1 files changed, 335 insertions, 57 deletions

diff --git a/src/validation.cpp b/src/validation.cpp
index 13860454dc..f7723764f9 100644
--- a/src/validation.cpp
+++ b/src/validation.cpp
@@ -21,6 +21,7 @@
 #include <index/txindex.h>
 #include <logging.h>
 #include <logging/timer.h>
+#include <node/coinstats.h>
 #include <node/ui_interface.h>
 #include <optional.h>
 #include <policy/policy.h>
@@ -381,10 +382,8 @@ static void UpdateMempoolForReorg(CTxMemPool& mempool, DisconnectedBlockTransact
     auto it = disconnectpool.queuedTx.get<insertion_order>().rbegin();
     while (it != disconnectpool.queuedTx.get<insertion_order>().rend()) {
         // ignore validation errors in resurrected transactions
-        TxValidationState stateDummy;
         if (!fAddToMempool || (*it)->IsCoinBase() ||
-            !AcceptToMemoryPool(mempool, stateDummy, *it,
-                                nullptr /* plTxnReplaced */, true /* bypass_limits */)) {
+            AcceptToMemoryPool(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);
@@ -464,9 +463,7 @@ public:
     // around easier.
     struct ATMPArgs {
         const CChainParams& m_chainparams;
-        TxValidationState &m_state;
         const int64_t m_accept_time;
-        std::list<CTransactionRef>* m_replaced_transactions;
         const bool m_bypass_limits;
         /*
          * Return any outpoints which were not previously present in the coins
@@ -477,11 +474,10 @@ public:
          */
         std::vector<COutPoint>& m_coins_to_uncache;
         const bool m_test_accept;
-        CAmount* m_fee_out;
     };
 
     // Single transaction acceptance
-    bool AcceptSingleTransaction(const CTransactionRef& ptx, ATMPArgs& args) EXCLUSIVE_LOCKS_REQUIRED(cs_main);
+    MempoolAcceptResult AcceptSingleTransaction(const CTransactionRef& ptx, ATMPArgs& args) EXCLUSIVE_LOCKS_REQUIRED(cs_main);
 
 private:
     // All the intermediate state that gets passed between the various levels
@@ -492,14 +488,17 @@ private:
         CTxMemPool::setEntries m_all_conflicting;
         CTxMemPool::setEntries m_ancestors;
         std::unique_ptr<CTxMemPoolEntry> m_entry;
+        std::list<CTransactionRef> m_replaced_transactions;
 
         bool m_replacement_transaction;
+        CAmount m_base_fees;
         CAmount m_modified_fees;
         CAmount m_conflicting_fees;
         size_t m_conflicting_size;
 
         const CTransactionRef& m_ptx;
         const uint256& m_hash;
+        TxValidationState m_state;
     };
 
     // Run the policy checks on a given transaction, excluding any script checks.
@@ -510,18 +509,18 @@ private:
 
     // Run the script checks using our policy flags. As this can be slow, we should
     // only invoke this on transactions that have otherwise passed policy checks.
-    bool PolicyScriptChecks(ATMPArgs& args, const Workspace& ws, PrecomputedTransactionData& txdata) EXCLUSIVE_LOCKS_REQUIRED(cs_main, m_pool.cs);
+    bool PolicyScriptChecks(const ATMPArgs& args, Workspace& ws, PrecomputedTransactionData& txdata) EXCLUSIVE_LOCKS_REQUIRED(cs_main, m_pool.cs);
 
     // Re-run the script checks, using consensus flags, and try to cache the
     // result in the scriptcache. This should be done after
     // PolicyScriptChecks(). This requires that all inputs either be in our
     // utxo set or in the mempool.
-    bool ConsensusScriptChecks(ATMPArgs& args, const Workspace& ws, PrecomputedTransactionData &txdata) EXCLUSIVE_LOCKS_REQUIRED(cs_main, m_pool.cs);
+    bool ConsensusScriptChecks(const ATMPArgs& args, Workspace& ws, PrecomputedTransactionData &txdata) EXCLUSIVE_LOCKS_REQUIRED(cs_main, m_pool.cs);
 
