coins: pass linked list of flagged entries to BatchWrite

BatchWrite now iterates through the linked
list of flagged entries instead of the entire
coinsCache map.

Co-Authored-By: l0rinc <pap.lorinc@gmail.com>

src/coins.cpp

@@ -12,7 +12,7 @@
 bool CCoinsView::GetCoin(const COutPoint &outpoint, Coin &coin) const { return false; }
 uint256 CCoinsView::GetBestBlock() const { return uint256(); }
 std::vector<uint256> CCoinsView::GetHeadBlocks() const { return std::vector<uint256>(); }
-bool CCoinsView::BatchWrite(CCoinsMap &mapCoins, const uint256 &hashBlock, bool erase) { return false; }
+bool CCoinsView::BatchWrite(CoinsViewCacheCursor& cursor, const uint256 &hashBlock) { return false; }
 std::unique_ptr<CCoinsViewCursor> CCoinsView::Cursor() const { return nullptr; }
 
 bool CCoinsView::HaveCoin(const COutPoint &outpoint) const
@@ -27,7 +27,7 @@ bool CCoinsViewBacked::HaveCoin(const COutPoint &outpoint) const { return base->
 uint256 CCoinsViewBacked::GetBestBlock() const { return base->GetBestBlock(); }
 std::vector<uint256> CCoinsViewBacked::GetHeadBlocks() const { return base->GetHeadBlocks(); }
 void CCoinsViewBacked::SetBackend(CCoinsView &viewIn) { base = &viewIn; }
-bool CCoinsViewBacked::BatchWrite(CCoinsMap &mapCoins, const uint256 &hashBlock, bool erase) { return base->BatchWrite(mapCoins, hashBlock, erase); }
+bool CCoinsViewBacked::BatchWrite(CoinsViewCacheCursor& cursor, const uint256 &hashBlock) { return base->BatchWrite(cursor, hashBlock); }
 std::unique_ptr<CCoinsViewCursor> CCoinsViewBacked::Cursor() const { return base->Cursor(); }
 size_t CCoinsViewBacked::EstimateSize() const { return base->EstimateSize(); }
 
@@ -183,10 +183,8 @@ void CCoinsViewCache::SetBestBlock(const uint256 &hashBlockIn) {
     hashBlock = hashBlockIn;
 }
 
-bool CCoinsViewCache::BatchWrite(CCoinsMap &mapCoins, const uint256 &hashBlockIn, bool erase) {
+bool CCoinsViewCache::BatchWrite(CoinsViewCacheCursor& cursor, const uint256 &hashBlockIn) {
-    for (CCoinsMap::iterator it = mapCoins.begin();
+    for (auto it{cursor.Begin()}; it != cursor.End(); it = cursor.NextAndMaybeErase(*it)) {
-            it != mapCoins.end();
-            it = erase ? mapCoins.erase(it) : std::next(it)) {
         // Ignore non-dirty entries (optimization).
         if (!it->second.IsDirty()) {
             continue;
@@ -200,10 +198,9 @@ bool CCoinsViewCache::BatchWrite(CCoinsMap &mapCoins, const uint256 &hashBlockIn
                 // and mark it as dirty.
                 itUs = cacheCoins.try_emplace(it->first).first;
                 CCoinsCacheEntry& entry{itUs->second};
-                if (erase) {
+                if (cursor.WillErase(*it)) {
-                    // The `move` call here is purely an optimization; we rely on the
+                    // Since this entry will be erased,
-                    // `mapCoins.erase` call in the `for` expression to actually remove
+                    // we can move the coin into us instead of copying it
-                    // the entry from the child map.
                     entry.coin = std::move(it->second.coin);
                 } else {
                     entry.coin = it->second.coin;
@@ -235,10 +232,9 @@ bool CCoinsViewCache::BatchWrite(CCoinsMap &mapCoins, const uint256 &hashBlockIn
             } else {
                 // A normal modification.
                 cachedCoinsUsage -= itUs->second.coin.DynamicMemoryUsage();
-                if (erase) {
+                if (cursor.WillErase(*it)) {
-                    // The `move` call here is purely an optimization; we rely on the
+                    // Since this entry will be erased,
-                    // `mapCoins.erase` call in the `for` expression to actually remove
+                    // we can move the coin into us instead of copying it
-                    // the entry from the child map.
                     itUs->second.coin = std::move(it->second.coin);
                 } else {
                     itUs->second.coin = it->second.coin;
@@ -257,12 +253,10 @@ bool CCoinsViewCache::BatchWrite(CCoinsMap &mapCoins, const uint256 &hashBlockIn
 }
 
