coins: test chainstate flush baseline

Add `CDBWrapper::GetProperty()` and expose it through `CCoinsViewDB::GetDBProperty()` so coins tests can inspect LevelDB runtime properties through the coins view.
Use it in a coins DB flush baseline that records the LevelDB layout after flushing while keeping readback coverage for the flushed coin and best block.

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

src/dbwrapper.cpp

@@ -301,11 +301,16 @@ void CDBWrapper::WriteBatch(CDBBatch& batch, bool fSync)
     }
 }
 
+std::optional<std::string> CDBWrapper::GetProperty(const std::string& property) const
+{
+    if (std::string value; DBContext().pdb->GetProperty(property, &value)) return value;
+    return std::nullopt;
+}
+
 size_t CDBWrapper::DynamicMemoryUsage() const
 {
-    std::string memory;
     std::optional<size_t> parsed;
-    if (!DBContext().pdb->GetProperty("leveldb.approximate-memory-usage", &memory) || !(parsed = ToIntegral<size_t>(memory))) {
+    if (auto memory{GetProperty("leveldb.approximate-memory-usage")}; !memory || !(parsed = ToIntegral<size_t>(*memory))) {
         LogDebug(BCLog::LEVELDB, "Failed to get approximate-memory-usage property\n");
         return 0;
     }

src/dbwrapper.h

@@ -258,6 +258,9 @@ public:
 
     void WriteBatch(CDBBatch& batch, bool fSync = false);
 
+    //! Return a LevelDB property value, if available.
+    std::optional<std::string> GetProperty(const std::string& property) const;
+
     // Get an estimate of LevelDB memory usage (in bytes).
     size_t DynamicMemoryUsage() const;
 

src/test/coins_tests.cpp

@@ -14,6 +14,7 @@
 #include <uint256.h>
 #include <undo.h>
 #include <util/byte_units.h>
+#include <util/check.h>
 #include <util/strencodings.h>
 
 #include <map>
@@ -1060,6 +1061,28 @@ BOOST_FIXTURE_TEST_CASE(ccoins_flush_behavior, FlushTest)
     }
 }
 
+BOOST_FIXTURE_TEST_CASE(coins_db_leveldb_layout, FlushTest)
+{
+    auto level2_files{[](CCoinsViewDB& base) {
+        return *Assert(ToIntegral<int>(*Assert(base.GetDBProperty("leveldb.num-files-at-level2"))));
+    }};
+    const COutPoint outpoint{Txid::FromUint256(m_rng.rand256()), 0};
+    const Coin coin{MakeCoin()};
+    const uint256 block_hash{m_rng.rand256()};
+
+    CCoinsViewDB base{{.path = m_args.GetDataDirBase() / "coins_db_leveldb_layout", .cache_bytes = 1_MiB, .wipe_data = true}, {}};
+    CCoinsViewCache cache{&base};
+
+    cache.EmplaceCoinInternalDANGER(COutPoint{outpoint}, Coin{coin});
+    cache.SetBestBlock(block_hash);
+    cache.Sync();
+
+    BOOST_CHECK_EQUAL(level2_files(base), 0);
+
+    BOOST_CHECK(*Assert(base.GetCoin(outpoint)) == coin);
+    BOOST_CHECK_EQUAL(base.GetBestBlock(), block_hash);
+}
+
 BOOST_AUTO_TEST_CASE(coins_resource_is_used)
 {
     CCoinsMapMemoryResource resource;

src/txdb.cpp

@@ -175,6 +175,11 @@ size_t CCoinsViewDB::EstimateSize() const
     return m_db->EstimateSize(DB_COIN, uint8_t(DB_COIN + 1));
 }
 
+std::optional<std::string> CCoinsViewDB::GetDBProperty(const std::string& property)
+{
+    return m_db->GetProperty(property);
+}
+
 /** Specialization of CCoinsViewCursor to iterate over a CCoinsViewDB */
 class CCoinsViewDBCursor: public CCoinsViewCursor
 {

src/txdb.h

@@ -17,6 +17,7 @@
 #include <cstdint>
 #include <memory>
 #include <optional>
+#include <string>
 #include <vector>
 
 class COutPoint;
@@ -54,6 +55,9 @@ public:
 
     //! Dynamically alter the underlying leveldb cache size.
     void ResizeCache(size_t new_cache_size) EXCLUSIVE_LOCKS_REQUIRED(cs_main);
+
+    //! Return an underlying LevelDB property value, if available.
+    std::optional<std::string> GetDBProperty(const std::string& property);
 };
 
 #endif // BITCOIN_TXDB_H