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Merge bitcoin/bitcoin#32313: coins: fix cachedCoinsUsage accounting in CCoinsViewCache

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24d861da78 coins: only adjust `cachedCoinsUsage` on `EmplaceCoinInternalDANGER` insert (Lőrinc)
d7c9d6c291 coins: fix `cachedCoinsUsage` accounting to prevent underflow (Lőrinc)
39cf8bb3d0 refactor: remove redundant usage tracking from `CoinsViewCacheCursor` (Lőrinc)
67cff8bec9 refactor: assert newly-created parent cache entry has zero memory usage (Lőrinc)

Pull request description:

  ### Summary

  This PR fixes `cachedCoinsUsage` accounting bugs in `CCoinsViewCache` that caused UBSan `unsigned-integer-overflow` violations during testing. The issues stemmed from incorrect decrement timing in `AddCoin()`, unconditional reset in `Flush()` on failure, and incorrect increment in `EmplaceCoinInternalDANGER()` when insertion fails.

  ### Problems Fixed

  **1. `AddCoin()` underflow on exception**
  - Previously decremented `cachedCoinsUsage` *before* the `possible_overwrite` validation
  - If validation threw, the map entry remained unchanged but counter was decremented
  - This corrupted accounting and later caused underflow
  - **Impact**: Test-only in current codebase, but unsound accounting that could affect future changes

  **2. `Flush()` accounting drift on failure**
  - Unconditionally reset `cachedCoinsUsage` to 0, even when `BatchWrite()` failed
  - Left the map populated while the counter read zero
  - **Impact**: Test-only (production `BatchWrite()` returns `true`), but broke accounting consistency

  **3. Cursor redundant usage tracking**
  - `CoinsViewCacheCursor::NextAndMaybeErase()` subtracted usage when erasing spent entries
  - However, `SpendCoin()` already decremented and cleared the `scriptPubKey`, leaving `DynamicMemoryUsage()` at 0
  - **Impact**: Redundant code that obscured actual accounting behavior

  **4. `EmplaceCoinInternalDANGER()` double-counting**
  - Incremented `cachedCoinsUsage` even when `try_emplace` did not insert (duplicate key)
  - Inflated the counter on duplicate attempts
  - **Impact**: Mostly test-reachable (AssumeUTXO doesn't overwrite in production), but incorrect accounting

  ### Testing

  To reproduce the historical UBSan failures on the referenced baseline and to verify the fix, run:
  ```
  MAKEJOBS="-j$(nproc)" FILE_ENV="./ci/test/00_setup_env_native_fuzz.sh" ./ci/test_run_all.sh
  ```

  The change was tested with the related unit and fuzz test, and asserted before/after each `cachedCoinsUsage` change (in production code and fuzz) that the calculations are still correct by recalculating them from scratch.

  <details>
  <summary>Details</summary>

  ```C++
  bool CCoinsViewCache::CacheUsageValid() const
  {
      size_t actual{0};
      for (auto& entry : cacheCoins | std::views::values) actual += entry.coin.DynamicMemoryUsage();
      return actual == cachedCoinsUsage;
  }
  ```
  or
  ```patch
  diff --git a/src/coins.cpp b/src/coins.cpp
  --- a/src/coins.cpp(revision fd3b1a7f4bb2ac527f23d4eb4cfa40a3215906e5)
  +++ b/src/coins.cpp(revision 872a05633bfdbd06ad82190d7fe34b42d13ebfe9)
  @@ -96,6 +96,7 @@
           fresh = !it->second.IsDirty();
       }
       if (!inserted) {
  +        Assert(cachedCoinsUsage >= it->second.coin.DynamicMemoryUsage());
           cachedCoinsUsage -= it->second.coin.DynamicMemoryUsage();
       }
       it->second.coin = std::move(coin);
  @@ -133,6 +134,7 @@
   bool CCoinsViewCache::SpendCoin(const COutPoint &outpoint, Coin* moveout) {
       CCoinsMap::iterator it = FetchCoin(outpoint);
       if (it == cacheCoins.end()) return false;
  +    Assert(cachedCoinsUsage >= it->second.coin.DynamicMemoryUsage());
       cachedCoinsUsage -= it->second.coin.DynamicMemoryUsage();
       TRACEPOINT(utxocache, spent,
              outpoint.hash.data(),
  @@ -226,10 +228,12 @@
               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.
  +                Assert(cachedCoinsUsage >= itUs->second.coin.DynamicMemoryUsage());
                   cachedCoinsUsage -= itUs->second.coin.DynamicMemoryUsage();
                   cacheCoins.erase(itUs);
               } else {
                   // A normal modification.
  +                Assert(cachedCoinsUsage >= itUs->second.coin.DynamicMemoryUsage());
                   cachedCoinsUsage -= itUs->second.coin.DynamicMemoryUsage();
                   if (cursor.WillErase(*it)) {
                       // Since this entry will be erased,
  @@ -279,6 +283,7 @@
   {
       CCoinsMap::iterator it = cacheCoins.find(hash);
       if (it != cacheCoins.end() && !it->second.IsDirty() && !it->second.IsFresh()) {
  +        Assert(cachedCoinsUsage >= it->second.coin.DynamicMemoryUsage());
           cachedCoinsUsage -= it->second.coin.DynamicMemoryUsage();
           TRACEPOINT(utxocache, uncache,
                  hash.hash.data(),
  ```

