Merge bitcoin/bitcoin#22415: Make m_mempool optional in CChainState

ceb7b35a39145717e2d9d356fd382bd1f95d2a5a refactor: move UpdateTip into CChainState (James O'Beirne)
4abf0779d6594e97222279110c328b75b5f3db7b refactor: no mempool arg to GetCoinsCacheSizeState (James O'Beirne)
46e3efd1e4ae2f058ecfffdaee7e882c4305eb35 refactor: move UpdateMempoolForReorg into CChainState (James O'Beirne)
617661703ac29e0744f21de74501d033fdc53ff6 validation: make CChainState::m_mempool optional (James O'Beirne)

Pull request description:

  Make `CChainState::m_mempool` optional by making it a pointer instead of a reference. This will allow a simplification to assumeutxo semantics (see https://github.com/bitcoin/bitcoin/pull/15606#pullrequestreview-692965905) and help facilitate the `-nomempool` option.

ACKs for top commit:
  jnewbery:
    ACK ceb7b35a39145717e2d9d356fd382bd1f95d2a5a
  naumenkogs:
    ACK ceb7b35a39145717e2d9d356fd382bd1f95d2a5a
  ryanofsky:
    Code review ACK ceb7b35a39145717e2d9d356fd382bd1f95d2a5a (just minor style and test tweaks since last review)
  lsilva01:
    Code review ACK and tested on Signet ACK https://github.com/bitcoin/bitcoin/pull/22415/commits/ceb7b35a39145717e2d9d356fd382bd1f95d2a5a
  MarcoFalke:
    review ACK ceb7b35a39145717e2d9d356fd382bd1f95d2a5a 😌

Tree-SHA512: cc445ad33439d5918cacf80a6354eea8f3d33bb7719573ed5b970fad1a0dab410bcd70be44c862b8aba1b71263b82d79876688c553e339362653dfb3d8ec81e6

7 files changed, 118 insertions, 86 deletions

diff --git a/src/init.cpp b/src/init.cpp
index 9a4475466c..9afd76d62d 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..22aafcaa6c 100644
--- a/src/test/validation_flush_tests.cpp
+++ b/src/test/validation_flush_tests.cpp
@@ -20,10 +20,9 @@ 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{};
 
     constexpr bool is_64_bit = sizeof(void*) == 8;
 
@@ -57,7 +56,7 @@ BOOST_AUTO_TEST_CASE(getcoinscachesizestate)
 
     // Without any coins in the cache, we shouldn't need to flush.
     BOOST_CHECK_EQUAL(
-        chainstate.GetCoinsCacheSizeState(&tx_pool, MAX_COINS_CACHE_BYTES, /*max_mempool_size_bytes*/ 0),
+        chainstate.GetCoinsCacheSizeState(MAX_COINS_CACHE_BYTES, /*max_mempool_size_bytes*/ 0),
         CoinsCacheSizeState::OK);
 
     // If the initial memory allocations of cacheCoins don't match these common
@@ -72,7 +71,7 @@ BOOST_AUTO_TEST_CASE(getcoinscachesizestate)
         }
 
         BOOST_CHECK_EQUAL(
-            chainstate.GetCoinsCacheSizeState(&tx_pool, MAX_COINS_CACHE_BYTES, /*max_mempool_size_bytes*/ 0),
+            chainstate.GetCoinsCacheSizeState(MAX_COINS_CACHE_BYTES, /*max_mempool_size_bytes*/ 0),
             CoinsCacheSizeState::CRITICAL);
 
         BOOST_TEST_MESSAGE("Exiting cache flush tests early due to unsupported arch");
@@ -93,7 +92,7 @@ BOOST_AUTO_TEST_CASE(getcoinscachesizestate)
         print_view_mem_usage(view);
         BOOST_CHECK_EQUAL(view.AccessCoin(res).DynamicMemoryUsage(), COIN_SIZE);
         BOOST_CHECK_EQUAL(
-            chainstate.GetCoinsCacheSizeState(&tx_pool, MAX_COINS_CACHE_BYTES, /*max_mempool_size_bytes*/ 0),
+            chainstate.GetCoinsCacheSizeState(MAX_COINS_CACHE_BYTES, /*max_mempool_size_bytes*/ 0),
             CoinsCacheSizeState::OK);
     }
 