     // Try to add the transaction to the mempool, removing any conflicts first.
     // Returns true if the transaction is in the mempool after any size
     // limiting is performed, false otherwise.
-    bool Finalize(ATMPArgs& args, Workspace& ws) EXCLUSIVE_LOCKS_REQUIRED(cs_main, m_pool.cs);
+    bool Finalize(const ATMPArgs& args, Workspace& ws) EXCLUSIVE_LOCKS_REQUIRED(cs_main, m_pool.cs);
 
     // Compare a package's feerate against minimum allowed.
     bool CheckFeeRate(size_t package_size, CAmount package_fee, TxValidationState& state) EXCLUSIVE_LOCKS_REQUIRED(cs_main, m_pool.cs)
@@ -559,12 +558,12 @@ bool MemPoolAccept::PreChecks(ATMPArgs& args, Workspace& ws)
     const uint256& hash = ws.m_hash;
 
     // Copy/alias what we need out of args
-    TxValidationState &state = args.m_state;
     const int64_t nAcceptTime = args.m_accept_time;
     const bool bypass_limits = args.m_bypass_limits;
     std::vector<COutPoint>& coins_to_uncache = args.m_coins_to_uncache;
 
     // Alias what we need out of ws
+    TxValidationState& state = ws.m_state;
     std::set<uint256>& setConflicts = ws.m_conflicts;
     CTxMemPool::setEntries& allConflicting = ws.m_all_conflicting;
     CTxMemPool::setEntries& setAncestors = ws.m_ancestors;
@@ -684,16 +683,10 @@ bool MemPoolAccept::PreChecks(ATMPArgs& args, Workspace& ws)
     if (!CheckSequenceLocks(m_pool, tx, STANDARD_LOCKTIME_VERIFY_FLAGS, &lp))
         return state.Invalid(TxValidationResult::TX_PREMATURE_SPEND, "non-BIP68-final");
 
-    CAmount nFees = 0;
-    if (!Consensus::CheckTxInputs(tx, state, m_view, g_chainman.m_blockman.GetSpendHeight(m_view), nFees)) {
+    if (!Consensus::CheckTxInputs(tx, state, m_view, g_chainman.m_blockman.GetSpendHeight(m_view), ws.m_base_fees)) {
         return false; // state filled in by CheckTxInputs
     }
 
-    // If fee_out is passed, return the fee to the caller
-    if (args.m_fee_out) {
-        *args.m_fee_out = nFees;
-    }
-
     // Check for non-standard pay-to-script-hash in inputs
     const auto& params = args.m_chainparams.GetConsensus();
     auto taproot_state = VersionBitsState(::ChainActive().Tip(), params, Consensus::DEPLOYMENT_TAPROOT, versionbitscache);
@@ -708,7 +701,7 @@ bool MemPoolAccept::PreChecks(ATMPArgs& args, Workspace& ws)
     int64_t nSigOpsCost = GetTransactionSigOpCost(tx, m_view, STANDARD_SCRIPT_VERIFY_FLAGS);
 
     // nModifiedFees includes any fee deltas from PrioritiseTransaction
-    nModifiedFees = nFees;
+    nModifiedFees = ws.m_base_fees;
     m_pool.ApplyDelta(hash, nModifiedFees);
 
     // Keep track of transactions that spend a coinbase, which we re-scan
@@ -722,7 +715,7 @@ bool MemPoolAccept::PreChecks(ATMPArgs& args, Workspace& ws)
         }
     }
 
-    entry.reset(new CTxMemPoolEntry(ptx, nFees, nAcceptTime, ::ChainActive().Height(),
+    entry.reset(new CTxMemPoolEntry(ptx, ws.m_base_fees, nAcceptTime, ::ChainActive().Height(),
             fSpendsCoinbase, nSigOpsCost, lp));
     unsigned int nSize = entry->GetTxSize();
 
