simplify ChainstateManager::SnapshotBlockhash() return semantics

Don't return null snapshotblockhash values to avoid caller complexity/confusion.

6 files changed, 347 insertions, 3 deletions

diff --git a/src/chain.h b/src/chain.h
index 43e8a39f36..04a5db5a17 100644
--- a/src/chain.h
+++ b/src/chain.h
@@ -163,14 +163,27 @@ public:
 
     //! Number of transactions in this block.
     //! Note: in a potential headers-first mode, this number cannot be relied upon
+    //! Note: this value is faked during UTXO snapshot load to ensure that
+    //! LoadBlockIndex() will load index entries for blocks that we lack data for.
+    //! @sa ActivateSnapshot
     unsigned int nTx{0};
 
     //! (memory only) Number of transactions in the chain up to and including this block.
     //! This value will be non-zero only if and only if transactions for this block and all its parents are available.
     //! Change to 64-bit type when necessary; won't happen before 2030
+    //!
+    //! Note: this value is faked during use of a UTXO snapshot because we don't
+    //! have the underlying block data available during snapshot load.
+    //! @sa AssumeutxoData
+    //! @sa ActivateSnapshot
     unsigned int nChainTx{0};
 
     //! Verification status of this block. See enum BlockStatus
+    //!
+    //! Note: this value is modified to show BLOCK_OPT_WITNESS during UTXO snapshot
+    //! load to avoid the block index being spuriously rewound.
+    //! @sa RewindBlockIndex
+    //! @sa ActivateSnapshot
     uint32_t nStatus{0};
 
     //! block header
diff --git a/src/coins.cpp b/src/coins.cpp
index dd84e720e7..d52851cadd 100644
--- a/src/coins.cpp
+++ b/src/coins.cpp
@@ -97,6 +97,14 @@ void CCoinsViewCache::AddCoin(const COutPoint &outpoint, Coin&& coin, bool possi
     cachedCoinsUsage += it->second.coin.DynamicMemoryUsage();
 }
 
+void CCoinsViewCache::EmplaceCoinInternalDANGER(COutPoint&& outpoint, Coin&& coin) {
+    cachedCoinsUsage += coin.DynamicMemoryUsage();
+    cacheCoins.emplace(
+        std::piecewise_construct,
+        std::forward_as_tuple(std::move(outpoint)),
+        std::forward_as_tuple(std::move(coin), CCoinsCacheEntry::DIRTY));
+}
+
 void AddCoins(CCoinsViewCache& cache, const CTransaction &tx, int nHeight, bool check_for_overwrite) {
     bool fCoinbase = tx.IsCoinBase();
     const uint256& txid = tx.GetHash();
diff --git a/src/coins.h b/src/coins.h
index d2eb42d8cf..feb441fd6a 100644
--- a/src/coins.h
+++ b/src/coins.h
@@ -20,6 +20,8 @@
 #include <functional>
 #include <unordered_map>
 
+class ChainstateManager;
+
 /**
  * A UTXO entry.
  *
@@ -125,6 +127,7 @@ struct CCoinsCacheEntry
 
     CCoinsCacheEntry() : flags(0) {}
     explicit CCoinsCacheEntry(Coin&& coin_) : coin(std::move(coin_)), flags(0) {}
+    CCoinsCacheEntry(Coin&& coin_, unsigned char flag) : coin(std::move(coin_)), flags(flag) {}
 };
 
 typedef std::unordered_map<COutPoint, CCoinsCacheEntry, SaltedOutpointHasher> CCoinsMap;
@@ -263,6 +266,15 @@ public:
     void AddCoin(const COutPoint& outpoint, Coin&& coin, bool possible_overwrite);
 
     /**
+     * Emplace a coin into cacheCoins without performing any checks, marking
+     * the emplaced coin as dirty.
+     *
+     * NOT FOR GENERAL USE. Used only when loading coins from a UTXO snapshot.
+     * @sa ChainstateManager::PopulateAndValidateSnapshot()
+     */
+    void EmplaceCoinInternalDANGER(COutPoint&& outpoint, Coin&& coin);
+
+    /**
      * Spend a coin. Pass moveto in order to get the deleted data.
      * If no unspent output exists for the passed outpoint, this call
      * has no effect.
diff --git a/src/test/validation_chainstatemanager_tests.cpp b/src/test/validation_chainstatemanager_tests.cpp
index 3d8570e27c..d02b7a733d 100644
--- a/src/test/validation_chainstatemanager_tests.cpp
+++ b/src/test/validation_chainstatemanager_tests.cpp
@@ -28,6 +28,8 @@ BOOST_AUTO_TEST_CASE(chainstatemanager)
     std::vector<CChainState*> chainstates;
     const CChainParams& chainparams = Params();
 
