use ChainstateManager to initialize chainstate

This allows us to easily initialize multiple chainstates on startup in future
commits. It retires the g_chainstate global in lieu of g_chainman.

src/validation.h

@@ -592,8 +592,7 @@ private:
     std::unique_ptr<CoinsViews> m_coins_views;
 
 public:
-    explicit CChainState(BlockManager& blockman) : m_blockman(blockman) {}
-    explicit CChainState(uint256 from_snapshot_blockhash = uint256());
+    explicit CChainState(BlockManager& blockman, uint256 from_snapshot_blockhash = uint256());
 
     /**
      * Initialize the CoinsViews UTXO set database management data structures. The in-memory
@@ -839,9 +838,14 @@ private:
     bool m_snapshot_validated{false};
 
     // For access to m_active_chainstate.
+    friend CChainState& ChainstateActive();
     friend CChain& ChainActive();
 
 public:
+    //! A single BlockManager instance is shared across each constructed
+    //! chainstate to avoid duplicating block metadata.
+    BlockManager m_blockman GUARDED_BY(::cs_main);
+
     //! Instantiate a new chainstate and assign it based upon whether it is
     //! from a snapshot.
     //!
@@ -858,6 +862,11 @@ public:
     int ActiveHeight() const { return ActiveChain().Height(); }
     CBlockIndex* ActiveTip() const { return ActiveChain().Tip(); }
 
+    BlockMap& BlockIndex() EXCLUSIVE_LOCKS_REQUIRED(::cs_main)
+    {
+        return m_blockman.m_block_index;
+    }
+
     bool IsSnapshotActive() const;
 
     Optional<uint256> SnapshotBlockhash() const;
@@ -885,6 +894,8 @@ public:
     void Reset();
 };
 
+extern ChainstateManager g_chainman;
+
 /** @returns the most-work valid chainstate. */
 CChainState& ChainstateActive();
 
@@ -894,11 +905,6 @@ CChain& ChainActive();
 /** @returns the global block index map. */
 BlockMap& BlockIndex();
 
-// 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;