Cache block index entry corresponding to assumeutxo snapshot base blockhash

This is to (a) avoid repeated lookups into the block index for an entry that
should never change and (b) emphasize that the snapshot base should always
exist when set and not change during the runtime of the program.

Thanks to Russ Yanofsky for suggesting this approach.

src/validation.cpp

@@ -1579,6 +1579,13 @@ Chainstate::Chainstate(
       m_chainman(chainman),
       m_from_snapshot_blockhash(from_snapshot_blockhash) {}
 
+const CBlockIndex* Chainstate::SnapshotBase()
+{
+    if (!m_from_snapshot_blockhash) return nullptr;
+    if (!m_cached_snapshot_base) m_cached_snapshot_base = Assert(m_chainman.m_blockman.LookupBlockIndex(*m_from_snapshot_blockhash));
+    return m_cached_snapshot_base;
+}
+
 void Chainstate::InitCoinsDB(
     size_t cache_size_bytes,
     bool in_memory,
@@ -3434,7 +3441,7 @@ void Chainstate::TryAddBlockIndexCandidate(CBlockIndex* pindex)
         // For the background chainstate, we only consider connecting blocks
         // towards the snapshot base (which can't be nullptr or else we'll
         // never make progress).
-        const CBlockIndex* snapshot_base = Assert(m_chainman.GetSnapshotBaseBlock());
+        const CBlockIndex* snapshot_base{Assert(m_chainman.GetSnapshotBaseBlock())};
         if (snapshot_base->GetAncestor(pindex->nHeight) == pindex) {
             setBlockIndexCandidates.insert(pindex);
         }
@@ -5704,9 +5711,7 @@ util::Result<void> Chainstate::InvalidateCoinsDBOnDisk()
 
 const CBlockIndex* ChainstateManager::GetSnapshotBaseBlock() const
 {
-    const auto blockhash_op = this->SnapshotBlockhash();
+    return m_active_chainstate ? m_active_chainstate->SnapshotBase() : nullptr;
-    if (!blockhash_op) return nullptr;
-    return Assert(m_blockman.LookupBlockIndex(*blockhash_op));
 }
 
 std::optional<int> ChainstateManager::GetSnapshotBaseHeight() const

src/validation.h

@@ -500,6 +500,9 @@ protected:
     //! is set to true on the snapshot chainstate.
     bool m_disabled GUARDED_BY(::cs_main) {false};
 
+    //! Cached result of LookupBlockIndex(*m_from_snapshot_blockhash)
+    const CBlockIndex* m_cached_snapshot_base GUARDED_BY(::cs_main) {nullptr};
+
 public:
     //! Reference to a BlockManager instance which itself is shared across all
     //! Chainstate instances.
@@ -551,6 +554,13 @@ public:
      */
     const std::optional<uint256> m_from_snapshot_blockhash;
 
+    /**
+     * The base of the snapshot this chainstate was created from.
+     *
+     * nullptr if this chainstate was not created from a snapshot.
+     */
+    const CBlockIndex* SnapshotBase() EXCLUSIVE_LOCKS_REQUIRED(::cs_main);
+
     //! Return true if this chainstate relies on blocks that are assumed-valid. In
     //! practice this means it was created based on a UTXO snapshot.
     bool reliesOnAssumedValid() { return m_from_snapshot_blockhash.has_value(); }