@@ -926,11 +919,10 @@ bool MemPoolAccept::PreChecks(ATMPArgs& args, Workspace& ws)
     return true;
 }
 
-bool MemPoolAccept::PolicyScriptChecks(ATMPArgs& args, const Workspace& ws, PrecomputedTransactionData& txdata)
+bool MemPoolAccept::PolicyScriptChecks(const ATMPArgs& args, Workspace& ws, PrecomputedTransactionData& txdata)
 {
     const CTransaction& tx = *ws.m_ptx;
-
-    TxValidationState &state = args.m_state;
+    TxValidationState& state = ws.m_state;
 
     constexpr unsigned int scriptVerifyFlags = STANDARD_SCRIPT_VERIFY_FLAGS;
 
@@ -953,12 +945,11 @@ bool MemPoolAccept::PolicyScriptChecks(ATMPArgs& args, const Workspace& ws, Prec
     return true;
 }
 
-bool MemPoolAccept::ConsensusScriptChecks(ATMPArgs& args, const Workspace& ws, PrecomputedTransactionData& txdata)
+bool MemPoolAccept::ConsensusScriptChecks(const ATMPArgs& args, Workspace& ws, PrecomputedTransactionData& txdata)
 {
     const CTransaction& tx = *ws.m_ptx;
     const uint256& hash = ws.m_hash;
-
-    TxValidationState &state = args.m_state;
+    TxValidationState& state = ws.m_state;
     const CChainParams& chainparams = args.m_chainparams;
 
     // Check again against the current block tip's script verification
@@ -985,11 +976,11 @@ bool MemPoolAccept::ConsensusScriptChecks(ATMPArgs& args, const Workspace& ws, P
     return true;
 }
 
-bool MemPoolAccept::Finalize(ATMPArgs& args, Workspace& ws)
+bool MemPoolAccept::Finalize(const ATMPArgs& args, Workspace& ws)
 {
     const CTransaction& tx = *ws.m_ptx;
     const uint256& hash = ws.m_hash;
-    TxValidationState &state = args.m_state;
+    TxValidationState& state = ws.m_state;
     const bool bypass_limits = args.m_bypass_limits;
 
     CTxMemPool::setEntries& allConflicting = ws.m_all_conflicting;
@@ -1008,8 +999,7 @@ bool MemPoolAccept::Finalize(ATMPArgs& args, Workspace& ws)
                 hash.ToString(),
                 FormatMoney(nModifiedFees - nConflictingFees),
                 (int)entry->GetTxSize() - (int)nConflictingSize);
-        if (args.m_replaced_transactions)
-            args.m_replaced_transactions->push_back(it->GetSharedTx());
+        ws.m_replaced_transactions.push_back(it->GetSharedTx());
     }
     m_pool.RemoveStaged(allConflicting, false, MemPoolRemovalReason::REPLACED);
 
@@ -1032,14 +1022,14 @@ bool MemPoolAccept::Finalize(ATMPArgs& args, Workspace& ws)
     return true;
 }
 
-bool MemPoolAccept::AcceptSingleTransaction(const CTransactionRef& ptx, ATMPArgs& args)
+MempoolAcceptResult MemPoolAccept::AcceptSingleTransaction(const CTransactionRef& ptx, ATMPArgs& args)
 {
     AssertLockHeld(cs_main);
     LOCK(m_pool.cs); // mempool "read lock" (held through GetMainSignals().TransactionAddedToMempool())
 
-    Workspace workspace(ptx);
+    Workspace ws(ptx);
 
-    if (!PreChecks(args, workspace)) return false;
+    if (!PreChecks(args, ws)) return MempoolAcceptResult(ws.m_state);
 
