refactor: Remove manual CDBBatch size estimation

Remove the manual batch size estimation logic (`SizeEstimate()` method and `size_estimate` member) from `CDBBatch`.
Size is now determined solely by the `ApproximateSize()` method introduced in the previous commit, which delegates to the native LevelDB function.

The manual calculation is no longer necessary as LevelDB now provides this functionality directly, and the previous commit verified that the native function's results matched the manual estimation.

Assertions comparing the two methods are removed from `txdb.cpp`.

Co-authored-by: Wladimir J. van der Laan <laanwj@protonmail.com>

src/dbwrapper.cpp

@@ -168,8 +168,6 @@ CDBBatch::~CDBBatch() = default;
 void CDBBatch::Clear()
 {
     m_impl_batch->batch.Clear();
-    assert(m_impl_batch->batch.ApproximateSize() == kHeader);
-    size_estimate = kHeader; // TODO remove
 }
 
 void CDBBatch::WriteImpl(std::span<const std::byte> key, DataStream& ssValue)
@@ -178,26 +176,12 @@ void CDBBatch::WriteImpl(std::span<const std::byte> key, DataStream& ssValue)
     ssValue.Xor(dbwrapper_private::GetObfuscateKey(parent));
     leveldb::Slice slValue(CharCast(ssValue.data()), ssValue.size());
     m_impl_batch->batch.Put(slKey, slValue);
-    // LevelDB serializes writes as:
-    // - byte: header
-    // - varint: key length (1 byte up to 127B, 2 bytes up to 16383B, ...)
-    // - byte[]: key
-    // - varint: value length
-    // - byte[]: value
-    // The formula below assumes the key and value are both less than 16k.
-    size_estimate += 3 + (slKey.size() > 127) + slKey.size() + (slValue.size() > 127) + slValue.size();
 }
 
 void CDBBatch::EraseImpl(std::span<const std::byte> key)
 {
     leveldb::Slice slKey(CharCast(key.data()), key.size());
     m_impl_batch->batch.Delete(slKey);
-    // LevelDB serializes erases as:
-    // - byte: header
-    // - varint: key length
-    // - byte[]: key
-    // The formula below assumes the key is less than 16kB.
-    size_estimate += 2 + (slKey.size() > 127) + slKey.size();
 }
 
 size_t CDBBatch::ApproximateSize() const

src/dbwrapper.h

@@ -75,8 +75,6 @@ class CDBBatch
     friend class CDBWrapper;
 
 private:
-    static constexpr size_t kHeader{12}; // See: src/leveldb/db/write_batch.cc#L27
-
     const CDBWrapper &parent;
 
     struct WriteBatchImpl;
@@ -85,8 +83,6 @@ private:
     DataStream ssKey{};
     DataStream ssValue{};
 
-    size_t size_estimate{0};
-
     void WriteImpl(std::span<const std::byte> key, DataStream& ssValue);
     void EraseImpl(std::span<const std::byte> key);
 
@@ -120,7 +116,6 @@ public:
     }
 
     size_t ApproximateSize() const;
-    size_t SizeEstimate() const { return size_estimate; } // TODO replace with ApproximateSize
 };
 
 class CDBIterator

src/txdb.cpp

@@ -113,39 +113,27 @@ bool CCoinsViewDB::BatchWrite(CoinsViewCacheCursor& cursor, const uint256 &hashB
     // transition from old_tip to hashBlock.
     // A vector is used for future extensibility, as we may want to support
     // interrupting after partial writes from multiple independent reorgs.
-    assert(batch.ApproximateSize() == batch.SizeEstimate()); // TODO remove
     batch.Erase(DB_BEST_BLOCK);
-    assert(batch.ApproximateSize() == batch.SizeEstimate()); // TODO remove
     batch.Write(DB_HEAD_BLOCKS, Vector(hashBlock, old_tip));
-    assert(batch.ApproximateSize() == batch.SizeEstimate()); // TODO remove
 
     for (auto it{cursor.Begin()}; it != cursor.End();) {
         if (it->second.IsDirty()) {
             CoinEntry entry(&it->first);
             if (it->second.coin.IsSpent()) {
-                assert(batch.ApproximateSize() == batch.SizeEstimate()); // TODO remove
                 batch.Erase(entry);
-                assert(batch.ApproximateSize() == batch.SizeEstimate()); // TODO remove
             } else {
-                assert(batch.ApproximateSize() == batch.SizeEstimate()); // TODO remove
                 batch.Write(entry, it->second.coin);
-                assert(batch.ApproximateSize() == batch.SizeEstimate()); // TODO remove
             }
 
             changed++;
         }
         count++;
         it = cursor.NextAndMaybeErase(*it);
-        assert(batch.ApproximateSize() == batch.SizeEstimate()); // TODO remove
-        if (batch.SizeEstimate() > m_options.batch_write_bytes) {
-            LogDebug(BCLog::COINDB, "Writing partial batch of %.2f MiB\n", batch.SizeEstimate() * (1.0 / 1048576.0));
+        if (batch.ApproximateSize() > m_options.batch_write_bytes) {
+            LogDebug(BCLog::COINDB, "Writing partial batch of %.2f MiB\n", batch.ApproximateSize() * (1.0 / 1048576.0));
 
-            assert(batch.ApproximateSize() == batch.SizeEstimate()); // TODO remove
             m_db->WriteBatch(batch);
-            assert(batch.ApproximateSize() == batch.SizeEstimate()); // TODO remove
             batch.Clear();
-            assert(batch.ApproximateSize() == batch.SizeEstimate()); // TODO remove
-
             if (m_options.simulate_crash_ratio) {
                 static FastRandomContext rng;
                 if (rng.randrange(m_options.simulate_crash_ratio) == 0) {
@@ -157,15 +145,11 @@ bool CCoinsViewDB::BatchWrite(CoinsViewCacheCursor& cursor, const uint256 &hashB
     }
 
     // In the last batch, mark the database as consistent with hashBlock again.
-    assert(batch.ApproximateSize() == batch.SizeEstimate()); // TODO remove
     batch.Erase(DB_HEAD_BLOCKS);
-    assert(batch.ApproximateSize() == batch.SizeEstimate()); // TODO remove
     batch.Write(DB_BEST_BLOCK, hashBlock);
-    assert(batch.ApproximateSize() == batch.SizeEstimate()); // TODO remove
 
-    LogDebug(BCLog::COINDB, "Writing final batch of %.2f MiB\n", batch.SizeEstimate() * (1.0 / 1048576.0));
+    LogDebug(BCLog::COINDB, "Writing final batch of %.2f MiB\n", batch.ApproximateSize() * (1.0 / 1048576.0));
     bool ret = m_db->WriteBatch(batch);
-    assert(batch.ApproximateSize() == batch.SizeEstimate()); // TODO remove
     LogDebug(BCLog::COINDB, "Committed %u changed transaction outputs (out of %u) to coin database...\n", (unsigned int)changed, (unsigned int)count);
     return ret;
 }