 bool CCoinsViewCache::Flush() {
-    bool fOk = base->BatchWrite(cacheCoins, hashBlock, /*erase=*/true);
+    auto cursor{CoinsViewCacheCursor(cachedCoinsUsage, m_sentinel, cacheCoins, /*will_erase=*/true)};
+    bool fOk = base->BatchWrite(cursor, hashBlock);
     if (fOk) {
-        if (!cacheCoins.empty()) {
+        cacheCoins.clear();
-            /* BatchWrite must erase all cacheCoins elements when erase=true. */
-            throw std::logic_error("Not all cached coins were erased");
-        }
         ReallocateCache();
     }
     cachedCoinsUsage = 0;
@@ -271,7 +265,8 @@ bool CCoinsViewCache::Flush() {
 
 bool CCoinsViewCache::Sync()
 {
-    bool fOk = base->BatchWrite(cacheCoins, hashBlock, /*erase=*/false);
+    auto cursor{CoinsViewCacheCursor(cachedCoinsUsage, m_sentinel, cacheCoins, /*will_erase=*/false)};
+    bool fOk = base->BatchWrite(cursor, hashBlock);
     // Instead of clearing `cacheCoins` as we would in Flush(), just clear the
     // FRESH/DIRTY flags of any coin that isn't spent.
     for (auto it = cacheCoins.begin(); it != cacheCoins.end(); ) {

src/coins.h

@@ -243,6 +243,30 @@ private:
     uint256 hashBlock;
 };
 
+/**
+ * Cursor for iterating over the linked list of flagged entries in CCoinsViewCache.
+ */
+struct CoinsViewCacheCursor
+{
+    CoinsViewCacheCursor(size_t& usage LIFETIMEBOUND,
+                        CoinsCachePair& sentinel LIFETIMEBOUND,
+                        CCoinsMap& map LIFETIMEBOUND,
+                        bool will_erase) noexcept
+        : m_usage(usage), m_sentinel(sentinel), m_map(map), m_will_erase(will_erase) {}
+
+    inline CoinsCachePair* Begin() const noexcept { return m_sentinel.second.Next(); }
+    inline CoinsCachePair* End() const noexcept { return &m_sentinel; }
+
+    inline CoinsCachePair* NextAndMaybeErase(CoinsCachePair& current) noexcept { return current.second.Next(); }
+
+    inline bool WillErase(CoinsCachePair& current) const noexcept { return m_will_erase; }
+private:
+    size_t& m_usage;
+    CoinsCachePair& m_sentinel;
+    CCoinsMap& m_map;
+    bool m_will_erase;
+};
+
 /** Abstract view on the open txout dataset. */
 class CCoinsView
 {
@@ -266,8 +290,8 @@ public:
     virtual std::vector<uint256> GetHeadBlocks() const;
 
     //! Do a bulk modification (multiple Coin changes + BestBlock change).
-    //! The passed mapCoins can be modified.
+    //! The passed cursor is used to iterate through the coins.
-    virtual bool BatchWrite(CCoinsMap &mapCoins, const uint256 &hashBlock, bool erase = true);
+    virtual bool BatchWrite(CoinsViewCacheCursor& cursor, const uint256& hashBlock);
 
