coins: Keep track of number of dirty entries in `CCoinsViewCache`

diff --git a/src/coins.cpp b/src/coins.cpp
index fc33c521..1bc01d20 100644
--- a/src/coins.cpp
+++ b/src/coins.cpp
@@ -98,10 +98,12 @@ void CCoinsViewCache::AddCoin(const COutPoint &outpoint, Coin&& coin, bool possi
         fresh = !it->second.IsDirty();
     }
     if (!inserted) {
+        m_dirty_count -= it->second.IsDirty();
         cachedCoinsUsage -= it->second.coin.DynamicMemoryUsage();
     }
     it->second.coin = std::move(coin);
     CCoinsCacheEntry::SetDirty(*it, m_sentinel);
+    ++m_dirty_count;
     if (fresh) CCoinsCacheEntry::SetFresh(*it, m_sentinel);
     cachedCoinsUsage += it->second.coin.DynamicMemoryUsage();
     TRACEPOINT(utxocache, add,
@@ -117,6 +119,7 @@ void CCoinsViewCache::EmplaceCoinInternalDANGER(COutPoint&& outpoint, Coin&& coi
     auto [it, inserted] = cacheCoins.try_emplace(std::move(outpoint), std::move(coin));
     if (inserted) {
         CCoinsCacheEntry::SetDirty(*it, m_sentinel);
+        ++m_dirty_count;
         cachedCoinsUsage += mem_usage;
     }
 }
@@ -135,6 +138,7 @@ void AddCoins(CCoinsViewCache& cache, const CTransaction &tx, int nHeight, bool
 bool CCoinsViewCache::SpendCoin(const COutPoint &outpoint, Coin* moveout) {
     CCoinsMap::iterator it = FetchCoin(outpoint);
     if (it == cacheCoins.end()) return false;
+    m_dirty_count -= it->second.IsDirty();
     cachedCoinsUsage -= it->second.coin.DynamicMemoryUsage();
     TRACEPOINT(utxocache, spent,
            outpoint.hash.data(),
@@ -149,6 +153,7 @@ bool CCoinsViewCache::SpendCoin(const COutPoint &outpoint, Coin* moveout) {
         cacheCoins.erase(it);
     } else {
         CCoinsCacheEntry::SetDirty(*it, m_sentinel);
+        ++m_dirty_count;
         it->second.coin.Clear();
     }
     return true;
@@ -207,8 +212,9 @@ void CCoinsViewCache::BatchWrite(CoinsViewCacheCursor& cursor, const uint256& ha
                 } else {
                     entry.coin = it->second.coin;
                 }
-                cachedCoinsUsage += entry.coin.DynamicMemoryUsage();
                 CCoinsCacheEntry::SetDirty(*itUs, m_sentinel);
+                ++m_dirty_count;
+                cachedCoinsUsage += entry.coin.DynamicMemoryUsage();
                 // We can mark it FRESH in the parent if it was FRESH in the child
                 // Otherwise it might have just been flushed from the parent's cache
                 // and already exist in the grandparent
@@ -227,6 +233,7 @@ void CCoinsViewCache::BatchWrite(CoinsViewCacheCursor& cursor, const uint256& ha
             if (itUs->second.IsFresh() && it->second.coin.IsSpent()) {
                 // The grandparent cache does not have an entry, and the coin
                 // has been spent. We can just delete it from the parent cache.
+                m_dirty_count -= itUs->second.IsDirty();
                 cachedCoinsUsage -= itUs->second.coin.DynamicMemoryUsage();
                 cacheCoins.erase(itUs);
             } else {
@@ -240,7 +247,10 @@ void CCoinsViewCache::BatchWrite(CoinsViewCacheCursor& cursor, const uint256& ha
                     itUs->second.coin = it->second.coin;
                 }
                 cachedCoinsUsage += itUs->second.coin.DynamicMemoryUsage();
-                CCoinsCacheEntry::SetDirty(*itUs, m_sentinel);
+                if (!itUs->second.IsDirty()) {
+                    CCoinsCacheEntry::SetDirty(*itUs, m_sentinel);
+                    ++m_dirty_count;
+                }
                 // NOTE: It isn't safe to mark the coin as FRESH in the parent
                 // cache. If it already existed and was spent in the parent
                 // cache then marking it FRESH would prevent that spentness
@@ -253,8 +263,9 @@ void CCoinsViewCache::BatchWrite(CoinsViewCacheCursor& cursor, const uint256& ha
 
 void CCoinsViewCache::Flush(bool reallocate_cache)
 {
-    auto cursor{CoinsViewCacheCursor(m_sentinel, cacheCoins, /*will_erase=*/true)};
+    auto cursor{CoinsViewCacheCursor(m_dirty_count, m_sentinel, cacheCoins, /*will_erase=*/true)};
     base->BatchWrite(cursor, hashBlock);
+    Assume(m_dirty_count == 0);
     cacheCoins.clear();
     if (reallocate_cache) {
         ReallocateCache();
@@ -264,8 +275,9 @@ void CCoinsViewCache::Flush(bool reallocate_cache)
 