  </details>

ACKs for top commit:
  optout21:
    reACK 24d861da78
  andrewtoth:
    ACK 24d861da78
  sipa:
    ACK 24d861da78
  w0xlt:
    ACK 24d861da78

Tree-SHA512: ff1b756b46220f278ab6c850626a0f376bed64389ef7f66a95c994e1c7cceec1d1843d2b24e8deabe10e2bdade2a274d9654ac60eb2b9bf471a71db8a2ff496c
This commit is contained in:
merge-script
2025-10-15 09:48:04 -04:00
parent 40e7d4cd0d 24d861da78
commit f76e1ae389
5 changed files with 66 additions and 41 deletions
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30
src/test/fuzz/coins_view.cpp
View File

@@ -59,25 +59,19 @@ void TestCoinsView(FuzzedDataProvider& fuzzed_data_provider, CCoinsView& backend
if (random_coin.IsSpent()) {
return;
}
Coin coin = random_coin;
bool expected_code_path = false;
const bool possible_overwrite = fuzzed_data_provider.ConsumeBool();
try {
coins_view_cache.AddCoin(random_out_point, std::move(coin), possible_overwrite);
expected_code_path = true;
} catch (const std::logic_error& e) {
if (e.what() == std::string{"Attempted to overwrite an unspent coin (when possible_overwrite is false)"}) {
COutPoint outpoint{random_out_point};
Coin coin{random_coin};
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);
expected_code_path = true;
// AddCoin() decreases cachedCoinsUsage by the memory usage of the old coin at the beginning and
// increases it by the value of the new coin at the end. If it throws in the process, the value
// of cachedCoinsUsage would have been incorrectly decreased, leading to an underflow later on.
// To avoid this, use Flush() to reset the value of cachedCoinsUsage in sync with the cacheCoins
// mapping.
(void)coins_view_cache.Flush();
}
} else {
coins_view_cache.EmplaceCoinInternalDANGER(std::move(outpoint), std::move(coin));
}
assert(expected_code_path);
},
[&] {
(void)coins_view_cache.Flush();
@@ -131,7 +125,6 @@ void TestCoinsView(FuzzedDataProvider& fuzzed_data_provider, CCoinsView& backend
[&] {
CoinsCachePair sentinel{};
sentinel.second.SelfRef(sentinel);
size_t usage{0};
CCoinsMapMemoryResource resource;
CCoinsMap coins_map{0, SaltedOutpointHasher{/*deterministic=*/true}, CCoinsMap::key_equal{}, &resource};
LIMITED_WHILE(good_data && fuzzed_data_provider.ConsumeBool(), 10'000)
@@ -152,11 +145,10 @@ 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);
usage += it->second.coin.DynamicMemoryUsage();
}
bool expected_code_path = false;
try {
auto cursor{CoinsViewCacheCursor(usage, sentinel, coins_map, /*will_erase=*/true)};
auto cursor{CoinsViewCacheCursor(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().