@@ -101,26 +100,26 @@ BOOST_AUTO_TEST_CASE(getcoinscachesizestate)
     for (int i{0}; i < 4; ++i) {
         add_coin(view);
         print_view_mem_usage(view);
-        if (chainstate.GetCoinsCacheSizeState(&tx_pool, MAX_COINS_CACHE_BYTES, /*max_mempool_size_bytes*/ 0) ==
+        if (chainstate.GetCoinsCacheSizeState(MAX_COINS_CACHE_BYTES, /*max_mempool_size_bytes*/ 0) ==
             CoinsCacheSizeState::CRITICAL) {
             break;
         }
     }
 
     BOOST_CHECK_EQUAL(
-        chainstate.GetCoinsCacheSizeState(&tx_pool, MAX_COINS_CACHE_BYTES, /*max_mempool_size_bytes*/ 0),
+        chainstate.GetCoinsCacheSizeState(MAX_COINS_CACHE_BYTES, /*max_mempool_size_bytes*/ 0),
         CoinsCacheSizeState::CRITICAL);
 
     // Passing non-zero max mempool usage should allow us more headroom.
     BOOST_CHECK_EQUAL(
-        chainstate.GetCoinsCacheSizeState(&tx_pool, MAX_COINS_CACHE_BYTES, /*max_mempool_size_bytes*/ 1 << 10),
+        chainstate.GetCoinsCacheSizeState(MAX_COINS_CACHE_BYTES, /*max_mempool_size_bytes*/ 1 << 10),
         CoinsCacheSizeState::OK);
 
     for (int i{0}; i < 3; ++i) {
         add_coin(view);
         print_view_mem_usage(view);
         BOOST_CHECK_EQUAL(
-            chainstate.GetCoinsCacheSizeState(&tx_pool, MAX_COINS_CACHE_BYTES, /*max_mempool_size_bytes*/ 1 << 10),
+            chainstate.GetCoinsCacheSizeState(MAX_COINS_CACHE_BYTES, /*max_mempool_size_bytes*/ 1 << 10),
             CoinsCacheSizeState::OK);
     }
 
@@ -136,7 +135,7 @@ BOOST_AUTO_TEST_CASE(getcoinscachesizestate)
         BOOST_CHECK(usage_percentage >= 0.9);
         BOOST_CHECK(usage_percentage < 1);
         BOOST_CHECK_EQUAL(
-            chainstate.GetCoinsCacheSizeState(&tx_pool, MAX_COINS_CACHE_BYTES, 1 << 10),
+            chainstate.GetCoinsCacheSizeState(MAX_COINS_CACHE_BYTES, 1 << 10),
             CoinsCacheSizeState::LARGE);
     }
 
@@ -144,7 +143,7 @@ BOOST_AUTO_TEST_CASE(getcoinscachesizestate)
     for (int i{0}; i < 1000; ++i) {
         add_coin(view);
         BOOST_CHECK_EQUAL(
-            chainstate.GetCoinsCacheSizeState(&tx_pool),
+            chainstate.GetCoinsCacheSizeState(),
             CoinsCacheSizeState::OK);
     }
 
@@ -152,7 +151,7 @@ BOOST_AUTO_TEST_CASE(getcoinscachesizestate)
     // preallocated memory that doesn't get reclaimed even after flush.
 
     BOOST_CHECK_EQUAL(
-        chainstate.GetCoinsCacheSizeState(&tx_pool, MAX_COINS_CACHE_BYTES, 0),
+        chainstate.GetCoinsCacheSizeState(MAX_COINS_CACHE_BYTES, 0),
         CoinsCacheSizeState::CRITICAL);
 
     view.SetBestBlock(InsecureRand256());
@@ -160,7 +159,7 @@ BOOST_AUTO_TEST_CASE(getcoinscachesizestate)
     print_view_mem_usage(view);
 
     BOOST_CHECK_EQUAL(
-        chainstate.GetCoinsCacheSizeState(&tx_pool, MAX_COINS_CACHE_BYTES, 0),
+        chainstate.GetCoinsCacheSizeState(MAX_COINS_CACHE_BYTES, 0),
         CoinsCacheSizeState::CRITICAL);
 }
 
diff --git a/src/validation.cpp b/src/validation.cpp
index c80f015143..26333d7026 100644
--- a/src/validation.cpp
+++ b/src/validation.cpp
@@ -329,23 +329,14 @@ static bool IsCurrentForFeeEstimation(CChainState& active_chainstate) EXCLUSIVE_
     return true;
 }
 
