coins: add Sync() method to allow flush without cacheCoins drop

In certain circumstances, we may want to flush to disk without
emptying `cacheCoins`, which affects performance. UTXO snapshot
activation is one such case.

This method is currently unused and this commit does not
change any behavior.

Incorporates feedback from John Newbery.

Co-authored-by: Suhas Daftuar <sdaftuar@gmail.com>

1 files changed, 15 insertions, 5 deletions

diff --git a/src/coins.h b/src/coins.h
index b0d6bdf333..4edc146a14 100644
--- a/src/coins.h
+++ b/src/coins.h
@@ -176,7 +176,7 @@ public:
 
     //! Do a bulk modification (multiple Coin changes + BestBlock change).
     //! The passed mapCoins can be modified.
-    virtual bool BatchWrite(CCoinsMap &mapCoins, const uint256 &hashBlock);
+    virtual bool BatchWrite(CCoinsMap &mapCoins, const uint256 &hashBlock, bool erase = true);
 
     //! Get a cursor to iterate over the whole state
     virtual std::unique_ptr<CCoinsViewCursor> Cursor() const;
@@ -202,7 +202,7 @@ public:
     uint256 GetBestBlock() const override;
     std::vector<uint256> GetHeadBlocks() const override;
     void SetBackend(CCoinsView &viewIn);
-    bool BatchWrite(CCoinsMap &mapCoins, const uint256 &hashBlock) override;
+    bool BatchWrite(CCoinsMap &mapCoins, const uint256 &hashBlock, bool erase = true) override;
     std::unique_ptr<CCoinsViewCursor> Cursor() const override;
     size_t EstimateSize() const override;
 };
@@ -235,7 +235,7 @@ public:
     bool HaveCoin(const COutPoint &outpoint) const override;
     uint256 GetBestBlock() const override;
     void SetBestBlock(const uint256 &hashBlock);
-    bool BatchWrite(CCoinsMap &mapCoins, const uint256 &hashBlock) override;
+    bool BatchWrite(CCoinsMap &mapCoins, const uint256 &hashBlock, bool erase = true) override;
     std::unique_ptr<CCoinsViewCursor> Cursor() const override {
         throw std::logic_error("CCoinsViewCache cursor iteration not supported.");
     }
@@ -282,13 +282,23 @@ public:
     bool SpendCoin(const COutPoint &outpoint, Coin* moveto = nullptr);
 
     /**
-     * Push the modifications applied to this cache to its base.
-     * Failure to call this method before destruction will cause the changes to be forgotten.
+     * Push the modifications applied to this cache to its base and wipe local state.
+     * Failure to call this method or Sync() 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();
 
     /**
+     * Push the modifications applied to this cache to its base while retaining
+     * the contents of this cache (except for spent coins, which we erase).
+     * Failure to call this method or Flush() 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 Sync();
+
+    /**
      * Removes the UTXO with the given outpoint from the cache, if it is
      * not modified.
      */