coins: only adjust cachedCoinsUsage on EmplaceCoinInternalDANGER insert

`EmplaceCoinInternalDANGER()` incremented `cachedCoinsUsage` even when `try_emplace` did not insert (duplicate key), inflating the counter.
This is mostly reachable in tests today since `AssumeUTXO` does not overwrite.

Increment only on successful insert, and capture `coin.DynamicMemoryUsage()` before the move so accounting uses the correct value.

Fuzz: add an `EmplaceCoinInternalDANGER` path to exercise insert-only accounting.
Unit test: emplace two different coins at the same outpoint (with different `DynamicMemoryUsage()`), verify `SelfTest()` passes and `AccessCoin(outpoint)` returns the first coin.

Co-authored-by: Andrew Toth <andrewstoth@gmail.com>
Co-authored-by: w0xlt <woltx@protonmail.com>

Author: 3 people

src/coins.cpp

@@ -111,9 +111,12 @@ void CCoinsViewCache::AddCoin(const COutPoint &outpoint, Coin&& coin, bool possi
 }
 
 void CCoinsViewCache::EmplaceCoinInternalDANGER(COutPoint&& outpoint, Coin&& coin) {
-    cachedCoinsUsage += coin.DynamicMemoryUsage();
+    const auto mem_usage{coin.DynamicMemoryUsage()};
     auto [it, inserted] = cacheCoins.try_emplace(std::move(outpoint), std::move(coin));
-    if (inserted) CCoinsCacheEntry::SetDirty(*it, m_sentinel);
+    if (inserted) {
+        CCoinsCacheEntry::SetDirty(*it, m_sentinel);
+        cachedCoinsUsage += mem_usage;
+    }
 }
 
 void AddCoins(CCoinsViewCache& cache, const CTransaction &tx, int nHeight, bool check_for_overwrite) {

src/test/coins_tests.cpp

@@ -1103,4 +1103,22 @@ BOOST_AUTO_TEST_CASE(ccoins_addcoin_exception_keeps_usage_balanced)
     BOOST_CHECK(cache.AccessCoin(outpoint) == coin1);
 }
 
+BOOST_AUTO_TEST_CASE(ccoins_emplace_duplicate_keeps_usage_balanced)
+{
+    CCoinsView root;
+    CCoinsViewCacheTest cache{&root};
+
+    const COutPoint outpoint{Txid::FromUint256(m_rng.rand256()), m_rng.rand32()};
+
+    const Coin coin1{CTxOut{m_rng.randrange(10), CScript{} << m_rng.randbytes(CScriptBase::STATIC_SIZE + 1)}, 1, false};
+    cache.EmplaceCoinInternalDANGER(COutPoint{outpoint}, Coin{coin1});
+    cache.SelfTest();
+
+    const Coin coin2{CTxOut{m_rng.randrange(20), CScript{} << m_rng.randbytes(CScriptBase::STATIC_SIZE + 2)}, 2, false};
+    cache.EmplaceCoinInternalDANGER(COutPoint{outpoint}, Coin{coin2});
+    cache.SelfTest();
+
+    BOOST_CHECK(cache.AccessCoin(outpoint) == coin1);
+}
+
 BOOST_AUTO_TEST_SUITE_END()

src/test/fuzz/coins_view.cpp

@@ -61,12 +61,16 @@ void TestCoinsView(FuzzedDataProvider& fuzzed_data_provider, CCoinsView& backend
                 }
                 COutPoint outpoint{random_out_point};
                 Coin coin{random_coin};
-                const bool possible_overwrite{fuzzed_data_provider.ConsumeBool()};
-                try {
-                    coins_view_cache.AddCoin(outpoint, std::move(coin), possible_overwrite);
-                } catch (const std::logic_error& e) {
-                    assert(e.what() == std::string{"Attempted to overwrite an unspent coin (when possible_overwrite is false)"});
-                    assert(!possible_overwrite);
+                if (fuzzed_data_provider.ConsumeBool()) {
+                    const bool possible_overwrite{fuzzed_data_provider.ConsumeBool()};
+                    try {
+                        coins_view_cache.AddCoin(outpoint, std::move(coin), possible_overwrite);
+                    } catch (const std::logic_error& e) {
+                        assert(e.what() == std::string{"Attempted to overwrite an unspent coin (when possible_overwrite is false)"});
+                        assert(!possible_overwrite);
+                    }
+                } else {
+                    coins_view_cache.EmplaceCoinInternalDANGER(std::move(outpoint), std::move(coin));
                 }
             },
             [&] {