fuzz: prevent invalid FRESH entries and surface BatchWrite errors

Modify fuzzer logic to avoid setting `FRESH` for an outpoint that already exists unspent in the parent view, and ensure `FRESH` implies `DIRTY`.
This keeps cursor invariants realistic and lets `BatchWrite` failures expose real bugs without resetting state.

src/test/fuzz/coins_view.cpp

@@ -77,18 +77,7 @@ public:
     {
         // Nothing must modify cacheCoins other than BatchWrite.
         assert(ComputeCacheCoinsSnapshot() == m_expected_snapshot);
-        try {
+        CCoinsViewCache::BatchWrite(cursor, block_hash);
-            CCoinsViewCache::BatchWrite(cursor, block_hash);
-        } catch (const std::logic_error& e) {
-            // This error is thrown if the cursor contains a fresh entry for an outpoint that we already have a fresh
-            // entry for. This can happen if the fuzzer calls AddCoin -> Flush -> AddCoin -> Flush on the child cache.
-            // There's not an easy way to prevent the fuzzer from reaching this, so we handle it here.
-            // Since it is thrown in the middle of the write, we reset our own state and iterate through
-            // the cursor so the caller's state is also reset.
-            assert(e.what() == std::string{"FRESH flag misapplied to coin that exists in parent cache"});
-            Reset();
-            for (auto it{cursor.Begin()}; it != cursor.End(); it = cursor.NextAndMaybeErase(*it)) {}
-        }
         m_expected_snapshot = ComputeCacheCoinsSnapshot();
     }
 
@@ -199,8 +188,6 @@ void TestCoinsView(FuzzedDataProvider& fuzzed_data_provider, CCoinsViewCache& co
                 LIMITED_WHILE(good_data && fuzzed_data_provider.ConsumeBool(), 10'000)
                 {
                     CCoinsCacheEntry coins_cache_entry;
-                    const auto dirty{fuzzed_data_provider.ConsumeBool()};
-                    const auto fresh{fuzzed_data_provider.ConsumeBool()};
                     if (fuzzed_data_provider.ConsumeBool()) {
                         coins_cache_entry.coin = random_coin;
                     } else {
@@ -211,26 +198,20 @@ void TestCoinsView(FuzzedDataProvider& fuzzed_data_provider, CCoinsViewCache& co
                         }
                         coins_cache_entry.coin = *opt_coin;
                     }
+                    // Avoid setting FRESH for an outpoint that already exists unspent in the parent view.
+                    bool fresh{!coins_view_cache.PeekCoin(random_out_point) && fuzzed_data_provider.ConsumeBool()};
+                    bool dirty{fresh || fuzzed_data_provider.ConsumeBool()};
                     auto it{coins_map.emplace(random_out_point, std::move(coins_cache_entry)).first};
                     if (dirty) CCoinsCacheEntry::SetDirty(*it, sentinel);
                     if (fresh) CCoinsCacheEntry::SetFresh(*it, sentinel);
                     dirty_count += dirty;
                 }
-                bool expected_code_path = false;
+                auto cursor{CoinsViewCacheCursor(dirty_count, sentinel, coins_map, /*will_erase=*/true)};
-                try {
+                uint256 best_block{coins_view_cache.GetBestBlock()};
-                    auto cursor{CoinsViewCacheCursor(dirty_count, sentinel, coins_map, /*will_erase=*/true)};
+                if (fuzzed_data_provider.ConsumeBool()) best_block = ConsumeUInt256(fuzzed_data_provider);
-                    uint256 best_block{coins_view_cache.GetBestBlock()};
+                // Set best block hash to non-null to satisfy the assertion in CCoinsViewDB::BatchWrite().
-                    if (fuzzed_data_provider.ConsumeBool()) best_block = ConsumeUInt256(fuzzed_data_provider);
+                if (is_db && best_block.IsNull()) best_block = uint256::ONE;
-                    // Set best block hash to non-null to satisfy the assertion in CCoinsViewDB::BatchWrite().
+                coins_view_cache.BatchWrite(cursor, best_block);
-                    if (is_db && best_block.IsNull()) best_block = uint256::ONE;
-                    coins_view_cache.BatchWrite(cursor, best_block);
-                    expected_code_path = true;
-                } catch (const std::logic_error& e) {
-                    if (e.what() == std::string{"FRESH flag misapplied to coin that exists in parent cache"}) {
-                        expected_code_path = true;
-                    }
-                }
-                assert(expected_code_path);
             });
     }