     // Only compute the precomputed transaction data if we need to verify
     // scripts (ie, other policy checks pass). We perform the inexpensive
@@ -1047,31 +1037,35 @@ bool MemPoolAccept::AcceptSingleTransaction(const CTransactionRef& ptx, ATMPArgs
     // checks pass, to mitigate CPU exhaustion denial-of-service attacks.
     PrecomputedTransactionData txdata;
 
-    if (!PolicyScriptChecks(args, workspace, txdata)) return false;
+    if (!PolicyScriptChecks(args, ws, txdata)) return MempoolAcceptResult(ws.m_state);
 
-    if (!ConsensusScriptChecks(args, workspace, txdata)) return false;
+    if (!ConsensusScriptChecks(args, ws, txdata)) return MempoolAcceptResult(ws.m_state);
 
     // Tx was accepted, but not added
-    if (args.m_test_accept) return true;
+    if (args.m_test_accept) {
+        return MempoolAcceptResult(std::move(ws.m_replaced_transactions), ws.m_base_fees);
+    }
 
-    if (!Finalize(args, workspace)) return false;
+    if (!Finalize(args, ws)) return MempoolAcceptResult(ws.m_state);
 
     GetMainSignals().TransactionAddedToMempool(ptx, m_pool.GetAndIncrementSequence());
 
-    return true;
+    return MempoolAcceptResult(std::move(ws.m_replaced_transactions), ws.m_base_fees);
 }
 
 } // anon namespace
 
 /** (try to) add transaction to memory pool with a specified acceptance time **/
-static bool AcceptToMemoryPoolWithTime(const CChainParams& chainparams, CTxMemPool& pool, TxValidationState &state, const CTransactionRef &tx,
-                        int64_t nAcceptTime, std::list<CTransactionRef>* plTxnReplaced,
-                        bool bypass_limits, bool test_accept, CAmount* fee_out=nullptr) EXCLUSIVE_LOCKS_REQUIRED(cs_main)
+static MempoolAcceptResult AcceptToMemoryPoolWithTime(const CChainParams& chainparams, CTxMemPool& pool,
+                                                      const CTransactionRef &tx, int64_t nAcceptTime,
+                                                      bool bypass_limits, bool test_accept)
+                                                      EXCLUSIVE_LOCKS_REQUIRED(cs_main)
 {
     std::vector<COutPoint> coins_to_uncache;
-    MemPoolAccept::ATMPArgs args { chainparams, state, nAcceptTime, plTxnReplaced, bypass_limits, coins_to_uncache, test_accept, fee_out };
-    bool res = MemPoolAccept(pool).AcceptSingleTransaction(tx, args);
-    if (!res) {
+    MemPoolAccept::ATMPArgs args { chainparams, nAcceptTime, bypass_limits, coins_to_uncache, test_accept };
+
+    const MempoolAcceptResult result = MemPoolAccept(pool).AcceptSingleTransaction(tx, args);
+    if (result.m_result_type != MempoolAcceptResult::ResultType::VALID) {
         // Remove coins that were not present in the coins cache before calling ATMPW;
         // this is to prevent memory DoS in case we receive a large number of
         // invalid transactions that attempt to overrun the in-memory coins cache
@@ -1083,15 +1077,12 @@ static bool AcceptToMemoryPoolWithTime(const CChainParams& chainparams, CTxMemPo
     // After we've (potentially) uncached entries, ensure our coins cache is still within its size limits
     BlockValidationState state_dummy;
     ::ChainstateActive().FlushStateToDisk(chainparams, state_dummy, FlushStateMode::PERIODIC);
-    return res;
+    return result;
 }
 
