Merge bitcoin/bitcoin#22263: refactor: wrap CCoinsViewCursor in unique_ptr

7ad414f4bfa74595ee5726e66f3527045c02a977 doc: add comment about CCoinsViewDBCursor constructor (James O'Beirne)
0f8a5a4dd530549d37c43da52c923ac3b2af1a03 move-only(ish): don't expose CCoinsViewDBCursor (James O'Beirne)
615c1adfb07b9b466173166dc2e53ace540e4b32 refactor: wrap CCoinsViewCursor in unique_ptr (James O'Beirne)

Pull request description:

  I tripped over this one for a few hours at the beginning of the week, so I've sort of got a personal vendetta against `CCoinsView::Cursor()` returning a raw pointer.

  Specifically in the case of CCoinsViewDB, if a raw cursor is allocated and not freed, a cryptic leveldb assertion failure occurs on CCoinsViewDB destruction (`Assertion 'dummy_versions_.next_ == &dummy_versions_' failed.`).

  This is a pretty simple change.

  Related to: https://github.com/bitcoin/bitcoin/issues/21766
  See also: https://github.com/google/leveldb/issues/142#issuecomment-414418135

ACKs for top commit:
  MarcoFalke:
    review ACK 7ad414f4bfa74595ee5726e66f3527045c02a977 🔎
  jonatack:
    re-ACK 7ad414f4bfa74595ee5726e66f3527045c02a977 modulo suggestion
  ryanofsky:
    Code review ACK 7ad414f4bfa74595ee5726e66f3527045c02a977. Two new commits look good and thanks for clarifying constructor comment

Tree-SHA512: 6471d03e2de674d84b1ea0d31e25f433d52aa1aa4996f7b4aab1bd02b6bc340b15e64cc8ea07bbefefa3b5da35384ca5400cc230434e787c30931b8574c672f9

6 files changed, 37 insertions, 34 deletions

diff --git a/src/coins.cpp b/src/coins.cpp
index d52851cadd..ce0b131de6 100644
--- a/src/coins.cpp
+++ b/src/coins.cpp
@@ -13,7 +13,7 @@ bool CCoinsView::GetCoin(const COutPoint &outpoint, Coin &coin) const { return f
 uint256 CCoinsView::GetBestBlock() const { return uint256(); }
 std::vector<uint256> CCoinsView::GetHeadBlocks() const { return std::vector<uint256>(); }
 bool CCoinsView::BatchWrite(CCoinsMap &mapCoins, const uint256 &hashBlock) { return false; }
-CCoinsViewCursor *CCoinsView::Cursor() const { return nullptr; }
+std::unique_ptr<CCoinsViewCursor> CCoinsView::Cursor() const { return nullptr; }
 
 bool CCoinsView::HaveCoin(const COutPoint &outpoint) const
 {
@@ -28,7 +28,7 @@ uint256 CCoinsViewBacked::GetBestBlock() const { return base->GetBestBlock(); }
 std::vector<uint256> CCoinsViewBacked::GetHeadBlocks() const { return base->GetHeadBlocks(); }
 void CCoinsViewBacked::SetBackend(CCoinsView &viewIn) { base = &viewIn; }
 bool CCoinsViewBacked::BatchWrite(CCoinsMap &mapCoins, const uint256 &hashBlock) { return base->BatchWrite(mapCoins, hashBlock); }
-CCoinsViewCursor *CCoinsViewBacked::Cursor() const { return base->Cursor(); }
+std::unique_ptr<CCoinsViewCursor> CCoinsViewBacked::Cursor() const { return base->Cursor(); }
 size_t CCoinsViewBacked::EstimateSize() const { return base->EstimateSize(); }
 
 CCoinsViewCache::CCoinsViewCache(CCoinsView *baseIn) : CCoinsViewBacked(baseIn), cachedCoinsUsage(0) {}
diff --git a/src/coins.h b/src/coins.h
index 816b4864a3..3151a260d9 100644
--- a/src/coins.h
+++ b/src/coins.h
@@ -180,7 +180,7 @@ public:
     virtual bool BatchWrite(CCoinsMap &mapCoins, const uint256 &hashBlock);
 
     //! Get a cursor to iterate over the whole state
-    virtual CCoinsViewCursor *Cursor() const;
+    virtual std::unique_ptr<CCoinsViewCursor> Cursor() const;
 
     //! As we use CCoinsViews polymorphically, have a virtual destructor
     virtual ~CCoinsView() {}
@@ -204,7 +204,7 @@ public:
     std::vector<uint256> GetHeadBlocks() const override;
     void SetBackend(CCoinsView &viewIn);
     bool BatchWrite(CCoinsMap &mapCoins, const uint256 &hashBlock) override;
-    CCoinsViewCursor *Cursor() const override;
+    std::unique_ptr<CCoinsViewCursor> Cursor() const override;
     size_t EstimateSize() const override;
 };
 
@@ -237,7 +237,7 @@ public:
     uint256 GetBestBlock() const override;
     void SetBestBlock(const uint256 &hashBlock);
     bool BatchWrite(CCoinsMap &mapCoins, const uint256 &hashBlock) override;
-    CCoinsViewCursor* Cursor() const override {
+    std::unique_ptr<CCoinsViewCursor> Cursor() const override {
         throw std::logic_error("CCoinsViewCache cursor iteration not supported.");
     }
 
diff --git a/src/rpc/blockchain.cpp b/src/rpc/blockchain.cpp
index 63897e0e05..4d4d3fb7d0 100644
--- a/src/rpc/blockchain.cpp
+++ b/src/rpc/blockchain.cpp
@@ -2391,7 +2391,7 @@ static RPCHelpMan scantxoutset()
             LOCK(cs_main);
             CChainState& active_chainstate = chainman.ActiveChainstate();
             active_chainstate.ForceFlushStateToDisk();
-            pcursor = std::unique_ptr<CCoinsViewCursor>(active_chainstate.CoinsDB().Cursor());
+            pcursor = active_chainstate.CoinsDB().Cursor();
             CHECK_NONFATAL(pcursor);
             tip = active_chainstate.m_chain.Tip();
             CHECK_NONFATAL(tip);
@@ -2590,7 +2590,7 @@ UniValue CreateUTXOSnapshot(NodeContext& node, CChainState& chainstate, CAutoFil
             throw JSONRPCError(RPC_INTERNAL_ERROR, "Unable to read UTXO set");
         }
 
