refactor: have CCoins* data managed under CChainState

This change encapsulates UTXO set data within CChainState instances, removing
global data `pcoinsTip` and `pcoinsviewdb`. This is necessary if we want to
maintain multiple chainstates with their own rendering of the UTXO set.

We introduce a class CoinsViews which consolidates the construction of a
CCoins* hierarchy. Construction of its various pieces (db, coinscatcher,
in-memory cache) is split up so that we avoid flushing bad state to disk if
startup is interrupted.

We also introduce `CChainState::CanFlushToDisk()` which tells us when it is
safe to flush the chainstate based on this partial construction.

This commit could be broken into smaller pieces, but it would require more
ephemeral diffs to, e.g., temporarily change CCoinsViewDB's constructor
invocations.

Other changes:

- A parameter has been added to the CCoinsViewDB constructor that allows the
  name of the corresponding leveldb directory to be specified.

Thanks to Russell Yanofsky and Marco Falke for helpful feedback.

1 files changed, 86 insertions, 8 deletions

diff --git a/src/validation.h b/src/validation.h
index 6661369bf8..2a268d8cab 100644
--- a/src/validation.h
+++ b/src/validation.h
@@ -19,6 +19,7 @@
 #include <script/script_error.h>
 #include <sync.h>
 #include <txmempool.h> // For CTxMemPool::cs
+#include <txdb.h>
 #include <versionbits.h>
 
 #include <algorithm>
@@ -37,7 +38,6 @@ class CBlockIndex;
 class CBlockTreeDB;
 class CBlockUndo;
 class CChainParams;
-class CCoinsViewDB;
 class CInv;
 class CConnman;
 class CScriptCheck;
@@ -505,8 +505,40 @@ public:
         CBlockIndex** ppindex) EXCLUSIVE_LOCKS_REQUIRED(cs_main);
 };
 
-/** Global variable that points to the active CCoinsView (protected by cs_main) */
-extern std::unique_ptr<CCoinsViewCache> pcoinsTip;
+/**
+ * A convenience class for constructing the CCoinsView* hierarchy used
+ * to facilitate access to the UTXO set.
+ *
+ * This class consists of an arrangement of layered CCoinsView objects,
+ * preferring to store and retrieve coins in memory via `m_cacheview` but
+ * ultimately falling back on cache misses to the canonical store of UTXOs on
+ * disk, `m_dbview`.
+ */
+class CoinsViews {
+
+public:
+    //! The lowest level of the CoinsViews cache hierarchy sits in a leveldb database on disk.
+    //! All unspent coins reside in this store.
+    CCoinsViewDB m_dbview;
+
+    //! This view wraps access to the leveldb instance and handles read errors gracefully.
+    CCoinsViewErrorCatcher m_catcherview;
+
+    //! This is the top layer of the cache hierarchy - it keeps as many coins in memory as
+    //! can fit per the dbcache setting.
+    std::unique_ptr<CCoinsViewCache> m_cacheview;
+
+    //! This constructor initializes CCoinsViewDB and CCoinsViewErrorCatcher instances, but it
+    //! *does not* create a CCoinsViewCache instance by default. This is done separately because the
+    //! presence of the cache has implications on whether or not we're allowed to flush the cache's
+    //! state to disk, which should not be done until the health of the database is verified.
+    //!
+    //! All arguments forwarded onto CCoinsViewDB.
+    CoinsViews(std::string ldb_name, size_t cache_size_bytes, bool in_memory, bool should_wipe);
+
+    //! Initialize the CCoinsViewCache member.
+    void InitCache();
+};
 
 /**
  * CChainState stores and provides an API to update our local knowledge of the
@@ -556,12 +588,39 @@ private:
     //! easily as opposed to referencing a global.
     BlockManager& m_blockman;
 
+    //! Manages the UTXO set, which is a reflection of the contents of `m_chain`.
+    std::unique_ptr<CoinsViews> m_coins_views;
+
 public:
-    CChainState(BlockManager& blockman) : m_blockman(blockman) { }
+    CChainState(BlockManager& blockman) : m_blockman(blockman) {}
+    CChainState();
+
+    /**
+     * Initialize the CoinsViews UTXO set database management data structures. The in-memory
+     * cache is initialized separately.
+     *
+     * All parameters forwarded to CoinsViews.
+     */
+    void InitCoinsDB(
+        size_t cache_size_bytes,
+        bool in_memory,
+        bool should_wipe,
+        std::string leveldb_name = "chainstate");
+
+    //! Initialize the in-memory coins cache (to be done after the health of the on-disk database
+    //! is verified).
+    void InitCoinsCache();
+
+    //! @returns whether or not the CoinsViews object has been fully initialized and we can
+    //!          safely flush this object to disk.
+    bool CanFlushToDisk() {
+        return m_coins_views && m_coins_views->m_cacheview;
+    }
 
     //! The current chain of blockheaders we consult and build on.
     //! @see CChain, CBlockIndex.
     CChain m_chain;
+
     /**
      * The set of all CBlockIndex entries with BLOCK_VALID_TRANSACTIONS (for itself and all ancestors) and
      * as good as our current tip or better. Entries may be failed, though, and pruning nodes may be
@@ -572,9 +631,26 @@ public:
     //! @returns A reference to the in-memory cache of the UTXO set.
     CCoinsViewCache& CoinsTip()
     {
-        return *::pcoinsTip;
+        assert(m_coins_views->m_cacheview);
+        return *m_coins_views->m_cacheview.get();
+    }
+
+    //! @returns A reference to the on-disk UTXO set database.
+    CCoinsViewDB& CoinsDB()
+    {
+        return m_coins_views->m_dbview;
     }
 
+    //! @returns A reference to a wrapped view of the in-memory UTXO set that
+    //!     handles disk read errors gracefully.
+    CCoinsViewErrorCatcher& CoinsErrorCatcher()
+    {
+        return m_coins_views->m_catcherview;
+    }
+
+    //! Destructs all objects related to accessing the UTXO set.
+    void ResetCoinsViews() { m_coins_views.reset(); }
+
     /**
      * Update the on-disk chain state.
      * The caches and indexes are flushed depending on the mode we're called with
@@ -606,7 +682,7 @@ public:
     bool ConnectBlock(const CBlock& block, CValidationState& state, CBlockIndex* pindex,
                       CCoinsViewCache& view, const CChainParams& chainparams, bool fJustCheck = false) EXCLUSIVE_LOCKS_REQUIRED(cs_main);
 
-    // Block disconnection on our pcoinsTip:
+    // Apply the effects of a block disconnection on the UTXO set.
     bool DisconnectTip(CValidationState& state, const CChainParams& chainparams, DisconnectedBlockTransactions* disconnectpool) EXCLUSIVE_LOCKS_REQUIRED(cs_main, ::mempool.cs);
 
     // Manual block validity manipulation:
@@ -668,8 +744,10 @@ CChain& ChainActive();
 /** @returns the global block index map. */
 BlockMap& BlockIndex();
 
-/** Global variable that points to the coins database (protected by cs_main) */
-extern std::unique_ptr<CCoinsViewDB> pcoinsdbview;
+// Most often ::ChainstateActive() should be used instead of this, but some code
+// may not be able to assume that this has been initialized yet and so must use it
+// directly, e.g. init.cpp.
+extern std::unique_ptr<CChainState> g_chainstate;
 
 /** Global variable that points to the active block tree (protected by cs_main) */
 extern std::unique_ptr<CBlockTreeDB> pblocktree;