-bool AcceptToMemoryPool(CTxMemPool& pool, TxValidationState &state, const CTransactionRef &tx,
-                        std::list<CTransactionRef>* plTxnReplaced,
-                        bool bypass_limits, bool test_accept, CAmount* fee_out)
+MempoolAcceptResult AcceptToMemoryPool(CTxMemPool& pool, const CTransactionRef &tx, bool bypass_limits, bool test_accept)
 {
-    const CChainParams& chainparams = Params();
-    return AcceptToMemoryPoolWithTime(chainparams, pool, state, tx, GetTime(), plTxnReplaced, bypass_limits, test_accept, fee_out);
+    return AcceptToMemoryPoolWithTime(Params(), pool, tx, GetTime(), bypass_limits, test_accept);
 }
 
 CTransactionRef GetTransaction(const CBlockIndex* const block_index, const CTxMemPool* const mempool, const uint256& hash, const Consensus::Params& consensusParams, uint256& hashBlock)
@@ -3228,7 +3219,7 @@ static bool FindBlockPos(FlatFilePos &pos, unsigned int nAddSize, unsigned int n
 
     if ((int)nFile != nLastBlockFile) {
         if (!fKnown) {
-            LogPrintf("Leaving block file %i: %s\n", nLastBlockFile, vinfoBlockFile[nLastBlockFile].ToString());
+            LogPrint(BCLog::VALIDATION, "Leaving block file %i: %s\n", nLastBlockFile, vinfoBlockFile[nLastBlockFile].ToString());
         }
         FlushBlockFile(!fKnown, finalize_undo);
         nLastBlockFile = nFile;
@@ -5038,13 +5029,10 @@ bool LoadMempool(CTxMemPool& pool)
             if (amountdelta) {
                 pool.PrioritiseTransaction(tx->GetHash(), amountdelta);
             }
-            TxValidationState state;
             if (nTime > nNow - nExpiryTimeout) {
                 LOCK(cs_main);
-                AcceptToMemoryPoolWithTime(chainparams, pool, state, tx, nTime,
-                                           nullptr /* plTxnReplaced */, false /* bypass_limits */,
-                                           false /* test_accept */);
-                if (state.IsValid()) {
+                if (AcceptToMemoryPoolWithTime(chainparams, pool, tx, nTime, false /* bypass_limits */,
+                                               false /* test_accept */).m_result_type == MempoolAcceptResult::ResultType::VALID) {
                     ++count;
                 } else {
                     // mempool may contain the transaction already, e.g. from
@@ -5169,7 +5157,8 @@ double GuessVerificationProgress(const ChainTxData& data, const CBlockIndex *pin
 
 Optional<uint256> ChainstateManager::SnapshotBlockhash() const {
     LOCK(::cs_main);
-    if (m_active_chainstate != nullptr) {
+    if (m_active_chainstate != nullptr &&
+            !m_active_chainstate->m_from_snapshot_blockhash.IsNull()) {
         // If a snapshot chainstate exists, it will always be our active.
         return m_active_chainstate->m_from_snapshot_blockhash;
     }
@@ -5214,6 +5203,295 @@ CChainState& ChainstateManager::InitializeChainstate(CTxMemPool& mempool, const
     return *to_modify;
 }
 
