From 032223f403d320c3c47c7d5932e333f306efcdc3 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?L=C5=91rinc?= <pap.lorinc@gmail.com>
Date: Fri, 24 Apr 2026 22:14:05 +0200
Subject: [PATCH] dbwrapper: reuse iterator scratch stream

`CDBIterator::Seek()` and `CDBIterator::GetValue()` only need a temporary owning buffer for immediate use.

Keep one preallocated `DataStream` scratch member on the non-copyable iterator and guard each use with `ScopedDataStreamUsage`.
`SeekImpl()` consumes the serialized key during the immediate LevelDB seek, and `GetValue()` still copies LevelDB value bytes into the owning scratch stream before in-place deobfuscation.

A single stream is enough because these methods cannot overlap on the same iterator without re-entering or concurrently using it, which the guard asserts against.
The preceding test covers repeated seeks and failed value deserialization followed by a successful read from the same entry.

Co-authored-by: Andrew Toth <andrewstoth@gmail.com>
---
 src/dbwrapper.cpp |  5 ++++-
 src/dbwrapper.h   | 15 ++++++++-------
 2 files changed, 12 insertions(+), 8 deletions(-)

diff --git a/src/dbwrapper.cpp b/src/dbwrapper.cpp
index 5bfdc75e654..3743d434820 100644
--- a/src/dbwrapper.cpp
+++ b/src/dbwrapper.cpp
@@ -360,7 +360,10 @@ struct CDBIterator::IteratorImpl {
 };
 
 CDBIterator::CDBIterator(const CDBWrapper& _parent, std::unique_ptr<IteratorImpl> _piter) : parent(_parent),
-                                                                                            m_impl_iter(std::move(_piter)) {}
+                                                                                            m_impl_iter(std::move(_piter))
+{
+    m_scratch.reserve(DBWRAPPER_PREALLOC_KEY_SIZE);
+}
 
 CDBIterator* CDBWrapper::NewIterator()
 {
diff --git a/src/dbwrapper.h b/src/dbwrapper.h
index 6957f32721d..51775ee6365 100644
--- a/src/dbwrapper.h
+++ b/src/dbwrapper.h
@@ -121,6 +121,7 @@ public:
 private:
     const CDBWrapper &parent;
     const std::unique_ptr<IteratorImpl> m_impl_iter;
+    DataStream m_scratch{};
 
     void SeekImpl(std::span<const std::byte> key);
     std::span<const std::byte> GetKeyImpl() const;
@@ -140,10 +141,9 @@ public:
     void SeekToFirst();
 
     template<typename K> void Seek(const K& key) {
-        DataStream ssKey{};
-        ssKey.reserve(DBWRAPPER_PREALLOC_KEY_SIZE);
-        ssKey << key;
-        SeekImpl(ssKey);
+        ScopedDataStreamUsage scoped_scratch{m_scratch};
+        m_scratch << key;
+        SeekImpl(m_scratch);
     }
 
     void Next();
@@ -160,9 +160,10 @@ public:
 
     template<typename V> bool GetValue(V& value) {
         try {
-            DataStream ssValue{GetValueImpl()};
-            dbwrapper_private::GetObfuscation(parent)(ssValue);
-            ssValue >> value;
+            ScopedDataStreamUsage scoped_scratch{m_scratch};
+            m_scratch.write(GetValueImpl());
+            dbwrapper_private::GetObfuscation(parent)(m_scratch);
+            m_scratch >> value;
         } catch (const std::exception&) {
             return false;
         }