     //! Get a cursor to iterate over the whole state
     virtual std::unique_ptr<CCoinsViewCursor> Cursor() const;
@@ -293,7 +317,7 @@ public:
     uint256 GetBestBlock() const override;
     std::vector<uint256> GetHeadBlocks() const override;
     void SetBackend(CCoinsView &viewIn);
-    bool BatchWrite(CCoinsMap &mapCoins, const uint256 &hashBlock, bool erase = true) override;
+    bool BatchWrite(CoinsViewCacheCursor& cursor, const uint256 &hashBlock) override;
     std::unique_ptr<CCoinsViewCursor> Cursor() const override;
     size_t EstimateSize() const override;
 };
@@ -332,7 +356,7 @@ public:
     bool HaveCoin(const COutPoint &outpoint) const override;
     uint256 GetBestBlock() const override;
     void SetBestBlock(const uint256 &hashBlock);
-    bool BatchWrite(CCoinsMap &mapCoins, const uint256 &hashBlock, bool erase = true) override;
+    bool BatchWrite(CoinsViewCacheCursor& cursor, const uint256 &hashBlock) override;
     std::unique_ptr<CCoinsViewCursor> Cursor() const override {
         throw std::logic_error("CCoinsViewCache cursor iteration not supported.");
     }

src/test/coins_tests.cpp

@@ -55,9 +55,9 @@ public:
 
     uint256 GetBestBlock() const override { return hashBestBlock_; }
 
-    bool BatchWrite(CCoinsMap& mapCoins, const uint256& hashBlock, bool erase = true) override
+    bool BatchWrite(CoinsViewCacheCursor& cursor, const uint256& hashBlock) override
     {
-        for (CCoinsMap::iterator it = mapCoins.begin(); it != mapCoins.end(); it = erase ? mapCoins.erase(it) : std::next(it)) {
+        for (auto it{cursor.Begin()}; it != cursor.End(); it = cursor.NextAndMaybeErase(*it)){
             if (it->second.IsDirty()) {
                 // Same optimization used in CCoinsViewDB is to only write dirty entries.
                 map_[it->first] = it->second.coin;
@@ -618,8 +618,9 @@ void WriteCoinsViewEntry(CCoinsView& view, CAmount value, char flags)
     sentinel.second.SelfRef(sentinel);
     CCoinsMapMemoryResource resource;
     CCoinsMap map{0, CCoinsMap::hasher{}, CCoinsMap::key_equal{}, &resource};
-    InsertCoinsMapEntry(map, sentinel, value, flags);
+    auto usage{InsertCoinsMapEntry(map, sentinel, value, flags)};
-    BOOST_CHECK(view.BatchWrite(map, {}));
+    auto cursor{CoinsViewCacheCursor(usage, sentinel, map, /*will_erase=*/true)};
+    BOOST_CHECK(view.BatchWrite(cursor, {}));
 }
 
 class SingleEntryCacheTest

src/test/fuzz/coins_view.cpp

@@ -122,6 +122,7 @@ FUZZ_TARGET(coins_view, .init = initialize_coins_view)
             [&] {
                 CoinsCachePair sentinel{};
                 sentinel.second.SelfRef(sentinel);
+                size_t usage{0};
                 CCoinsMapMemoryResource resource;
                 CCoinsMap coins_map{0, SaltedOutpointHasher{/*deterministic=*/true}, CCoinsMap::key_equal{}, &resource};
                 LIMITED_WHILE(good_data && fuzzed_data_provider.ConsumeBool(), 10'000)
@@ -140,10 +141,12 @@ FUZZ_TARGET(coins_view, .init = initialize_coins_view)
                     }
                     auto it{coins_map.emplace(random_out_point, std::move(coins_cache_entry)).first};
                     it->second.AddFlags(flags, *it, sentinel);
+                    usage += it->second.coin.DynamicMemoryUsage();
                 }
                 bool expected_code_path = false;
                 try {
-                    coins_view_cache.BatchWrite(coins_map, fuzzed_data_provider.ConsumeBool() ? ConsumeUInt256(fuzzed_data_provider) : coins_view_cache.GetBestBlock());
+                    auto cursor{CoinsViewCacheCursor(usage, sentinel, coins_map, /*will_erase=*/true)};
+                    coins_view_cache.BatchWrite(cursor, fuzzed_data_provider.ConsumeBool() ? ConsumeUInt256(fuzzed_data_provider) : coins_view_cache.GetBestBlock());
                     expected_code_path = true;
                 } catch (const std::logic_error& e) {
                     if (e.what() == std::string{"FRESH flag misapplied to coin that exists in parent cache"}) {