 void CCoinsViewCache::Sync()
 {
-    auto cursor{CoinsViewCacheCursor(m_sentinel, cacheCoins, /*will_erase=*/false)};
+    auto cursor{CoinsViewCacheCursor(m_dirty_count, m_sentinel, cacheCoins, /*will_erase=*/false)};
     base->BatchWrite(cursor, hashBlock);
+    Assume(m_dirty_count == 0);
     if (m_sentinel.second.Next() != &m_sentinel) {
         /* BatchWrite must clear flags of all entries */
         throw std::logic_error("Not all unspent flagged entries were cleared");
@@ -276,6 +288,7 @@ void CCoinsViewCache::Reset() noexcept
 {
     cacheCoins.clear();
     cachedCoinsUsage = 0;
+    m_dirty_count = 0;
     SetBestBlock(uint256::ZERO);
 }
 
@@ -348,7 +361,7 @@ void CCoinsViewCache::SanityCheck() const
         // Count the number of entries actually in the list.
         ++count_linked;
     }
-    assert(count_linked == count_dirty);
+    assert(count_dirty == count_linked && count_dirty == m_dirty_count);
     assert(recomputed_usage == cachedCoinsUsage);
 }
 
diff --git a/src/coins.h b/src/coins.h
index 4b39c0ba..9efd23c5 100644
--- a/src/coins.h
+++ b/src/coins.h
@@ -265,10 +265,11 @@ struct CoinsViewCacheCursor
     //! This is an optimization compared to erasing all entries as the cursor iterates them when will_erase is set.
     //! Calling CCoinsMap::clear() afterwards is faster because a CoinsCachePair cannot be coerced back into a
     //! CCoinsMap::iterator to be erased, and must therefore be looked up again by key in the CCoinsMap before being erased.
-    CoinsViewCacheCursor(CoinsCachePair& sentinel LIFETIMEBOUND,
+    CoinsViewCacheCursor(size_t& dirty_count LIFETIMEBOUND,
+                         CoinsCachePair& sentinel LIFETIMEBOUND,
                          CCoinsMap& map LIFETIMEBOUND,
                          bool will_erase) noexcept
-        : m_sentinel(sentinel), m_map(map), m_will_erase(will_erase) {}
+        : m_dirty_count(dirty_count), m_sentinel(sentinel), m_map(map), m_will_erase(will_erase) {}
 
     inline CoinsCachePair* Begin() const noexcept { return m_sentinel.second.Next(); }
     inline CoinsCachePair* End() const noexcept { return &m_sentinel; }
@@ -277,6 +278,7 @@ struct CoinsViewCacheCursor
     inline CoinsCachePair* NextAndMaybeErase(CoinsCachePair& current) noexcept
     {
         const auto next_entry{current.second.Next()};
+        m_dirty_count -= current.second.IsDirty();
         // If we are not going to erase the cache, we must still erase spent entries.
         // Otherwise, clear the state of the entry.
         if (!m_will_erase) {
@@ -292,6 +294,7 @@ struct CoinsViewCacheCursor
 
     inline bool WillErase(CoinsCachePair& current) const noexcept { return m_will_erase || current.second.coin.IsSpent(); }
 private:
+    size_t& m_dirty_count;
     CoinsCachePair& m_sentinel;
     CCoinsMap& m_map;
     bool m_will_erase;
@@ -369,6 +372,8 @@ protected:
 
     /* Cached dynamic memory usage for the inner Coin objects. */
     mutable size_t cachedCoinsUsage{0};
+    /* Running count of dirty Coin cache entries. */
+    mutable size_t m_dirty_count{0};
 
     /**
      * Discard all modifications made to this cache without flushing to the base view.
@@ -458,9 +463,12 @@ public:
      */
     void Uncache(const COutPoint &outpoint);
 
-    //! Calculate the size of the cache (in number of transaction outputs)
+    //! Size of the cache (in number of transaction outputs)
     unsigned int GetCacheSize() const;
 
+    //! Number of dirty cache entries (transaction outputs)
+    size_t GetDirtyCount() const noexcept { return m_dirty_count; }
+
     //! Calculate the size of the cache (in bytes)
     size_t DynamicMemoryUsage() const;
 
diff --git a/src/test/coins_tests.cpp b/src/test/coins_tests.cpp
index d5219fce..92935ba6 100644
--- a/src/test/coins_tests.cpp
+++ b/src/test/coins_tests.cpp
@@ -95,6 +95,7 @@ public:
     CCoinsMap& map() const { return cacheCoins; }
     CoinsCachePair& sentinel() const { return m_sentinel; }
     size_t& usage() const { return cachedCoinsUsage; }
+    size_t& dirty() const { return m_dirty_count; }
 };
 
