txdb: add CCoinsViewDB::ChangeCacheSize

We'll need this to dynamically update the cache size of the existing
CCoinsViewDB instance when we create a new one during snapshot activation.

This requires us to keep the CDBWrapper instance as a pointer instead of
a reference so that we're able to destruct it and create a new instance
when the cache size changes.

Also renames `db` to `m_db` since we're already modifying each usage.

Includes feedback from Russ Yanofsky.

2 files changed, 36 insertions, 17 deletions

diff --git a/src/txdb.cpp b/src/txdb.cpp
index 129697f0e7..898cd1996b 100644
--- a/src/txdb.cpp
+++ b/src/txdb.cpp
@@ -10,6 +10,7 @@
 #include <shutdown.h>
 #include <ui_interface.h>
 #include <uint256.h>
+#include <util/memory.h>
 #include <util/system.h>
 #include <util/translation.h>
 #include <util/vector.h>
@@ -41,35 +42,45 @@ struct CoinEntry {
 
 }
 
-CCoinsViewDB::CCoinsViewDB(fs::path ldb_path, size_t nCacheSize, bool fMemory, bool fWipe) : db(ldb_path, nCacheSize, fMemory, fWipe, true)
+CCoinsViewDB::CCoinsViewDB(fs::path ldb_path, size_t nCacheSize, bool fMemory, bool fWipe) :
+    m_db(MakeUnique<CDBWrapper>(ldb_path, nCacheSize, fMemory, fWipe, true)),
+    m_ldb_path(ldb_path),
+    m_is_memory(fMemory) { }
+
+void CCoinsViewDB::ResizeCache(size_t new_cache_size)
 {
+    // Have to do a reset first to get the original `m_db` state to release its
+    // filesystem lock.
+    m_db.reset();
+    m_db = MakeUnique<CDBWrapper>(
+        m_ldb_path, new_cache_size, m_is_memory, /*fWipe*/ false, /*obfuscate*/ true);
 }
 
 bool CCoinsViewDB::GetCoin(const COutPoint &outpoint, Coin &coin) const {
-    return db.Read(CoinEntry(&outpoint), coin);
+    return m_db->Read(CoinEntry(&outpoint), coin);
 }
 
 bool CCoinsViewDB::HaveCoin(const COutPoint &outpoint) const {
-    return db.Exists(CoinEntry(&outpoint));
+    return m_db->Exists(CoinEntry(&outpoint));
 }
 
 uint256 CCoinsViewDB::GetBestBlock() const {
     uint256 hashBestChain;
-    if (!db.Read(DB_BEST_BLOCK, hashBestChain))
+    if (!m_db->Read(DB_BEST_BLOCK, hashBestChain))
         return uint256();
     return hashBestChain;
 }
 
 std::vector<uint256> CCoinsViewDB::GetHeadBlocks() const {
     std::vector<uint256> vhashHeadBlocks;
-    if (!db.Read(DB_HEAD_BLOCKS, vhashHeadBlocks)) {
+    if (!m_db->Read(DB_HEAD_BLOCKS, vhashHeadBlocks)) {
         return std::vector<uint256>();
     }
     return vhashHeadBlocks;
 }
 
 bool CCoinsViewDB::BatchWrite(CCoinsMap &mapCoins, const uint256 &hashBlock) {
-    CDBBatch batch(db);
+    CDBBatch batch(*m_db);
     size_t count = 0;
     size_t changed = 0;
     size_t batch_size = (size_t)gArgs.GetArg("-dbbatchsize", nDefaultDbBatchSize);
@@ -107,7 +118,7 @@ bool CCoinsViewDB::BatchWrite(CCoinsMap &mapCoins, const uint256 &hashBlock) {
         mapCoins.erase(itOld);
         if (batch.SizeEstimate() > batch_size) {
             LogPrint(BCLog::COINDB, "Writing partial batch of %.2f MiB\n", batch.SizeEstimate() * (1.0 / 1048576.0));
-            db.WriteBatch(batch);
+            m_db->WriteBatch(batch);
             batch.Clear();
             if (crash_simulate) {
                 static FastRandomContext rng;
@@ -124,14 +135,14 @@ bool CCoinsViewDB::BatchWrite(CCoinsMap &mapCoins, const uint256 &hashBlock) {
     batch.Write(DB_BEST_BLOCK, hashBlock);
 
