coins: make CCoinsView methods pure virtual

`CCoinsView` provided default no-op implementations, which allowed constructing a bare view and silently getting dummy behavior.
Make all interface methods pure virtual and remove the legacy default definitions from `coins.cpp` so callers must choose an explicit implementation.
Move the virtual destructor to the beginning to avoid mixing it between the methods.
No-op backing behavior remains available via `CoinsViewEmpty`.

src/coins.cpp

@@ -14,22 +14,6 @@ TRACEPOINT_SEMAPHORE(utxocache, add);
 TRACEPOINT_SEMAPHORE(utxocache, spent);
 TRACEPOINT_SEMAPHORE(utxocache, uncache);
 
-std::optional<Coin> CCoinsView::GetCoin(const COutPoint& outpoint) const { return std::nullopt; }
-std::optional<Coin> CCoinsView::PeekCoin(const COutPoint& outpoint) const { return GetCoin(outpoint); }
-uint256 CCoinsView::GetBestBlock() const { return uint256(); }
-std::vector<uint256> CCoinsView::GetHeadBlocks() const { return std::vector<uint256>(); }
-void CCoinsView::BatchWrite(CoinsViewCacheCursor& cursor, const uint256& block_hash)
-{
-    for (auto it{cursor.Begin()}; it != cursor.End(); it = cursor.NextAndMaybeErase(*it)) { }
-}
-
-std::unique_ptr<CCoinsViewCursor> CCoinsView::Cursor() const { return nullptr; }
-
-bool CCoinsView::HaveCoin(const COutPoint& outpoint) const
-{
-    return GetCoin(outpoint).has_value();
-}
-
 CCoinsViewBacked::CCoinsViewBacked(CCoinsView* in_view) : base(in_view) { }
 std::optional<Coin> CCoinsViewBacked::GetCoin(const COutPoint& outpoint) const { return base->GetCoin(outpoint); }
 std::optional<Coin> CCoinsViewBacked::PeekCoin(const COutPoint& outpoint) const { return base->PeekCoin(outpoint); }

src/coins.h

@@ -303,43 +303,43 @@ private:
     bool m_will_erase;
 };
 
-/** Abstract view on the open txout dataset. */
+/** Pure abstract view on the open txout dataset. */
 class CCoinsView
 {
 public:
+    //! As we use CCoinsViews polymorphically, have a virtual destructor
+    virtual ~CCoinsView() = default;
+
     //! Retrieve the Coin (unspent transaction output) for a given outpoint.
     //! May populate the cache. Use PeekCoin() to perform a non-caching lookup.
-    virtual std::optional<Coin> GetCoin(const COutPoint& outpoint) const;
+    virtual std::optional<Coin> GetCoin(const COutPoint& outpoint) const = 0;
 
     //! Retrieve the Coin (unspent transaction output) for a given outpoint, without caching results.
     //! Does not populate the cache. Use GetCoin() to cache the result.
-    virtual std::optional<Coin> PeekCoin(const COutPoint& outpoint) const;
+    virtual std::optional<Coin> PeekCoin(const COutPoint& outpoint) const = 0;
 
     //! Just check whether a given outpoint is unspent.
     //! May populate the cache. Use PeekCoin() to perform a non-caching lookup.
-    virtual bool HaveCoin(const COutPoint& outpoint) const;
+    virtual bool HaveCoin(const COutPoint& outpoint) const = 0;
 
     //! Retrieve the block hash whose state this CCoinsView currently represents
-    virtual uint256 GetBestBlock() const;
+    virtual uint256 GetBestBlock() const = 0;
 
     //! Retrieve the range of blocks that may have been only partially written.
     //! If the database is in a consistent state, the result is the empty vector.
     //! Otherwise, a two-element vector is returned consisting of the new and
     //! the old block hash, in that order.
-    virtual std::vector<uint256> GetHeadBlocks() const;
+    virtual std::vector<uint256> GetHeadBlocks() const = 0;
 