 } // namespace
@@ -656,8 +657,10 @@ static void WriteCoinsViewEntry(CCoinsView& view, const MaybeCoin& cache_coin)
     CCoinsMapMemoryResource resource;
     CCoinsMap map{0, CCoinsMap::hasher{}, CCoinsMap::key_equal{}, &resource};
     if (cache_coin) InsertCoinsMapEntry(map, sentinel, *cache_coin);
-    auto cursor{CoinsViewCacheCursor(sentinel, map, /*will_erase=*/true)};
+    size_t dirty_count{cache_coin && cache_coin->IsDirty()};
+    auto cursor{CoinsViewCacheCursor(dirty_count, sentinel, map, /*will_erase=*/true)};
     view.BatchWrite(cursor, {});
+    BOOST_CHECK_EQUAL(dirty_count, 0U);
 }
 
 class SingleEntryCacheTest
@@ -667,7 +670,10 @@ public:
     {
         auto base_cache_coin{base_value == ABSENT ? MISSING : CoinEntry{base_value, CoinEntry::State::DIRTY}};
         WriteCoinsViewEntry(base, base_cache_coin);
-        if (cache_coin) cache.usage() += InsertCoinsMapEntry(cache.map(), cache.sentinel(), *cache_coin);
+        if (cache_coin) {
+            cache.usage() += InsertCoinsMapEntry(cache.map(), cache.sentinel(), *cache_coin);
+            cache.dirty() += cache_coin->IsDirty();
+        }
     }
 
     CCoinsView root;
@@ -1128,6 +1134,7 @@ BOOST_AUTO_TEST_CASE(ccoins_reset_guard)
 
     const Coin coin{CTxOut{m_rng.randrange(10), CScript{} << m_rng.randbytes(CScriptBase::STATIC_SIZE + 1)}, 1, false};
     cache.EmplaceCoinInternalDANGER(COutPoint{outpoint}, Coin{coin});
+    BOOST_CHECK_EQUAL(cache.GetDirtyCount(), 1U);
 
     uint256 cache_best_block{m_rng.rand256()};
     cache.SetBestBlock(cache_best_block);
@@ -1137,12 +1144,14 @@ BOOST_AUTO_TEST_CASE(ccoins_reset_guard)
         BOOST_CHECK(cache.AccessCoin(outpoint) == coin);
         BOOST_CHECK(!cache.AccessCoin(outpoint).IsSpent());
         BOOST_CHECK_EQUAL(cache.GetCacheSize(), 1);
+        BOOST_CHECK_EQUAL(cache.GetDirtyCount(), 1);
         BOOST_CHECK_EQUAL(cache.GetBestBlock(), cache_best_block);
         BOOST_CHECK(!root_cache.HaveCoinInCache(outpoint));
     }
 
     BOOST_CHECK(cache.AccessCoin(outpoint).IsSpent());
     BOOST_CHECK_EQUAL(cache.GetCacheSize(), 0);
+    BOOST_CHECK_EQUAL(cache.GetDirtyCount(), 0);
     BOOST_CHECK_EQUAL(cache.GetBestBlock(), base_best_block);
     BOOST_CHECK(!root_cache.HaveCoinInCache(outpoint));
 
@@ -1153,8 +1162,13 @@ BOOST_AUTO_TEST_CASE(ccoins_reset_guard)
 
     BOOST_CHECK(cache.AccessCoin(outpoint).IsSpent());
     BOOST_CHECK_EQUAL(cache.GetCacheSize(), 0);
+    BOOST_CHECK_EQUAL(cache.GetDirtyCount(), 0U);
     BOOST_CHECK_EQUAL(cache.GetBestBlock(), base_best_block);
     BOOST_CHECK(!root_cache.HaveCoinInCache(outpoint));
+
+    // Flush should be a no-op after reset.
+    cache.Flush();
+    BOOST_CHECK_EQUAL(cache.GetDirtyCount(), 0U);
 }
 
 BOOST_AUTO_TEST_SUITE_END()
diff --git a/src/test/fuzz/coins_view.cpp b/src/test/fuzz/coins_view.cpp
index 699a45e2..ec10dbb3 100644
--- a/src/test/fuzz/coins_view.cpp
+++ b/src/test/fuzz/coins_view.cpp
@@ -139,6 +139,7 @@ void TestCoinsView(FuzzedDataProvider& fuzzed_data_provider, CCoinsView& backend
             [&] {
                 CoinsCachePair sentinel{};
                 sentinel.second.SelfRef(sentinel);
+                size_t dirty_count{0};
                 CCoinsMapMemoryResource resource;
                 CCoinsMap coins_map{0, SaltedOutpointHasher{/*deterministic=*/true}, CCoinsMap::key_equal{}, &resource};
                 LIMITED_WHILE(good_data && fuzzed_data_provider.ConsumeBool(), 10'000)
@@ -159,10 +160,11 @@ void TestCoinsView(FuzzedDataProvider& fuzzed_data_provider, CCoinsView& backend
                     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;
                 try {
-                    auto cursor{CoinsViewCacheCursor(sentinel, coins_map, /*will_erase=*/true)};
+                    auto cursor{CoinsViewCacheCursor(dirty_count, sentinel, coins_map, /*will_erase=*/true)};
                     uint256 best_block{coins_view_cache.GetBestBlock()};
                     if (fuzzed_data_provider.ConsumeBool()) best_block = ConsumeUInt256(fuzzed_data_provider);
                     // Set best block hash to non-null to satisfy the assertion in CCoinsViewDB::BatchWrite().