+    BOOST_CHECK(!manager.SnapshotBlockhash().has_value());
+
     // Create a legacy (IBD) chainstate.
     //
     CChainState& c1 = WITH_LOCK(::cs_main, return manager.InitializeChainstate(mempool));
@@ -54,10 +56,17 @@ BOOST_AUTO_TEST_CASE(chainstatemanager)
     auto& validated_cs = manager.ValidatedChainstate();
     BOOST_CHECK_EQUAL(&validated_cs, &c1);
 
+    BOOST_CHECK(!manager.SnapshotBlockhash().has_value());
+
     // Create a snapshot-based chainstate.
     //
-    CChainState& c2 = WITH_LOCK(::cs_main, return manager.InitializeChainstate(mempool, GetRandHash()));
+    const uint256 snapshot_blockhash = GetRandHash();
+    CChainState& c2 = WITH_LOCK(::cs_main, return manager.InitializeChainstate(
+        mempool, snapshot_blockhash));
     chainstates.push_back(&c2);
+
+    BOOST_CHECK_EQUAL(manager.SnapshotBlockhash().value(), snapshot_blockhash);
+
     c2.InitCoinsDB(
         /* cache_size_bytes */ 1 << 23, /* in_memory */ true, /* should_wipe */ false);
     WITH_LOCK(::cs_main, c2.InitCoinsCache(1 << 23));
diff --git a/src/validation.cpp b/src/validation.cpp
index 993b4cdb65..31609ea3e5 100644
--- a/src/validation.cpp
+++ b/src/validation.cpp
@@ -20,9 +20,9 @@
 #include <index/txindex.h>
 #include <logging.h>
 #include <logging/timer.h>
+#include <node/coinstats.h>
 #include <node/ui_interface.h>
 #include <optional.h>
-#include <node/coinstats.h>
 #include <policy/policy.h>
 #include <policy/settings.h>
 #include <pow.h>
@@ -5148,7 +5148,8 @@ 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;
     }
@@ -5205,6 +5206,283 @@ const AssumeutxoData* ExpectedAssumeutxo(
     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);
diff --git a/src/validation.h b/src/validation.h
index 00fc87878c..5ec5a549ce 100644
--- a/src/validation.h
+++ b/src/validation.h
@@ -11,10 +11,12 @@
 #endif
 
 #include <amount.h>
+#include <attributes.h>
 #include <coins.h>
 #include <consensus/validation.h>
 #include <crypto/common.h> // for ReadLE64
 #include <fs.h>
+#include <node/utxo_snapshot.h>
 #include <optional.h>
 #include <policy/feerate.h>
 #include <protocol.h> // For CMessageHeader::MessageStartChars
@@ -870,6 +872,12 @@ private:
     //! by the background validation chainstate.
     bool m_snapshot_validated{false};
 
+    //! Internal helper for ActivateSnapshot().
+    [[nodiscard]] bool PopulateAndValidateSnapshot(
+        CChainState& snapshot_chainstate,
+        CAutoFile& coins_file,
+        const SnapshotMetadata& metadata);
+
     // For access to m_active_chainstate.
     friend CChainState& ChainstateActive();
     friend CChain& ChainActive();
@@ -900,6 +908,22 @@ public:
     //! Get all chainstates currently being used.
     std::vector<CChainState*> GetAll();
 
+    //! Construct and activate a Chainstate on the basis of UTXO snapshot data.
+    //!
+    //! Steps:
+    //!
+    //! - Initialize an unused CChainState.
+    //! - Load its `CoinsViews` contents from `coins_file`.
+    //! - Verify that the hash of the resulting coinsdb matches the expected hash
+    //!   per assumeutxo chain parameters.
+    //! - Wait for our headers chain to include the base block of the snapshot.
+    //! - "Fast forward" the tip of the new chainstate to the base of the snapshot,
+    //!   faking nTx* block index data along the way.
+    //! - Move the new chainstate to `m_snapshot_chainstate` and make it our
+    //!   ChainstateActive().
+    [[nodiscard]] bool ActivateSnapshot(
+        CAutoFile& coins_file, const SnapshotMetadata& metadata, bool in_memory);
+
     //! The most-work chain.
     CChainState& ActiveChainstate() const;
     CChain& ActiveChain() const { return ActiveChainstate().m_chain; }