From b0ff31084006ac7d4a7afba3190ca75f5f8441af Mon Sep 17 00:00:00 2001
From: Pieter Wuille <pieter@wuille.net>
Date: Wed, 1 Feb 2023 18:28:08 -0500
Subject: Add CCoinsViewCache::SanityCheck() and use it in fuzz test

---
 src/coins.cpp                    | 17 +++++++++++++++++
 src/coins.h                      |  3 +++
 src/test/fuzz/coinscache_sim.cpp |  9 ++++++++-
 3 files changed, 28 insertions(+), 1 deletion(-)

diff --git a/src/coins.cpp b/src/coins.cpp
index e98bf816ab..8d99019bb0 100644
--- a/src/coins.cpp
+++ b/src/coins.cpp
@@ -314,6 +314,23 @@ void CCoinsViewCache::ReallocateCache()
     ::new (&cacheCoins) CCoinsMap();
 }
 
+void CCoinsViewCache::SanityCheck() const
+{
+    size_t recomputed_usage = 0;
+    for (const auto& [_, entry] : cacheCoins) {
+        unsigned attr = 0;
+        if (entry.flags & CCoinsCacheEntry::DIRTY) attr |= 1;
+        if (entry.flags & CCoinsCacheEntry::FRESH) attr |= 2;
+        if (entry.coin.IsSpent()) attr |= 4;
+        // Only 5 combinations are possible.
+        assert(attr != 2 && attr != 4 && attr != 7);
+
+        // Recompute cachedCoinsUsage.
+        recomputed_usage += entry.coin.DynamicMemoryUsage();
+    }
+    assert(recomputed_usage == cachedCoinsUsage);
+}
+
 static const size_t MIN_TRANSACTION_OUTPUT_WEIGHT = WITNESS_SCALE_FACTOR * ::GetSerializeSize(CTxOut(), PROTOCOL_VERSION);
 static const size_t MAX_OUTPUTS_PER_BLOCK = MAX_BLOCK_WEIGHT / MIN_TRANSACTION_OUTPUT_WEIGHT;
 
diff --git a/src/coins.h b/src/coins.h
index 710b8c7c83..a2764d32bb 100644
--- a/src/coins.h
+++ b/src/coins.h
@@ -320,6 +320,9 @@ public:
     //! See: https://stackoverflow.com/questions/42114044/how-to-release-unordered-map-memory
     void ReallocateCache();
 
+    //! Run an internal sanity check on the cache data structure. */
+    void SanityCheck() const;
+
 private:
     /**
      * @note this is marked const, but may actually append to `cacheCoins`, increasing
diff --git a/src/test/fuzz/coinscache_sim.cpp b/src/test/fuzz/coinscache_sim.cpp
index c3f732f075..b794888ca2 100644
--- a/src/test/fuzz/coinscache_sim.cpp
+++ b/src/test/fuzz/coinscache_sim.cpp
@@ -51,7 +51,8 @@ struct PrecomputedData
             const uint8_t ser[4] = {uint8_t(i), uint8_t(i >> 8), uint8_t(i >> 16), uint8_t(i >> 24)};
             uint256 hash;
             CSHA256().Write(PREFIX_S, 1).Write(ser, sizeof(ser)).Finalize(hash.begin());
-            /* Convert hash to scriptPubkeys. */
+            /* Convert hash to scriptPubkeys (of different lengths, so SanityCheck's cached memory
+             * usage check has a chance to detect mismatches). */
             switch (i % 5U) {
             case 0: /* P2PKH */
                 coins[i].out.scriptPubKey.resize(25);
@@ -381,6 +382,7 @@ FUZZ_TARGET(coinscache_sim)
 
             [&]() { // Remove a cache level.
                 // Apply to real caches (this reduces caches.size(), implicitly doing the same on the simulation data).
+                caches.back()->SanityCheck();
                 caches.pop_back();
             },
 
@@ -420,6 +422,11 @@ FUZZ_TARGET(coinscache_sim)
         );
     }
 
+    // Sanity check all the remaining caches
+    for (const auto& cache : caches) {
+        cache->SanityCheck();
+    }
+
     // Full comparison between caches and simulation data, from bottom to top,
     // as AccessCoin on a higher cache may affect caches below it.
     for (unsigned sim_idx = 1; sim_idx <= caches.size(); ++sim_idx) {
-- 
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