refactor: Add ChainstateManager::ValidatedChainstate() method

ValidatedChainstate() accessor replaces GetChainstateForIndexing() with no
change in behavior.

src/index/base.cpp

@@ -109,7 +109,7 @@ bool BaseIndex::Init()
     // m_chainstate member gives indexing code access to node internals. It is
     // removed in followup https://github.com/bitcoin/bitcoin/pull/24230
     m_chainstate = WITH_LOCK(::cs_main,
-        return &m_chain->context()->chainman->GetChainstateForIndexing());
+                             return &m_chain->context()->chainman->ValidatedChainstate());
     // Register to validation interface before setting the 'm_synced' flag, so that
     // callbacks are not missed once m_synced is true.
     m_chain->context()->validation_signals->RegisterValidationInterface(this);

src/init.cpp

@@ -2229,7 +2229,7 @@ bool StartIndexBackgroundSync(NodeContext& node)
     std::optional<const CBlockIndex*> indexes_start_block;
     std::string older_index_name;
     ChainstateManager& chainman = *Assert(node.chainman);
-    const Chainstate& chainstate = WITH_LOCK(::cs_main, return chainman.GetChainstateForIndexing());
+    const Chainstate& chainstate = WITH_LOCK(::cs_main, return chainman.ValidatedChainstate());
     const CChain& index_chain = chainstate.m_chain;
 
     for (auto index : node.indexes) {

src/validation.cpp

@@ -6437,14 +6437,6 @@ bool ChainstateManager::ValidatedSnapshotCleanup(Chainstate& validated_cs, Chain
     return true;
 }
 
-Chainstate& ChainstateManager::GetChainstateForIndexing()
-{
-    // We can't always return `m_ibd_chainstate` because after background validation
-    // has completed, `m_snapshot_chainstate == m_active_chainstate`, but it can be
-    // indexed.
-    return (this->GetAll().size() > 1) ? *m_ibd_chainstate : *m_active_chainstate;
-}
-
 std::pair<int, int> ChainstateManager::GetPruneRange(const Chainstate& chainstate, int last_height_can_prune)
 {
     if (chainstate.m_chain.Height() <= 0) {

src/validation.h

@@ -1153,6 +1153,17 @@ public:
         return cs && cs->m_target_blockhash && !cs->m_target_utxohash ? cs : nullptr;
     }
 
+    //! Return fully validated chainstate that should be used for indexing, to
+    //! support indexes that need to index blocks in order and can't start from
+    //! the snapshot block.
+    Chainstate& ValidatedChainstate() const EXCLUSIVE_LOCKS_REQUIRED(GetMutex())
+    {
+        for (auto* cs : {&CurrentChainstate(), HistoricalChainstate()}) {
+            if (cs && cs->m_assumeutxo == Assumeutxo::VALIDATED) return *cs;
+        }
+        abort();
+    }
+
     //! Alternatives to CurrentChainstate() used by older code to query latest
     //! chainstate information without locking cs_main. Newer code should avoid
     //! querying ChainstateManager and use Chainstate objects directly, or
@@ -1344,16 +1355,6 @@ public:
     //! @sa node/chainstate:LoadChainstate()
     bool ValidatedSnapshotCleanup(Chainstate& validated_cs, Chainstate& unvalidated_cs) EXCLUSIVE_LOCKS_REQUIRED(::cs_main);
 
-    //! @returns the chainstate that indexes should consult when ensuring that an
-    //!   index is synced with a chain where we can expect block index entries to have
-    //!   BLOCK_HAVE_DATA beneath the tip.
-    //!
-    //!   In other words, give us the chainstate for which we can reasonably expect
-    //!   that all blocks beneath the tip have been indexed. In practice this means
-    //!   when using an assumed-valid chainstate based upon a snapshot, return only the
-    //!   fully validated chain.
-    Chainstate& GetChainstateForIndexing() EXCLUSIVE_LOCKS_REQUIRED(::cs_main);
-
     //! Return the [start, end] (inclusive) of block heights we can prune.
     //!
     //! start > end is possible, meaning no blocks can be pruned.