+const AssumeutxoData* ExpectedAssumeutxo(
+    const int height, const CChainParams& chainparams)
+{
+    const MapAssumeutxo& valid_assumeutxos_map = chainparams.Assumeutxo();
+    const auto assumeutxo_found = valid_assumeutxos_map.find(height);
+
+    if (assumeutxo_found != valid_assumeutxos_map.end()) {
+        return &assumeutxo_found->second;
+    }
+    return nullptr;
+}
+
+bool ChainstateManager::ActivateSnapshot(
+        CAutoFile& coins_file,
+        const SnapshotMetadata& metadata,
+        bool in_memory)
+{
+    uint256 base_blockhash = metadata.m_base_blockhash;
+
+    if (this->SnapshotBlockhash()) {
+        LogPrintf("[snapshot] can't activate a snapshot-based chainstate more than once\n");
+        return false;
+    }
+
+    int64_t current_coinsdb_cache_size{0};
+    int64_t current_coinstip_cache_size{0};
+
+    // Cache percentages to allocate to each chainstate.
+    //
+    // These particular percentages don't matter so much since they will only be
+    // relevant during snapshot activation; caches are rebalanced at the conclusion of
+    // this function. We want to give (essentially) all available cache capacity to the
+    // snapshot to aid the bulk load later in this function.
+    static constexpr double IBD_CACHE_PERC = 0.01;
+    static constexpr double SNAPSHOT_CACHE_PERC = 0.99;
+
+    {
+        LOCK(::cs_main);
+        // Resize the coins caches to ensure we're not exceeding memory limits.
+        //
+        // Allocate the majority of the cache to the incoming snapshot chainstate, since
+        // (optimistically) getting to its tip will be the top priority. We'll need to call
+        // `MaybeRebalanceCaches()` once we're done with this function to ensure
+        // the right allocation (including the possibility that no snapshot was activated
+        // and that we should restore the active chainstate caches to their original size).
+        //
+        current_coinsdb_cache_size = this->ActiveChainstate().m_coinsdb_cache_size_bytes;
+        current_coinstip_cache_size = this->ActiveChainstate().m_coinstip_cache_size_bytes;
+
+        // Temporarily resize the active coins cache to make room for the newly-created
+        // snapshot chain.
+        this->ActiveChainstate().ResizeCoinsCaches(
+            static_cast<size_t>(current_coinstip_cache_size * IBD_CACHE_PERC),
+            static_cast<size_t>(current_coinsdb_cache_size * IBD_CACHE_PERC));
+    }
+
+    auto snapshot_chainstate = WITH_LOCK(::cs_main, return MakeUnique<CChainState>(
+            this->ActiveChainstate().m_mempool, m_blockman, base_blockhash));
+
+    {
+        LOCK(::cs_main);
+        snapshot_chainstate->InitCoinsDB(
+            static_cast<size_t>(current_coinsdb_cache_size * SNAPSHOT_CACHE_PERC),
+            in_memory, false, "chainstate");
+        snapshot_chainstate->InitCoinsCache(
+            static_cast<size_t>(current_coinstip_cache_size * SNAPSHOT_CACHE_PERC));
+    }
+
+    const bool snapshot_ok = this->PopulateAndValidateSnapshot(
+        *snapshot_chainstate, coins_file, metadata);
+
+    if (!snapshot_ok) {
+        WITH_LOCK(::cs_main, this->MaybeRebalanceCaches());
+        return false;
+    }
+
+    {
+        LOCK(::cs_main);
+        assert(!m_snapshot_chainstate);
+        m_snapshot_chainstate.swap(snapshot_chainstate);
+        const bool chaintip_loaded = m_snapshot_chainstate->LoadChainTip(::Params());
+        assert(chaintip_loaded);
+
+        m_active_chainstate = m_snapshot_chainstate.get();
+
+        LogPrintf("[snapshot] successfully activated snapshot %s\n", base_blockhash.ToString());
+        LogPrintf("[snapshot] (%.2f MB)\n",
+            m_snapshot_chainstate->CoinsTip().DynamicMemoryUsage() / (1000 * 1000));
+
+        this->MaybeRebalanceCaches();
+    }
+    return true;
+}
+
+bool ChainstateManager::PopulateAndValidateSnapshot(
+    CChainState& snapshot_chainstate,
+    CAutoFile& coins_file,
