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.

1 files changed, 9 insertions, 1 deletions

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 */