-/* 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.
- *
- * Note: we assume that disconnectpool only contains transactions that are NOT
- * confirmed in the current chain nor already in the mempool (otherwise,
- * in-mempool descendants of such transactions would be removed).
- *
- * Passing fAddToMempool=false will skip trying to add the transactions back,
- * and instead just erase from the mempool as needed.
- */
-
-static void UpdateMempoolForReorg(CChainState& active_chainstate, CTxMemPool& mempool, DisconnectedBlockTransactions& disconnectpool, bool fAddToMempool) EXCLUSIVE_LOCKS_REQUIRED(cs_main, mempool.cs)
+void CChainState::MaybeUpdateMempoolForReorg(
+    DisconnectedBlockTransactions& disconnectpool,
+    bool fAddToMempool)
 {
+    if (!m_mempool) return;
+
     AssertLockHeld(cs_main);
-    AssertLockHeld(mempool.cs);
+    AssertLockHeld(m_mempool->cs);
     std::vector<uint256> vHashUpdate;
     // disconnectpool's insertion_order index sorts the entries from
     // oldest to newest, but the oldest entry will be the last tx from the
@@ -357,11 +348,13 @@ static void UpdateMempoolForReorg(CChainState& active_chainstate, CTxMemPool& me
     while (it != disconnectpool.queuedTx.get<insertion_order>().rend()) {
         // ignore validation errors in resurrected transactions
         if (!fAddToMempool || (*it)->IsCoinBase() ||
-            AcceptToMemoryPool(active_chainstate, mempool, *it, true /* bypass_limits */).m_result_type != MempoolAcceptResult::ResultType::VALID) {
+            AcceptToMemoryPool(
+                *this, *m_mempool, *it, true /* bypass_limits */).m_result_type !=
+                    MempoolAcceptResult::ResultType::VALID) {
             // If the transaction doesn't make it in to the mempool, remove any
             // transactions that depend on it (which would now be orphans).
-            mempool.removeRecursive(**it, MemPoolRemovalReason::REORG);
-        } else if (mempool.exists((*it)->GetHash())) {
+            m_mempool->removeRecursive(**it, MemPoolRemovalReason::REORG);
+        } else if (m_mempool->exists((*it)->GetHash())) {
             vHashUpdate.push_back((*it)->GetHash());
         }
         ++it;
@@ -372,12 +365,16 @@ static void UpdateMempoolForReorg(CChainState& active_chainstate, CTxMemPool& me
     // previously-confirmed transactions back to the mempool.
     // UpdateTransactionsFromBlock finds descendants of any transactions in
     // the disconnectpool that were added back and cleans up the mempool state.
-    mempool.UpdateTransactionsFromBlock(vHashUpdate);
+    m_mempool->UpdateTransactionsFromBlock(vHashUpdate);
 
     // We also need to remove any now-immature transactions
-    mempool.removeForReorg(active_chainstate, STANDARD_LOCKTIME_VERIFY_FLAGS);
+    m_mempool->removeForReorg(*this, STANDARD_LOCKTIME_VERIFY_FLAGS);
     // Re-limit mempool size, in case we added any transactions
-    LimitMempoolSize(mempool, active_chainstate.CoinsTip(), gArgs.GetArg("-maxmempool", DEFAULT_MAX_MEMPOOL_SIZE) * 1000000, std::chrono::hours{gArgs.GetArg("-mempoolexpiry", DEFAULT_MEMPOOL_EXPIRY)});
+    LimitMempoolSize(
+        *m_mempool,
+        this->CoinsTip(),
+        gArgs.GetArg("-maxmempool", DEFAULT_MAX_MEMPOOL_SIZE) * 1000000,
+        std::chrono::hours{gArgs.GetArg("-mempoolexpiry", DEFAULT_MEMPOOL_EXPIRY)});
 }
 
 /**
@@ -1212,7 +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),
@@ -2005,20 +2002,18 @@ bool CChainState::ConnectBlock(const CBlock& block, BlockValidationState& state,
     return true;
 }
 
-CoinsCacheSizeState CChainState::GetCoinsCacheSizeState(const CTxMemPool* tx_pool)
+CoinsCacheSizeState CChainState::GetCoinsCacheSizeState()
 {
     return this->GetCoinsCacheSizeState(
-        tx_pool,
         m_coinstip_cache_size_bytes,
         gArgs.GetArg("-maxmempool", DEFAULT_MAX_MEMPOOL_SIZE) * 1000000);
 }
 