+    const SnapshotMetadata& metadata)
+{
+    // It's okay to release cs_main before we're done using `coins_cache` because we know
+    // that nothing else will be referencing the newly created snapshot_chainstate yet.
+    CCoinsViewCache& coins_cache = *WITH_LOCK(::cs_main, return &snapshot_chainstate.CoinsTip());
+
+    uint256 base_blockhash = metadata.m_base_blockhash;
+
+    COutPoint outpoint;
+    Coin coin;
+    const uint64_t coins_count = metadata.m_coins_count;
+    uint64_t coins_left = metadata.m_coins_count;
+
+    LogPrintf("[snapshot] loading coins from snapshot %s\n", base_blockhash.ToString());
+    int64_t flush_now{0};
+    int64_t coins_processed{0};
+
+    while (coins_left > 0) {
+        try {
+            coins_file >> outpoint;
+        } catch (const std::ios_base::failure&) {
+            LogPrintf("[snapshot] bad snapshot - no coins left after deserializing %d coins\n",
+                coins_count - coins_left);
+            return false;
+        }
+        coins_file >> coin;
+        coins_cache.EmplaceCoinInternalDANGER(std::move(outpoint), std::move(coin));
+
+        --coins_left;
+        ++coins_processed;
+
+        if (coins_processed % 1000000 == 0) {
+            LogPrintf("[snapshot] %d coins loaded (%.2f%%, %.2f MB)\n",
+                coins_processed,
+                static_cast<float>(coins_processed) * 100 / static_cast<float>(coins_count),
+                coins_cache.DynamicMemoryUsage() / (1000 * 1000));
+        }
+
+        // Batch write and flush (if we need to) every so often.
+        //
+        // If our average Coin size is roughly 41 bytes, checking every 120,000 coins
+        // means <5MB of memory imprecision.
+        if (coins_processed % 120000 == 0) {
+            if (ShutdownRequested()) {
+                return false;
+            }
+
+            const auto snapshot_cache_state = WITH_LOCK(::cs_main,
+                return snapshot_chainstate.GetCoinsCacheSizeState(&snapshot_chainstate.m_mempool));
+
+            if (snapshot_cache_state >=
+                    CoinsCacheSizeState::CRITICAL) {
+                LogPrintf("[snapshot] flushing coins cache (%.2f MB)... ", /* Continued */
+                    coins_cache.DynamicMemoryUsage() / (1000 * 1000));
+                flush_now = GetTimeMillis();
+
+                // This is a hack - we don't know what the actual best block is, but that
+                // doesn't matter for the purposes of flushing the cache here. We'll set this
+                // to its correct value (`base_blockhash`) below after the coins are loaded.
+                coins_cache.SetBestBlock(GetRandHash());
+
+                coins_cache.Flush();
+                LogPrintf("done (%.2fms)\n", GetTimeMillis() - flush_now);
+            }
+        }
+    }
+
+    // Important that we set this. This and the coins_cache accesses above are
+    // sort of a layer violation, but either we reach into the innards of
+    // CCoinsViewCache here or we have to invert some of the CChainState to
+    // embed them in a snapshot-activation-specific CCoinsViewCache bulk load
+    // method.
+    coins_cache.SetBestBlock(base_blockhash);
+
+    bool out_of_coins{false};
+    try {
+        coins_file >> outpoint;
+    } catch (const std::ios_base::failure&) {
+        // We expect an exception since we should be out of coins.
+        out_of_coins = true;
+    }
+    if (!out_of_coins) {
+        LogPrintf("[snapshot] bad snapshot - coins left over after deserializing %d coins\n",
+            coins_count);
+        return false;
+    }
+
+    LogPrintf("[snapshot] loaded %d (%.2f MB) coins from snapshot %s\n",
+        coins_count,
+        coins_cache.DynamicMemoryUsage() / (1000 * 1000),
+        base_blockhash.ToString());
+
+    LogPrintf("[snapshot] flushing snapshot chainstate to disk\n");
+    // No need to acquire cs_main since this chainstate isn't being used yet.