     //! Do a bulk modification (multiple Coin changes + BestBlock change).
     //! The passed cursor is used to iterate through the coins.
-    virtual void BatchWrite(CoinsViewCacheCursor& cursor, const uint256& block_hash);
+    virtual void BatchWrite(CoinsViewCacheCursor& cursor, const uint256& block_hash) = 0;
 
-    //! Get a cursor to iterate over the whole state
-    virtual std::unique_ptr<CCoinsViewCursor> Cursor() const;
+    //! Get a cursor to iterate over the whole state. Implementations may return nullptr.
+    virtual std::unique_ptr<CCoinsViewCursor> Cursor() const = 0;
 
-    //! As we use CCoinsViews polymorphically, have a virtual destructor
-    virtual ~CCoinsView() = default;
-
-    //! Estimate database size (0 if not implemented)
-    virtual size_t EstimateSize() const { return 0; }
+    //! Estimate database size
+    virtual size_t EstimateSize() const = 0;
 };
 
 /** Noop coins view. */
@@ -353,6 +353,18 @@ public:
 
     CoinsViewEmpty(const CoinsViewEmpty&) = delete;
     CoinsViewEmpty& operator=(const CoinsViewEmpty&) = delete;
+
+    std::optional<Coin> GetCoin(const COutPoint&) const override { return {}; }
+    std::optional<Coin> PeekCoin(const COutPoint& outpoint) const override { return GetCoin(outpoint); }
+    bool HaveCoin(const COutPoint& outpoint) const override { return !!GetCoin(outpoint); }
+    uint256 GetBestBlock() const override { return {}; }
+    std::vector<uint256> GetHeadBlocks() const override { return {}; }
+    void BatchWrite(CoinsViewCacheCursor& cursor, const uint256&) override
+    {
+        for (auto it{cursor.Begin()}; it != cursor.End(); it = cursor.NextAndMaybeErase(*it)) { }
+    }
+    std::unique_ptr<CCoinsViewCursor> Cursor() const override { return {}; }
+    size_t EstimateSize() const override { return 0; }
 };
 
 /** CCoinsView backed by another CCoinsView */

src/test/fuzz/coinscache_sim.cpp

@@ -247,7 +247,7 @@ FUZZ_TARGET(coinscache_sim)
         CallOneOf(
             provider,
 
-            [&]() { // GetCoin
+            [&]() { // PeekCoin/GetCoin
                 uint32_t outpointidx = provider.ConsumeIntegralInRange<uint32_t>(0, NUM_OUTPOINTS - 1);
                 // Look up in simulation data.
                 auto sim = lookup(outpointidx);

src/txdb.cpp

@@ -78,6 +78,11 @@ std::optional<Coin> CCoinsViewDB::GetCoin(const COutPoint& outpoint) const
     return std::nullopt;
 }
 
+std::optional<Coin> CCoinsViewDB::PeekCoin(const COutPoint& outpoint) const
+{
+    return GetCoin(outpoint);
+}
+
 bool CCoinsViewDB::HaveCoin(const COutPoint& outpoint) const
 {
     return m_db->Exists(CoinEntry(&outpoint));

src/txdb.h

@@ -41,6 +41,7 @@ public:
     explicit CCoinsViewDB(DBParams db_params, CoinsViewOptions options);
 
     std::optional<Coin> GetCoin(const COutPoint& outpoint) const override;
+    std::optional<Coin> PeekCoin(const COutPoint& outpoint) const override;
     bool HaveCoin(const COutPoint& outpoint) const override;
     uint256 GetBestBlock() const override;
     std::vector<uint256> GetHeadBlocks() const override;

src/txmempool.h

@@ -768,7 +768,7 @@ protected:
 public:
     CCoinsViewMemPool(CCoinsView* baseIn, const CTxMemPool& mempoolIn);
     /** GetCoin, returning whether it exists and is not spent. Also updates m_non_base_coins if the
-     * coin is not fetched from base. */
+     * coin is not fetched from base. May populate the base view on cache misses. */
     std::optional<Coin> GetCoin(const COutPoint& outpoint) const override;
     /** Add the coins created by this transaction. These coins are only temporarily stored in
      * m_temp_added and cannot be flushed to the back end. Only used for package validation. */