-        pcursor = std::unique_ptr<CCoinsViewCursor>(chainstate.CoinsDB().Cursor());
+        pcursor = chainstate.CoinsDB().Cursor();
         tip = chainstate.m_blockman.LookupBlockIndex(stats.hashBlock);
         CHECK_NONFATAL(tip);
     }
diff --git a/src/test/fuzz/coins_view.cpp b/src/test/fuzz/coins_view.cpp
index 42f19d16c6..f452696689 100644
--- a/src/test/fuzz/coins_view.cpp
+++ b/src/test/fuzz/coins_view.cpp
@@ -183,8 +183,8 @@ FUZZ_TARGET_INIT(coins_view, initialize_coins_view)
     }
 
     {
-        const CCoinsViewCursor* coins_view_cursor = backend_coins_view.Cursor();
-        assert(coins_view_cursor == nullptr);
+        std::unique_ptr<CCoinsViewCursor> coins_view_cursor = backend_coins_view.Cursor();
+        assert(!coins_view_cursor);
         (void)backend_coins_view.EstimateSize();
         (void)backend_coins_view.GetBestBlock();
         (void)backend_coins_view.GetHeadBlocks();
diff --git a/src/txdb.cpp b/src/txdb.cpp
index 762f71feb1..4b76bee5ab 100644
--- a/src/txdb.cpp
+++ b/src/txdb.cpp
@@ -168,9 +168,34 @@ bool CBlockTreeDB::ReadLastBlockFile(int &nFile) {
     return Read(DB_LAST_BLOCK, nFile);
 }
 
-CCoinsViewCursor *CCoinsViewDB::Cursor() const
+/** Specialization of CCoinsViewCursor to iterate over a CCoinsViewDB */
+class CCoinsViewDBCursor: public CCoinsViewCursor
 {
-    CCoinsViewDBCursor *i = new CCoinsViewDBCursor(const_cast<CDBWrapper&>(*m_db).NewIterator(), GetBestBlock());
+public:
+    // Prefer using CCoinsViewDB::Cursor() since we want to perform some
+    // cache warmup on instantiation.
+    CCoinsViewDBCursor(CDBIterator* pcursorIn, const uint256&hashBlockIn):
+        CCoinsViewCursor(hashBlockIn), pcursor(pcursorIn) {}
+    ~CCoinsViewDBCursor() {}
+
+    bool GetKey(COutPoint &key) const override;
+    bool GetValue(Coin &coin) const override;
+    unsigned int GetValueSize() const override;
+
+    bool Valid() const override;
+    void Next() override;
+
+private:
+    std::unique_ptr<CDBIterator> pcursor;
+    std::pair<char, COutPoint> keyTmp;
+
+    friend class CCoinsViewDB;
+};
+
+std::unique_ptr<CCoinsViewCursor> CCoinsViewDB::Cursor() const
+{
+    auto i = std::make_unique<CCoinsViewDBCursor>(
+        const_cast<CDBWrapper&>(*m_db).NewIterator(), GetBestBlock());
     /* It seems that there are no "const iterators" for LevelDB.  Since we
        only need read operations on it, use a const-cast to get around
        that restriction.  */
diff --git a/src/txdb.h b/src/txdb.h
index 0cf7e2f1b8..845d80788f 100644
--- a/src/txdb.h
+++ b/src/txdb.h
@@ -60,7 +60,7 @@ public:
     uint256 GetBestBlock() const override;
     std::vector<uint256> GetHeadBlocks() const override;
     bool BatchWrite(CCoinsMap &mapCoins, const uint256 &hashBlock) override;
-    CCoinsViewCursor *Cursor() const override;
+    std::unique_ptr<CCoinsViewCursor> Cursor() const override;
 
     //! Attempt to update from an older database format. Returns whether an error occurred.
     bool Upgrade();
@@ -70,28 +70,6 @@ public:
     void ResizeCache(size_t new_cache_size) EXCLUSIVE_LOCKS_REQUIRED(cs_main);
 };
 
-/** Specialization of CCoinsViewCursor to iterate over a CCoinsViewDB */
-class CCoinsViewDBCursor: public CCoinsViewCursor
-{
-public:
-    ~CCoinsViewDBCursor() {}
-
-    bool GetKey(COutPoint &key) const override;
-    bool GetValue(Coin &coin) const override;
-    unsigned int GetValueSize() const override;
-
-    bool Valid() const override;
-    void Next() override;
-
-private:
-    CCoinsViewDBCursor(CDBIterator* pcursorIn, const uint256 &hashBlockIn):
-        CCoinsViewCursor(hashBlockIn), pcursor(pcursorIn) {}
-    std::unique_ptr<CDBIterator> pcursor;
-    std::pair<char, COutPoint> keyTmp;
-
-    friend class CCoinsViewDB;
-};
-
 /** Access to the block database (blocks/index/) */
 class CBlockTreeDB : public CDBWrapper
 {