refactor: Delegate to LevelDB for CDBBatch size estimation

Serialized batch size can be queried via the underlying LevelDB implementation calling the native `leveldb::WriteBatch::ApproximateSize()`.

The previous manual calculation was added in e66dbde6d1 as part of https://github.com/bitcoin/bitcoin/pull/10195. At that time (April 2017), the version of LevelDB used by Bitcoin Core (and even the latest source) lacked a native function for this. LevelDB added this capability in 69e2bd224b, merged later that year.

The old manual estimation method (`SizeEstimate()`) is kept temporarily in this commit, and assertions are added in `txdb.cpp` to verify its results against `ApproximateSize()` during batch writes. This ensures the native function behaves as expected before removing the manual calculation in the subsequent commit.

src/dbwrapper.cpp

@@ -200,6 +200,11 @@ void CDBBatch::EraseImpl(std::span<const std::byte> key)
     size_estimate += 2 + (slKey.size() > 127) + slKey.size();
 }
 
+size_t CDBBatch::ApproximateSize() const
+{
+    return m_impl_batch->batch.ApproximateSize();
+}
+
 struct LevelDBContext {
     //! custom environment this database is using (may be nullptr in case of default environment)
     leveldb::Env* penv;

src/dbwrapper.h

@@ -119,7 +119,8 @@ public:
         ssKey.clear();
     }
 
-    size_t SizeEstimate() const { return size_estimate; }
+    size_t ApproximateSize() const;
+    size_t SizeEstimate() const { return size_estimate; } // TODO replace with ApproximateSize
 };
 
 class CDBIterator

src/txdb.cpp

@@ -113,24 +113,39 @@ 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())
+            if (it->second.coin.IsSpent()) {
+                assert(batch.ApproximateSize() == batch.SizeEstimate()); // TODO remove
                 batch.Erase(entry);
-            else
+                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));
+
+            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) {
@@ -142,11 +157,15 @@ 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));
     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;
 }