From b0875eb3fea0934f3a6651fbc22aac12e33e15e5 Mon Sep 17 00:00:00 2001
From: Pieter Wuille <pieter.wuille@gmail.com>
Date: Sun, 24 Aug 2014 02:08:05 +0200
Subject: [PATCH] Allow BatchWrite to destroy its input, reducing copying

---
 src/coins.cpp | 16 +++++++++-------
 src/coins.h   | 10 ++++++----
 src/txdb.cpp  |  7 +++++--
 src/txdb.h    |  2 +-
 4 files changed, 21 insertions(+), 14 deletions(-)

diff --git a/src/coins.cpp b/src/coins.cpp
index fe40911db7d..6137b51c55c 100644
--- a/src/coins.cpp
+++ b/src/coins.cpp
@@ -57,7 +57,7 @@ bool CCoinsView::SetCoins(const uint256 &txid, const CCoins &coins) { return fal
 bool CCoinsView::HaveCoins(const uint256 &txid) { return false; }
 uint256 CCoinsView::GetBestBlock() { return uint256(0); }
 bool CCoinsView::SetBestBlock(const uint256 &hashBlock) { return false; }
-bool CCoinsView::BatchWrite(const CCoinsMap &mapCoins, const uint256 &hashBlock) { return false; }
+bool CCoinsView::BatchWrite(CCoinsMap &mapCoins, const uint256 &hashBlock) { return false; }
 bool CCoinsView::GetStats(CCoinsStats &stats) { return false; }
 
 
@@ -68,7 +68,7 @@ bool CCoinsViewBacked::HaveCoins(const uint256 &txid) { return base->HaveCoins(t
 uint256 CCoinsViewBacked::GetBestBlock() { return base->GetBestBlock(); }
 bool CCoinsViewBacked::SetBestBlock(const uint256 &hashBlock) { return base->SetBestBlock(hashBlock); }
 void CCoinsViewBacked::SetBackend(CCoinsView &viewIn) { base = &viewIn; }
-bool CCoinsViewBacked::BatchWrite(const CCoinsMap &mapCoins, const uint256 &hashBlock) { return base->BatchWrite(mapCoins, hashBlock); }
+bool CCoinsViewBacked::BatchWrite(CCoinsMap &mapCoins, const uint256 &hashBlock) { return base->BatchWrite(mapCoins, hashBlock); }
 bool CCoinsViewBacked::GetStats(CCoinsStats &stats) { return base->GetStats(stats); }
 
 CCoinsKeyHasher::CCoinsKeyHasher() : salt(GetRandHash()) {}
@@ -130,17 +130,19 @@ bool CCoinsViewCache::SetBestBlock(const uint256 &hashBlockIn) {
     return true;
 }
 
-bool CCoinsViewCache::BatchWrite(const CCoinsMap &mapCoins, const uint256 &hashBlockIn) {
-    for (CCoinsMap::const_iterator it = mapCoins.begin(); it != mapCoins.end(); it++)
-        cacheCoins[it->first] = it->second;
+bool CCoinsViewCache::BatchWrite(CCoinsMap &mapCoins, const uint256 &hashBlockIn) {
+    for (CCoinsMap::iterator it = mapCoins.begin(); it != mapCoins.end();) {
+        cacheCoins[it->first].swap(it->second);
+        CCoinsMap::iterator itOld = it++;
+        mapCoins.erase(itOld);
+    }
     hashBlock = hashBlockIn;
     return true;
 }
 
 bool CCoinsViewCache::Flush() {
     bool fOk = base->BatchWrite(cacheCoins, hashBlock);
-    if (fOk)
-        cacheCoins.clear();
+    cacheCoins.clear();
     return fOk;
 }
 
diff --git a/src/coins.h b/src/coins.h
index ff60288163b..2b657299e3a 100644
--- a/src/coins.h
+++ b/src/coins.h
@@ -291,8 +291,9 @@ public:
     // Modify the currently active block hash
     virtual bool SetBestBlock(const uint256 &hashBlock);
 
-    // Do a bulk modification (multiple SetCoins + one SetBestBlock)
-    virtual bool BatchWrite(const CCoinsMap &mapCoins, const uint256 &hashBlock);
+    // Do a bulk modification (multiple SetCoins + one SetBestBlock).
+    // The passed mapCoins can be modified.
+    virtual bool BatchWrite(CCoinsMap &mapCoins, const uint256 &hashBlock);
 
     // Calculate statistics about the unspent transaction output set
     virtual bool GetStats(CCoinsStats &stats);
@@ -316,7 +317,7 @@ public:
     uint256 GetBestBlock();
     bool SetBestBlock(const uint256 &hashBlock);
     void SetBackend(CCoinsView &viewIn);
-    bool BatchWrite(const CCoinsMap &mapCoins, const uint256 &hashBlock);
+    bool BatchWrite(CCoinsMap &mapCoins, const uint256 &hashBlock);
     bool GetStats(CCoinsStats &stats);
 };
 
@@ -337,7 +338,7 @@ public:
     bool HaveCoins(const uint256 &txid);
     uint256 GetBestBlock();
     bool SetBestBlock(const uint256 &hashBlock);
-    bool BatchWrite(const CCoinsMap &mapCoins, const uint256 &hashBlock);
+    bool BatchWrite(CCoinsMap &mapCoins, const uint256 &hashBlock);
 
     // Return a modifiable reference to a CCoins. Check HaveCoins first.
     // Many methods explicitly require a CCoinsViewCache because of this method, to reduce
@@ -346,6 +347,7 @@ public:
 
     // Push the modifications applied to this cache to its base.
     // Failure to call this method before destruction will cause the changes to be forgotten.
+    // If false is returned, the state of this cache (and its backing view) will be undefined.
     bool Flush();
 
     // Calculate the size of the cache (in number of transactions)
diff --git a/src/txdb.cpp b/src/txdb.cpp
index d3d05c58d70..7c0683aaf3d 100644
--- a/src/txdb.cpp
+++ b/src/txdb.cpp
@@ -54,12 +54,15 @@ bool CCoinsViewDB::SetBestBlock(const uint256 &hashBlock) {
     return db.WriteBatch(batch);
 }
 
-bool CCoinsViewDB::BatchWrite(const CCoinsMap &mapCoins, const uint256 &hashBlock) {
+bool CCoinsViewDB::BatchWrite(CCoinsMap &mapCoins, const uint256 &hashBlock) {
     LogPrint("coindb", "Committing %u changed transactions to coin database...\n", (unsigned int)mapCoins.size());
 
     CLevelDBBatch batch;
-    for (CCoinsMap::const_iterator it = mapCoins.begin(); it != mapCoins.end(); it++)
+    for (CCoinsMap::iterator it = mapCoins.begin(); it != mapCoins.end();) {
         BatchWriteCoins(batch, it->first, it->second);
+        CCoinsMap::iterator itOld = it++;
+        mapCoins.erase(itOld);
+    }
     if (hashBlock != uint256(0))
         BatchWriteHashBestChain(batch, hashBlock);
 
diff --git a/src/txdb.h b/src/txdb.h
index 7d670c25423..099f1517768 100644
--- a/src/txdb.h
+++ b/src/txdb.h
@@ -37,7 +37,7 @@ public:
     bool HaveCoins(const uint256 &txid);
     uint256 GetBestBlock();
     bool SetBestBlock(const uint256 &hashBlock);
-    bool BatchWrite(const CCoinsMap &mapCoins, const uint256 &hashBlock);
+    bool BatchWrite(CCoinsMap &mapCoins, const uint256 &hashBlock);
     bool GetStats(CCoinsStats &stats);
 };