refactor: Pimpl leveldb::batch for CDBBatch

Hide the leveldb::WriteBatch member variable with a pimpl in order not
to expose it directly in the header.

Also move CDBBatch::Clear to the dbwrapper implementation to use the new
impl_batch.

The context of this commit is an effort to decouple the dbwrapper header
file from leveldb includes. To this end, the includes are moved to the
dbwrapper implementation file. This is done as part of the kernel
project to reduce the number of required includes for users of the
kernel.

Author: TheCharlatan

src/dbwrapper.cpp

@@ -4,8 +4,10 @@
 
 #include <dbwrapper.h>
 
+#include <clientversion.h>
 #include <logging.h>
 #include <random.h>
+#include <serialize.h>
 #include <span.h>
 #include <streams.h>
 #include <tinyformat.h>
@@ -136,12 +138,28 @@ static leveldb::Options GetOptions(size_t nCacheSize)
     return options;
 }
 
+struct CDBBatch::WriteBatchImpl {
+    leveldb::WriteBatch batch;
+};
+
+CDBBatch::CDBBatch(const CDBWrapper& _parent) : parent(_parent),
+                                                m_impl_batch{std::make_unique<CDBBatch::WriteBatchImpl>()},
+                                                ssValue(SER_DISK, CLIENT_VERSION){};
+
+CDBBatch::~CDBBatch() = default;
+
+void CDBBatch::Clear()
+{
+    m_impl_batch->batch.Clear();
+    size_estimate = 0;
+}
+
 void CDBBatch::WriteImpl(Span<const std::byte> ssKey, CDataStream& ssValue)
 {
     leveldb::Slice slKey(CharCast(ssKey.data()), ssKey.size());
     ssValue.Xor(dbwrapper_private::GetObfuscateKey(parent));
     leveldb::Slice slValue(CharCast(ssValue.data()), ssValue.size());
-    batch.Put(slKey, slValue);
+    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, ...)
@@ -155,7 +173,7 @@ void CDBBatch::WriteImpl(Span<const std::byte> ssKey, CDataStream& ssValue)
 void CDBBatch::EraseImpl(Span<const std::byte> ssKey)
 {
     leveldb::Slice slKey(CharCast(ssKey.data()), ssKey.size());
-    batch.Delete(slKey);
+    m_impl_batch->batch.Delete(slKey);
     // LevelDB serializes erases as:
     // - byte: header
     // - varint: key length
@@ -241,7 +259,7 @@ bool CDBWrapper::WriteBatch(CDBBatch& batch, bool fSync)
     if (log_memory) {
         mem_before = DynamicMemoryUsage() / 1024.0 / 1024;
     }
-    leveldb::Status status = pdb->Write(fSync ? syncoptions : writeoptions, &batch.batch);
+    leveldb::Status status = pdb->Write(fSync ? syncoptions : writeoptions, &batch.m_impl_batch->batch);
     dbwrapper_private::HandleError(status);
     if (log_memory) {
         double mem_after = DynamicMemoryUsage() / 1024.0 / 1024;

src/dbwrapper.h

@@ -20,7 +20,7 @@
 #include <leveldb/options.h>
 #include <leveldb/slice.h>
 #include <leveldb/status.h>
-#include <leveldb/write_batch.h>
+#include <memory>
 #include <optional>
 #include <stdexcept>
 #include <string>
@@ -90,7 +90,9 @@ class CDBBatch
 
 private:
     const CDBWrapper &parent;
-    leveldb::WriteBatch batch;
+
+    struct WriteBatchImpl;
+    const std::unique_ptr<WriteBatchImpl> m_impl_batch;
 
     DataStream ssKey{};
     CDataStream ssValue;
@@ -104,13 +106,9 @@ class CDBBatch
     /**
      * @param[in] _parent   CDBWrapper that this batch is to be submitted to
      */
-    explicit CDBBatch(const CDBWrapper& _parent) : parent(_parent), ssValue(SER_DISK, CLIENT_VERSION){};
-
-    void Clear()
-    {
-        batch.Clear();
-        size_estimate = 0;
-    }
+    explicit CDBBatch(const CDBWrapper& _parent);
+    ~CDBBatch();
+    void Clear();
 
     template <typename K, typename V>
     void Write(const K& key, const V& value)