     LogPrint(BCLog::COINDB, "Writing final batch of %.2f MiB\n", batch.SizeEstimate() * (1.0 / 1048576.0));
-    bool ret = db.WriteBatch(batch);
+    bool ret = m_db->WriteBatch(batch);
     LogPrint(BCLog::COINDB, "Committed %u changed transaction outputs (out of %u) to coin database...\n", (unsigned int)changed, (unsigned int)count);
     return ret;
 }
 
 size_t CCoinsViewDB::EstimateSize() const
 {
-    return db.EstimateSize(DB_COIN, (char)(DB_COIN+1));
+    return m_db->EstimateSize(DB_COIN, (char)(DB_COIN+1));
 }
 
 CBlockTreeDB::CBlockTreeDB(size_t nCacheSize, bool fMemory, bool fWipe) : CDBWrapper(GetDataDir() / "blocks" / "index", nCacheSize, fMemory, fWipe) {
@@ -158,7 +169,7 @@ bool CBlockTreeDB::ReadLastBlockFile(int &nFile) {
 
 CCoinsViewCursor *CCoinsViewDB::Cursor() const
 {
-    CCoinsViewDBCursor *i = new CCoinsViewDBCursor(const_cast<CDBWrapper&>(db).NewIterator(), GetBestBlock());
+    CCoinsViewDBCursor *i = new CCoinsViewDBCursor(const_cast<CDBWrapper&>(*m_db).NewIterator(), GetBestBlock());
     /* It seems that there are no "const iterators" for LevelDB.  Since we
        only need read operations on it, use a const-cast to get around
        that restriction.  */
@@ -338,7 +349,7 @@ public:
  * Currently implemented: from the per-tx utxo model (0.8..0.14.x) to per-txout.
  */
 bool CCoinsViewDB::Upgrade() {
-    std::unique_ptr<CDBIterator> pcursor(db.NewIterator());
+    std::unique_ptr<CDBIterator> pcursor(m_db->NewIterator());
     pcursor->Seek(std::make_pair(DB_COINS, uint256()));
     if (!pcursor->Valid()) {
         return true;
@@ -349,7 +360,7 @@ bool CCoinsViewDB::Upgrade() {
     LogPrintf("[0%%]..."); /* Continued */
     uiInterface.ShowProgress(_("Upgrading UTXO database").translated, 0, true);
     size_t batch_size = 1 << 24;
-    CDBBatch batch(db);
+    CDBBatch batch(*m_db);
     int reportDone = 0;
     std::pair<unsigned char, uint256> key;
     std::pair<unsigned char, uint256> prev_key = {DB_COINS, uint256()};
@@ -384,9 +395,9 @@ bool CCoinsViewDB::Upgrade() {
             }
             batch.Erase(key);
             if (batch.SizeEstimate() > batch_size) {
-                db.WriteBatch(batch);
+                m_db->WriteBatch(batch);
                 batch.Clear();
-                db.CompactRange(prev_key, key);
+                m_db->CompactRange(prev_key, key);
                 prev_key = key;
             }
             pcursor->Next();
@@ -394,8 +405,8 @@ bool CCoinsViewDB::Upgrade() {
             break;
         }
     }
-    db.WriteBatch(batch);
-    db.CompactRange({DB_COINS, uint256()}, key);
+    m_db->WriteBatch(batch);
+    m_db->CompactRange({DB_COINS, uint256()}, key);
     uiInterface.ShowProgress("", 100, false);
     LogPrintf("[%s].\n", ShutdownRequested() ? "CANCELLED" : "DONE");
     return !ShutdownRequested();
diff --git a/src/txdb.h b/src/txdb.h
index 488c24f935..0cf7e2f1b8 100644
--- a/src/txdb.h
+++ b/src/txdb.h
@@ -39,11 +39,16 @@ static const int64_t max_filter_index_cache = 1024;
 //! Max memory allocated to coin DB specific cache (MiB)
 static const int64_t nMaxCoinsDBCache = 8;
 
+// Actually declared in validation.cpp; can't include because of circular dependency.
+extern RecursiveMutex cs_main;
+
 /** CCoinsView backed by the coin database (chainstate/) */
 class CCoinsViewDB final : public CCoinsView
 {
 protected:
-    CDBWrapper db;
+    std::unique_ptr<CDBWrapper> m_db;
+    fs::path m_ldb_path;
+    bool m_is_memory;
 public:
     /**
      * @param[in] ldb_path    Location in the filesystem where leveldb data will be stored.
@@ -60,6 +65,9 @@ public:
     //! Attempt to update from an older database format. Returns whether an error occurred.
     bool Upgrade();
     size_t EstimateSize() const override;
+
+    //! Dynamically alter the underlying leveldb cache size.
+    void ResizeCache(size_t new_cache_size) EXCLUSIVE_LOCKS_REQUIRED(cs_main);
 };
 
 /** Specialization of CCoinsViewCursor to iterate over a CCoinsViewDB */