src/test/fuzz/coinscache_sim.cpp

@@ -172,13 +172,13 @@ public:
     std::unique_ptr<CCoinsViewCursor> Cursor() const final { return {}; }
     size_t EstimateSize() const final { return m_data.size(); }
 
-    bool BatchWrite(CCoinsMap& data, const uint256&, bool erase) final
+    bool BatchWrite(CoinsViewCacheCursor& cursor, const uint256&) final
     {
-        for (auto it = data.begin(); it != data.end(); it = erase ? data.erase(it) : std::next(it)) {
+        for (auto it{cursor.Begin()}; it != cursor.End(); it = cursor.NextAndMaybeErase(*it)) {
             if (it->second.IsDirty()) {
                 if (it->second.coin.IsSpent() && (it->first.n % 5) != 4) {
                     m_data.erase(it->first);
-                } else if (erase) {
+                } else if (cursor.WillErase(*it)) {
                     m_data[it->first] = std::move(it->second.coin);
                 } else {
                     m_data[it->first] = it->second.coin;

src/txdb.cpp

@@ -88,7 +88,7 @@ std::vector<uint256> CCoinsViewDB::GetHeadBlocks() const {
     return vhashHeadBlocks;
 }
 
-bool CCoinsViewDB::BatchWrite(CCoinsMap &mapCoins, const uint256 &hashBlock, bool erase) {
+bool CCoinsViewDB::BatchWrite(CoinsViewCacheCursor& cursor, const uint256 &hashBlock) {
     CDBBatch batch(*m_db);
     size_t count = 0;
     size_t changed = 0;
@@ -114,7 +114,7 @@ bool CCoinsViewDB::BatchWrite(CCoinsMap &mapCoins, const uint256 &hashBlock, boo
     batch.Erase(DB_BEST_BLOCK);
     batch.Write(DB_HEAD_BLOCKS, Vector(hashBlock, old_tip));
 
-    for (CCoinsMap::iterator it = mapCoins.begin(); it != mapCoins.end();) {
+    for (auto it{cursor.Begin()}; it != cursor.End();) {
         if (it->second.IsDirty()) {
             CoinEntry entry(&it->first);
             if (it->second.coin.IsSpent())
@@ -124,7 +124,7 @@ bool CCoinsViewDB::BatchWrite(CCoinsMap &mapCoins, const uint256 &hashBlock, boo
             changed++;
         }
         count++;
-        it = erase ? mapCoins.erase(it) : std::next(it);
+        it = cursor.NextAndMaybeErase(*it);
         if (batch.SizeEstimate() > m_options.batch_write_bytes) {
             LogPrint(BCLog::COINDB, "Writing partial batch of %.2f MiB\n", batch.SizeEstimate() * (1.0 / 1048576.0));
             m_db->WriteBatch(batch);

src/txdb.h

@@ -63,7 +63,7 @@ public:
     bool HaveCoin(const COutPoint &outpoint) const override;
     uint256 GetBestBlock() const override;
     std::vector<uint256> GetHeadBlocks() const override;
-    bool BatchWrite(CCoinsMap &mapCoins, const uint256 &hashBlock, bool erase = true) override;
+    bool BatchWrite(CoinsViewCacheCursor& cursor, const uint256 &hashBlock) override;
     std::unique_ptr<CCoinsViewCursor> Cursor() const override;
 
     //! Whether an unsupported database format is used.