fuzz: keep coinscache_sim backend free of spent coins

`CoinsViewBottom` roughly simulates a memory-backed `CCoinsViewDB`, which never stores spent coins.

Stop returning spent coins from `GetCoin()`, erase spent entries in `BatchWrite()`, and tighten comparisons to expect `std::nullopt` when the simulator has no coin.

Co-authored-by: Lőrinc <pap.lorinc@gmail.com>

src/test/fuzz/coinscache_sim.cpp

@@ -138,8 +138,6 @@ struct CacheLevel
 /** Class for the base of the hierarchy (roughly simulating a memory-backed CCoinsViewDB).
  *
  * The initial state consists of the empty UTXO set.
- * Coins whose output index is 4 (mod 5) have GetCoin() always succeed after being spent.
- * This exercises code paths with spent, non-DIRTY cache entries.
  */
 class CoinsViewBottom final : public CCoinsView
 {
@@ -148,16 +146,13 @@ class CoinsViewBottom final : public CCoinsView
 public:
     std::optional<Coin> GetCoin(const COutPoint& outpoint) const final
     {
-        // TODO GetCoin shouldn't return spent coins
+        if (auto it{m_data.find(outpoint)}; it != m_data.end()) {
-        if (auto it = m_data.find(outpoint); it != m_data.end()) return it->second;
+            assert(!it->second.IsSpent());
+            return it->second;
+        }
         return std::nullopt;
     }
 
-    bool HaveCoin(const COutPoint& outpoint) const final
-    {
-        return m_data.contains(outpoint);
-    }
-
     uint256 GetBestBlock() const final { return {}; }
     std::vector<uint256> GetHeadBlocks() const final { return {}; }
     std::unique_ptr<CCoinsViewCursor> Cursor() const final { return {}; }
@@ -167,18 +162,20 @@ public:
     {
         for (auto it{cursor.Begin()}; it != cursor.End(); it = cursor.NextAndMaybeErase(*it)) {
             if (it->second.IsDirty()) {
-                if (it->second.coin.IsSpent() && (it->first.n % 5) != 4) {
+                if (it->second.coin.IsSpent()) {
                     m_data.erase(it->first);
-                } else if (cursor.WillErase(*it)) {
-                    m_data[it->first] = std::move(it->second.coin);
                 } else {
-                    m_data[it->first] = it->second.coin;
+                    if (cursor.WillErase(*it)) {
+                        m_data[it->first] = std::move(it->second.coin);
+                    } else {
+                        m_data[it->first] = it->second.coin;
+                    }
                 }
             } else {
                 /* For non-dirty entries being written, compare them with what we have. */
                 auto it2 = m_data.find(it->first);
                 if (it->second.coin.IsSpent()) {
-                    assert(it2 == m_data.end() || it2->second.IsSpent());
+                    assert(it2 == m_data.end());
                 } else {
                     assert(it2 != m_data.end());
                     assert(it->second.coin.out == it2->second.out);
@@ -263,7 +260,7 @@ FUZZ_TARGET(coinscache_sim)
                 auto realcoin = caches.back()->GetCoin(data.outpoints[outpointidx]);
                 // Compare results.
                 if (!sim.has_value()) {
-                    assert(!realcoin || realcoin->IsSpent());
+                    assert(!realcoin);
                 } else {
                     assert(realcoin && !realcoin->IsSpent());
                     const auto& simcoin = data.coins[sim->first];
@@ -449,7 +446,7 @@ FUZZ_TARGET(coinscache_sim)
         auto realcoin = bottom.GetCoin(data.outpoints[outpointidx]);
         auto sim = lookup(outpointidx, 0);
         if (!sim.has_value()) {
-            assert(!realcoin || realcoin->IsSpent());
+            assert(!realcoin);
         } else {
             assert(realcoin && !realcoin->IsSpent());
             assert(realcoin->out == data.coins[sim->first].out);