+    coins_cache.Flush(); // TODO: if #17487 is merged, add erase=false here for better performance.
+
+    assert(coins_cache.GetBestBlock() == base_blockhash);
+
+    CCoinsStats stats;
+    auto breakpoint_fnc = [] { /* TODO insert breakpoint here? */ };
+
+    // As above, okay to immediately release cs_main here since no other context knows
+    // about the snapshot_chainstate.
+    CCoinsViewDB* snapshot_coinsdb = WITH_LOCK(::cs_main, return &snapshot_chainstate.CoinsDB());
+
+    if (!GetUTXOStats(snapshot_coinsdb, stats, CoinStatsHashType::HASH_SERIALIZED, breakpoint_fnc)) {
+        LogPrintf("[snapshot] failed to generate coins stats\n");
+        return false;
+    }
+
+    // Ensure that the base blockhash appears in the known chain of valid headers. We're willing to
+    // wait a bit here because the snapshot may have been loaded on startup, before we've
+    // received headers from the network.
+
+    int max_secs_to_wait_for_headers = 60 * 10;
+    CBlockIndex* snapshot_start_block = nullptr;
+
+    while (max_secs_to_wait_for_headers > 0) {
+        snapshot_start_block = WITH_LOCK(::cs_main,
+            return m_blockman.LookupBlockIndex(base_blockhash));
+        --max_secs_to_wait_for_headers;
+
+        if (!snapshot_start_block) {
+            std::this_thread::sleep_for(std::chrono::seconds(1));
+        } else {
+            break;
+        }
+    }
+
+    if (snapshot_start_block == nullptr) {
+        LogPrintf("[snapshot] timed out waiting for snapshot start blockheader %s\n",
+            base_blockhash.ToString());
+        return false;
+    }
+
+    // Assert that the deserialized chainstate contents match the expected assumeutxo value.
+
+    int base_height = snapshot_start_block->nHeight;
+    auto maybe_au_data = ExpectedAssumeutxo(base_height, ::Params());
+
+    if (!maybe_au_data) {
+        LogPrintf("[snapshot] assumeutxo height in snapshot metadata not recognized " /* Continued */
+            "(%d) - refusing to load snapshot\n", base_height);
+        return false;
+    }
+
+    const AssumeutxoData& au_data = *maybe_au_data;
+
+    if (stats.hashSerialized != au_data.hash_serialized) {
+        LogPrintf("[snapshot] bad snapshot content hash: expected %s, got %s\n",
+            au_data.hash_serialized.ToString(), stats.hashSerialized.ToString());
+        return false;
+    }
+
+    snapshot_chainstate.m_chain.SetTip(snapshot_start_block);
+
+    // The remainder of this function requires modifying data protected by cs_main.
+    LOCK(::cs_main);
+
+    // Fake various pieces of CBlockIndex state:
+    //
+    //   - nChainTx: so that we accurately report IBD-to-tip progress
+    //   - nTx: so that LoadBlockIndex() loads assumed-valid CBlockIndex entries
+    //       (among other things)
+    //   - nStatus & BLOCK_OPT_WITNESS: so that RewindBlockIndex() doesn't zealously
+    //       unwind the assumed-valid chain.
+    //
+    CBlockIndex* index = nullptr;
+    for (int i = 0; i <= snapshot_chainstate.m_chain.Height(); ++i) {
+        index = snapshot_chainstate.m_chain[i];
+
+        if (!index->nTx) {
+            index->nTx = 1;
+        }
+        index->nChainTx = index->pprev ? index->pprev->nChainTx + index->nTx : 1;
+
+        // We need to fake this flag so that CChainState::RewindBlockIndex()
+        // won't try to rewind the entire assumed-valid chain on startup.
+        if (index->pprev && ::IsWitnessEnabled(index->pprev, ::Params().GetConsensus())) {
+            index->nStatus |= BLOCK_OPT_WITNESS;
+        }
+    }
+
+    assert(index);
+    index->nChainTx = metadata.m_nchaintx;
+    snapshot_chainstate.setBlockIndexCandidates.insert(snapshot_start_block);
+
+    LogPrintf("[snapshot] validated snapshot (%.2f MB)\n",
+        coins_cache.DynamicMemoryUsage() / (1000 * 1000));
+    return true;
+}
+
 CChainState& ChainstateManager::ActiveChainstate() const
 {
     LOCK(::cs_main);