // Lock for as long as disconnectpool is in scope to make sure UpdateMempoolForReorg is
+        // called after DisconnectTip without unlocking in between
+        LOCK(MempoolMutex());
         if (!m_chain.Contains(pindex)) break;
         pindex_was_in_chain = true;
         CBlockIndex *invalid_walk_tip = m_chain.Tip();
@@ -2806,7 +2814,9 @@ bool CChainState::InvalidateBlock(BlockValidationState& state, CBlockIndex* pind
         // transactions back to the mempool if disconnecting was successful,
         // and we're not doing a very deep invalidation (in which case
         // keeping the mempool up to date is probably futile anyway).
-        UpdateMempoolForReorg(*this, m_mempool, disconnectpool, /* fAddToMempool = */ (++disconnected <= 10) && ret);
+        if (m_mempool) {
+            UpdateMempoolForReorg(*this, *m_mempool, disconnectpool, /* fAddToMempool = */ (++disconnected <= 10) && ret);
+        }
         if (!ret) return false;
         assert(invalid_walk_tip->pprev == m_chain.Tip());
 
@@ -3817,10 +3827,11 @@ bool CChainState::LoadBlockIndexDB()
 
 void CChainState::LoadMempool(const ArgsManager& args)
 {
+    if (!m_mempool) return;
     if (args.GetArg("-persistmempool", DEFAULT_PERSIST_MEMPOOL)) {
-        ::LoadMempool(m_mempool, *this);
+        ::LoadMempool(*m_mempool, *this);
     }
-    m_mempool.SetIsLoaded(!ShutdownRequested());
+    m_mempool->SetIsLoaded(!ShutdownRequested());
 }
 
 bool CChainState::LoadChainTip()
@@ -4684,7 +4695,8 @@ std::vector<CChainState*> ChainstateManager::GetAll()
     return out;
 }
 
-CChainState& ChainstateManager::InitializeChainstate(CTxMemPool& mempool, const std::optional<uint256>& snapshot_blockhash)
+CChainState& ChainstateManager::InitializeChainstate(
+    CTxMemPool* mempool, const std::optional<uint256>& snapshot_blockhash)
 {
     bool is_snapshot = snapshot_blockhash.has_value();
     std::unique_ptr<CChainState>& to_modify =
@@ -4763,7 +4775,7 @@ bool ChainstateManager::ActivateSnapshot(
     }
 
     auto snapshot_chainstate = WITH_LOCK(::cs_main, return std::make_unique<CChainState>(
-            this->ActiveChainstate().m_mempool, m_blockman, base_blockhash));
+        /* mempool */ nullptr, m_blockman, base_blockhash));
 
     {
         LOCK(::cs_main);
@@ -4879,7 +4891,7 @@ bool ChainstateManager::PopulateAndValidateSnapshot(
             }
 
             const auto snapshot_cache_state = WITH_LOCK(::cs_main,
-                return snapshot_chainstate.GetCoinsCacheSizeState(&snapshot_chainstate.m_mempool));
+                return snapshot_chainstate.GetCoinsCacheSizeState(snapshot_chainstate.m_mempool));
 
             if (snapshot_cache_state >=
                     CoinsCacheSizeState::CRITICAL) {
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.