 CoinsCacheSizeState CChainState::GetCoinsCacheSizeState(
-    const CTxMemPool* tx_pool,
     size_t max_coins_cache_size_bytes,
     size_t max_mempool_size_bytes)
 {
-    const int64_t nMempoolUsage = tx_pool ? tx_pool->DynamicMemoryUsage() : 0;
+    const int64_t nMempoolUsage = m_mempool ? m_mempool->DynamicMemoryUsage() : 0;
     int64_t cacheSize = CoinsTip().DynamicMemoryUsage();
     int64_t nTotalSpace =
         max_coins_cache_size_bytes + std::max<int64_t>(max_mempool_size_bytes - nMempoolUsage, 0);
@@ -2057,7 +2052,7 @@ bool CChainState::FlushStateToDisk(
         bool fFlushForPrune = false;
         bool fDoFullFlush = false;
 
-        CoinsCacheSizeState cache_state = GetCoinsCacheSizeState(&m_mempool);
+        CoinsCacheSizeState cache_state = GetCoinsCacheSizeState();
         LOCK(cs_LastBlockFile);
         if (fPruneMode && (fCheckForPruning || nManualPruneHeight > 0) && !fReindex) {
             // make sure we don't prune above the blockfilterindexes bestblocks
@@ -2208,12 +2203,12 @@ static void AppendWarning(bilingual_str& res, const bilingual_str& warn)
     res += 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)
-    EXCLUSIVE_LOCKS_REQUIRED(::cs_main)
+void CChainState::UpdateTip(const CBlockIndex* pindexNew)
 {
     // New best block
-    mempool.AddTransactionsUpdated(1);
+    if (m_mempool) {
+        m_mempool->AddTransactionsUpdated(1);
+    }
 
     {
         LOCK(g_best_block_mutex);
@@ -2222,11 +2217,11 @@ static void UpdateTip(CTxMemPool& mempool, const CBlockIndex* pindexNew, const C
     }
 
     bilingual_str warning_messages;
-    if (!active_chainstate.IsInitialBlockDownload()) {
+    if (!this->IsInitialBlockDownload()) {
         const CBlockIndex* pindex = pindexNew;
         for (int bit = 0; bit < VERSIONBITS_NUM_BITS; bit++) {
             WarningBitsConditionChecker checker(bit);
-            ThresholdState state = checker.GetStateFor(pindex, chainParams.GetConsensus(), warningcache[bit]);
+            ThresholdState state = checker.GetStateFor(pindex, m_params.GetConsensus(), warningcache[bit]);
             if (state == ThresholdState::ACTIVE || state == ThresholdState::LOCKED_IN) {
                 const bilingual_str warning = strprintf(_("Unknown new rules activated (versionbit %i)"), bit);
                 if (state == ThresholdState::ACTIVE) {
@@ -2241,14 +2236,14 @@ static void UpdateTip(CTxMemPool& mempool, const CBlockIndex* pindexNew, const C
       pindexNew->GetBlockHash().ToString(), pindexNew->nHeight, pindexNew->nVersion,
       log(pindexNew->nChainWork.getdouble())/log(2.0), (unsigned long)pindexNew->nChainTx,
       FormatISO8601DateTime(pindexNew->GetBlockTime()),
-      GuessVerificationProgress(chainParams.TxData(), pindexNew), active_chainstate.CoinsTip().DynamicMemoryUsage() * (1.0 / (1<<20)), active_chainstate.CoinsTip().GetCacheSize(),
+      GuessVerificationProgress(m_params.TxData(), pindexNew), this->CoinsTip().DynamicMemoryUsage() * (1.0 / (1<<20)), this->CoinsTip().GetCacheSize(),
       !warning_messages.empty() ? strprintf(" warning='%s'", warning_messages.original) : "");
 }
 
 /** Disconnect m_chain's tip.
   * After calling, the mempool will be in an inconsistent state, with
   * transactions from disconnected blocks being added to disconnectpool.  You
-  * should make the mempool consistent again by calling UpdateMempoolForReorg.
+  * should make the mempool consistent again by calling MaybeUpdateMempoolForReorg.
   * with cs_main held.
   *
   * If disconnectpool is nullptr, then no disconnected transactions are added to
@@ -2258,7 +2253,7 @@ 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);
@@ -2284,7 +2279,7 @@ 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);
@@ -2292,14 +2287,14 @@ 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);
         }
     }
 
     m_chain.SetTip(pindexDelete->pprev);
 
