validation: introduce unused ChainstateManager

ChainstateManager is responsible for creating and managing multiple
chainstates, and will provide a high-level interface for accessing the
appropriate chainstate based upon a certain use.

Incorporates feedback from Marco Falke. Additional documentation written
by Russ Yanofsky.

Co-authored-by: Russell Yanofsky <russ@yanofsky.org>

src/validation.cpp

@@ -20,6 +20,7 @@
 #include <index/txindex.h>
 #include <logging.h>
 #include <logging/timer.h>
+#include <optional.h>
 #include <policy/fees.h>
 #include <policy/policy.h>
 #include <policy/settings.h>
@@ -4950,6 +4951,14 @@ void CChainState::CheckBlockIndex(const Consensus::Params& consensusParams)
     assert(nNodes == forward.size());
 }
 
+std::string CChainState::ToString()
+{
+    CBlockIndex* tip = m_chain.Tip();
+    return strprintf("Chainstate [%s] @ height %d (%s)",
+        m_from_snapshot_blockhash.IsNull() ? "ibd" : "snapshot",
+        tip ? tip->nHeight : -1, tip ? tip->GetBlockHash().ToString() : "null");
+}
+
 std::string CBlockFileInfo::ToString() const
 {
     return strprintf("CBlockFileInfo(blocks=%u, size=%u, heights=%u...%u, time=%s...%s)", nBlocks, nSize, nHeightFirst, nHeightLast, FormatISO8601Date(nTimeFirst), FormatISO8601Date(nTimeLast));
@@ -5144,3 +5153,90 @@ public:
     }
 };
 static CMainCleanup instance_of_cmaincleanup;
+
+Optional<uint256> ChainstateManager::SnapshotBlockhash() const {
+    if (m_active_chainstate != nullptr) {
+        // If a snapshot chainstate exists, it will always be our active.
+        return m_active_chainstate->m_from_snapshot_blockhash;
+    }
+    return {};
+}
+
+std::vector<CChainState*> ChainstateManager::GetAll()
+{
+    std::vector<CChainState*> out;
+
+    if (!IsSnapshotValidated() && m_ibd_chainstate) {
+        out.push_back(m_ibd_chainstate.get());
+    }
+
+    if (m_snapshot_chainstate) {
+        out.push_back(m_snapshot_chainstate.get());
+    }
+
+    return out;
+}
+
+CChainState& ChainstateManager::InitializeChainstate(const uint256& snapshot_blockhash)
+{
+    bool is_snapshot = !snapshot_blockhash.IsNull();
+    std::unique_ptr<CChainState>& to_modify =
+        is_snapshot ? m_snapshot_chainstate : m_ibd_chainstate;
+
+    if (to_modify) {
+        throw std::logic_error("should not be overwriting a chainstate");
+    }
+
+    to_modify.reset(new CChainState(snapshot_blockhash));
+
+    // Snapshot chainstates and initial IBD chaintates always become active.
+    if (is_snapshot || (!is_snapshot && !m_active_chainstate)) {
+        LogPrintf("Switching active chainstate to %s\n", to_modify->ToString());
+        m_active_chainstate = to_modify.get();
+    } else {
+        throw std::logic_error("unexpected chainstate activation");
+    }
+
+    return *to_modify;
+}
+
+CChain& ChainstateManager::ActiveChain() const
+{
+    assert(m_active_chainstate);
+    return m_active_chainstate->m_chain;
+}
+
+bool ChainstateManager::IsSnapshotActive() const
+{
+    return m_snapshot_chainstate && m_active_chainstate == m_snapshot_chainstate.get();
+}
+
+CChainState& ChainstateManager::ValidatedChainstate() const
+{
+    if (m_snapshot_chainstate && IsSnapshotValidated()) {
+        return *m_snapshot_chainstate.get();
+    }
+    assert(m_ibd_chainstate);
+    return *m_ibd_chainstate.get();
+}
+
+bool ChainstateManager::IsBackgroundIBD(CChainState* chainstate) const
+{
+    return (m_snapshot_chainstate && chainstate == m_ibd_chainstate.get());
+}
+
+void ChainstateManager::Unload()
+{
+    for (CChainState* chainstate : this->GetAll()) {
+        chainstate->m_chain.SetTip(nullptr);
+        chainstate->UnloadBlockIndex();
+    }
+}
+
+void ChainstateManager::Reset()
+{
+    m_ibd_chainstate.reset();
+    m_snapshot_chainstate.reset();
+    m_active_chainstate = nullptr;
+    m_snapshot_validated = false;
+}