[CCoinsViewMemPool] track non-base coins and allow Reset

Temporary coins should not be available in separate subpackage submissions.
Any mempool coins that are cached in m_view should be removed whenever
mempool contents change, as they may be spent or no longer exist.

src/txmempool.h

@@ -839,15 +839,27 @@ class CCoinsViewMemPool : public CCoinsViewBacked
     * validation, since we can access transaction outputs without submitting them to mempool.
     */
     std::unordered_map<COutPoint, Coin, SaltedOutpointHasher> m_temp_added;
+
+    /**
+     * Set of all coins that have been fetched from mempool or created using PackageAddTransaction
+     * (not base). Used to track the origin of a coin, see GetNonBaseCoins().
+     */
+    mutable std::unordered_set<COutPoint, SaltedOutpointHasher> m_non_base_coins;
 protected:
     const CTxMemPool& mempool;
 
 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. */
     bool GetCoin(const COutPoint &outpoint, Coin &coin) 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. */
     void PackageAddTransaction(const CTransactionRef& tx);
+    /** Get all coins in m_non_base_coins. */
+    std::unordered_set<COutPoint, SaltedOutpointHasher> GetNonBaseCoins() const { return m_non_base_coins; }
+    /** Clear m_temp_added and m_non_base_coins. */
+    void Reset();
 };
 
 /**