-    UpdateTip(m_mempool, pindexDelete->pprev, m_params, *this);
+    UpdateTip(pindexDelete->pprev);
     // Let wallets know transactions went from 1-confirmed to
     // 0-confirmed or conflicted:
     GetMainSignals().BlockDisconnected(pblock, pindexDelete);
@@ -2361,7 +2356,7 @@ 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.
@@ -2405,11 +2400,13 @@ 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);
+    UpdateTip(pindexNew);
 
     int64_t nTime6 = GetTimeMicros(); nTimePostConnect += nTime6 - nTime5; nTimeTotal += nTime6 - nTime1;
     LogPrint(BCLog::BENCH, "  - Connect postprocess: %.2fms [%.2fs (%.2fms/blk)]\n", (nTime6 - nTime5) * MILLI, nTimePostConnect * MICRO, nTimePostConnect * MILLI / nBlocksTotal);
@@ -2499,7 +2496,7 @@ 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);
@@ -2511,7 +2508,7 @@ 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);
+            MaybeUpdateMempoolForReorg(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
@@ -2555,7 +2552,7 @@ 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);
+                    MaybeUpdateMempoolForReorg(disconnectpool, false);
                     return false;
                 }
             } else {
@@ -2572,9 +2569,9 @@ bool CChainState::ActivateBestChainStep(BlockValidationState& state, CBlockIndex
     if (fBlocksDisconnected) {
         // If any blocks were disconnected, disconnectpool may be non empty.  Add
         // any disconnected transactions back to the mempool.
-        UpdateMempoolForReorg(*this, m_mempool, disconnectpool, true);
+        MaybeUpdateMempoolForReorg(disconnectpool, true);
     }
-    m_mempool.check(*this);
+    if (m_mempool) m_mempool->check(*this);
 
     CheckForkWarningConditions();
 
@@ -2646,7 +2643,8 @@ 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 {
@@ -2796,7 +2794,9 @@ 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 MaybeUpdateMempoolForReorg 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();
@@ -2810,7 +2810,7 @@ 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);
+        MaybeUpdateMempoolForReorg(disconnectpool, /* fAddToMempool = */ (++disconnected <= 10) && ret);
         if (!ret) return false;
         assert(invalid_walk_tip->pprev == m_chain.Tip());
 
@@ -3821,10 +3821,11 @@ 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()
@@ -4688,7 +4689,8 @@ 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 =
@@ -4767,7 +4769,7 @@ 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);
@@ -4883,7 +4885,7 @@ bool ChainstateManager::PopulateAndValidateSnapshot(
             }
 
             const auto snapshot_cache_state = WITH_LOCK(::cs_main,
-                return snapshot_chainstate.GetCoinsCacheSizeState(&snapshot_chainstate.m_mempool));
+                return snapshot_chainstate.GetCoinsCacheSizeState());
 
             if (snapshot_cache_state >=
                     CoinsCacheSizeState::CRITICAL) {
diff --git a/src/validation.h b/src/validation.h
index 3d66e3161d..9a2be3ad97 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.
@@ -773,19 +777,17 @@ public:
     //! Dictates whether we need to flush the cache to disk or not.
     //!
     //! @return the state of the size of the coins cache.
-    CoinsCacheSizeState GetCoinsCacheSizeState(const CTxMemPool* tx_pool)
-        EXCLUSIVE_LOCKS_REQUIRED(::cs_main);
+    CoinsCacheSizeState GetCoinsCacheSizeState() EXCLUSIVE_LOCKS_REQUIRED(::cs_main);
 
     CoinsCacheSizeState GetCoinsCacheSizeState(
-        const CTxMemPool* tx_pool,
         size_t max_coins_cache_size_bytes,
         size_t max_mempool_size_bytes) EXCLUSIVE_LOCKS_REQUIRED(::cs_main);
 
     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 +800,33 @@ 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;
+    }
+
+    /**
+     * 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.
+     *
+     * Note: we assume that disconnectpool only contains transactions that are NOT
+     * confirmed in the current chain nor already in the mempool (otherwise,
+     * in-mempool descendants of such transactions would be removed).
+     *
+     * Passing fAddToMempool=false will skip trying to add the transactions back,
+     * and instead just erase from the mempool as needed.
+     */
+    void MaybeUpdateMempoolForReorg(
+        DisconnectedBlockTransactions& disconnectpool,
+        bool fAddToMempool) EXCLUSIVE_LOCKS_REQUIRED(cs_main, m_mempool->cs);
+
+    /** Check warning conditions and do some notifications on new chain tip set. */
+    void UpdateTip(const CBlockIndex* pindexNew)
+        EXCLUSIVE_LOCKS_REQUIRED(::cs_main);
+
     friend ChainstateManager;
 };
 
@@ -907,7 +936,9 @@ 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.