init: add utxo snapshot detection

Add functionality for activating a snapshot-based chainstate if one is
detected on-disk.

Also cautiously initialize chainstate cache usages so that we don't
somehow blow past our cache allowances during initialization, then
rebalance at the end of init.

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

src/validation.cpp

@@ -4744,28 +4744,15 @@ std::vector<Chainstate*> ChainstateManager::GetAll()
     return out;
 }
 
-Chainstate& ChainstateManager::InitializeChainstate(
-    CTxMemPool* mempool, const std::optional<uint256>& snapshot_blockhash)
+Chainstate& ChainstateManager::InitializeChainstate(CTxMemPool* mempool)
 {
     AssertLockHeld(::cs_main);
-    bool is_snapshot = snapshot_blockhash.has_value();
-    std::unique_ptr<Chainstate>& to_modify =
-        is_snapshot ? m_snapshot_chainstate : m_ibd_chainstate;
+    assert(!m_ibd_chainstate);
+    assert(!m_active_chainstate);
 
-    if (to_modify) {
-        throw std::logic_error("should not be overwriting a chainstate");
-    }
-    to_modify.reset(new Chainstate(mempool, m_blockman, *this, 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;
+    m_ibd_chainstate = std::make_unique<Chainstate>(mempool, m_blockman, *this);
+    m_active_chainstate = m_ibd_chainstate.get();
+    return *m_active_chainstate;
 }
 
 const AssumeutxoData* ExpectedAssumeutxo(
@@ -5134,3 +5121,31 @@ ChainstateManager::~ChainstateManager()
         i.clear();
     }
 }
+
+bool ChainstateManager::DetectSnapshotChainstate(CTxMemPool* mempool)
+{
+    assert(!m_snapshot_chainstate);
+    std::optional<fs::path> path = node::FindSnapshotChainstateDir();
+    if (!path) {
+        return false;
+    }
+    std::optional<uint256> base_blockhash = node::ReadSnapshotBaseBlockhash(*path);
+    if (!base_blockhash) {
+        return false;
+    }
+    LogPrintf("[snapshot] detected active snapshot chainstate (%s) - loading\n",
+        fs::PathToString(*path));
+
+    this->ActivateExistingSnapshot(mempool, *base_blockhash);
+    return true;
+}
+
+Chainstate& ChainstateManager::ActivateExistingSnapshot(CTxMemPool* mempool, uint256 base_blockhash)
+{
+    assert(!m_snapshot_chainstate);
+    m_snapshot_chainstate =
+        std::make_unique<Chainstate>(mempool, m_blockman, *this, base_blockhash);
+    LogPrintf("[snapshot] switching active chainstate to %s\n", m_snapshot_chainstate->ToString());
+    m_active_chainstate = m_snapshot_chainstate.get();
+    return *m_snapshot_chainstate;
+}