refactor: Deduplicate Chainstate activation code

diff --git a/src/test/validation_chainstatemanager_tests.cpp b/src/test/validation_chainstatemanager_tests.cpp
index c6ae6088..03cec59c 100644
--- a/src/test/validation_chainstatemanager_tests.cpp
+++ b/src/test/validation_chainstatemanager_tests.cpp
@@ -69,7 +69,7 @@ BOOST_FIXTURE_TEST_CASE(chainstatemanager, TestChain100Setup)
     // Create a snapshot-based chainstate.
     //
     const uint256 snapshot_blockhash = active_tip->GetBlockHash();
-    Chainstate& c2 = WITH_LOCK(::cs_main, return manager.ActivateExistingSnapshot(snapshot_blockhash));
+    Chainstate& c2{WITH_LOCK(::cs_main, return manager.AddChainstate(std::make_unique<Chainstate>(nullptr, manager.m_blockman, manager, snapshot_blockhash)))};
     chainstates.push_back(&c2);
     c2.InitCoinsDB(
         /*cache_size_bytes=*/1 << 23, /*in_memory=*/true, /*should_wipe=*/false);
@@ -135,7 +135,7 @@ BOOST_FIXTURE_TEST_CASE(chainstatemanager_rebalance_caches, TestChain100Setup)
     // Create a snapshot-based chainstate.
     //
     CBlockIndex* snapshot_base{WITH_LOCK(manager.GetMutex(), return manager.ActiveChain()[manager.ActiveChain().Height() / 2])};
-    Chainstate& c2 = WITH_LOCK(cs_main, return manager.ActivateExistingSnapshot(*snapshot_base->phashBlock));
+    Chainstate& c2{WITH_LOCK(::cs_main, return manager.AddChainstate(std::make_unique<Chainstate>(nullptr, manager.m_blockman, manager, *snapshot_base->phashBlock)))};
     chainstates.push_back(&c2);
     c2.InitCoinsDB(
         /*cache_size_bytes=*/1 << 23, /*in_memory=*/true, /*should_wipe=*/false);
@@ -489,8 +489,7 @@ BOOST_FIXTURE_TEST_CASE(chainstatemanager_loadblockindex, TestChain100Setup)
     }
 
     // Note: cs2's tip is not set when ActivateExistingSnapshot is called.
-    Chainstate& cs2 = WITH_LOCK(::cs_main,
-        return chainman.ActivateExistingSnapshot(*assumed_base->phashBlock));
+    Chainstate& cs2{WITH_LOCK(::cs_main, return chainman.AddChainstate(std::make_unique<Chainstate>(nullptr, chainman.m_blockman, chainman, *assumed_base->phashBlock)))};
 
     // Set tip of the fully validated chain to be the validated tip
     cs1.m_chain.SetTip(*validated_tip);
diff --git a/src/validation.cpp b/src/validation.cpp
index 4c6432de..9e27da58 100644
--- a/src/validation.cpp
+++ b/src/validation.cpp
@@ -5796,28 +5796,14 @@ util::Result<CBlockIndex*> ChainstateManager::ActivateSnapshot(
         }
     }
 
-    assert(!m_snapshot_chainstate);
-    m_snapshot_chainstate.swap(snapshot_chainstate);
-    const bool chaintip_loaded = m_snapshot_chainstate->LoadChainTip();
+    Chainstate& chainstate{AddChainstate(std::move(snapshot_chainstate))};
+    const bool chaintip_loaded{chainstate.LoadChainTip()};
     assert(chaintip_loaded);
-
-    // Set snapshot block as the target block for the historical chainstate.
-    assert(m_ibd_chainstate.get());
-    assert(!m_ibd_chainstate->m_target_blockhash);
-    m_ibd_chainstate->SetTargetBlockHash(base_blockhash);
-
-    // Transfer possession of the mempool to the snapshot chainstate.
-    // Mempool is empty at this point because we're still in IBD.
-    Assert(m_active_chainstate->m_mempool->size() == 0);
-    Assert(!m_snapshot_chainstate->m_mempool);
-    m_snapshot_chainstate->m_mempool = m_active_chainstate->m_mempool;
-    m_active_chainstate->m_mempool = nullptr;
-    m_active_chainstate = m_snapshot_chainstate.get();
-    m_blockman.m_snapshot_height = this->GetSnapshotBaseHeight();
+    m_blockman.m_snapshot_height = Assert(chainstate.SnapshotBase())->nHeight;
 
     LogInfo("[snapshot] successfully activated snapshot %s", base_blockhash.ToString());
     LogInfo("[snapshot] (%.2f MB)",
-        m_snapshot_chainstate->CoinsTip().DynamicMemoryUsage() / (1000 * 1000));
+              chainstate.CoinsTip().DynamicMemoryUsage() / (1000 * 1000));
 
     this->MaybeRebalanceCaches();
     return snapshot_start_block;
@@ -6288,21 +6274,21 @@ bool ChainstateManager::DetectSnapshotChainstate()
     LogInfo("[snapshot] detected active snapshot chainstate (%s) - loading",
         fs::PathToString(*path));
 
-    this->ActivateExistingSnapshot(*base_blockhash);
+    auto snapshot_chainstate{std::make_unique<Chainstate>(nullptr, m_blockman, *this, base_blockhash)};
+    LogInfo("[snapshot] switching active chainstate to %s", snapshot_chainstate->ToString());
+    this->AddChainstate(std::move(snapshot_chainstate));
     return true;
 }
 
-Chainstate& ChainstateManager::ActivateExistingSnapshot(uint256 base_blockhash)
+Chainstate& ChainstateManager::AddChainstate(std::unique_ptr<Chainstate> chainstate)
 {
     assert(!m_snapshot_chainstate);
-    m_snapshot_chainstate =
-        std::make_unique<Chainstate>(nullptr, m_blockman, *this, base_blockhash);
-    LogInfo("[snapshot] switching active chainstate to %s", m_snapshot_chainstate->ToString());
+    m_snapshot_chainstate = std::move(chainstate);
 
     // Set target block for historical chainstate to snapshot block.
     assert(m_ibd_chainstate.get());
     assert(!m_ibd_chainstate->m_target_blockhash);
-    m_ibd_chainstate->SetTargetBlockHash(base_blockhash);
+    m_ibd_chainstate->SetTargetBlockHash(*Assert(m_snapshot_chainstate->m_from_snapshot_blockhash));
 
     // Transfer possession of the mempool to the chainstate.
     // Mempool is empty at this point because we're still in IBD.
diff --git a/src/validation.h b/src/validation.h
index d2d9e419..5e6d11d1 100644
--- a/src/validation.h
+++ b/src/validation.h
@@ -1315,16 +1315,15 @@ public:
     //! snapshot that is in the process of being validated.
     bool DetectSnapshotChainstate() EXCLUSIVE_LOCKS_REQUIRED(::cs_main);
 
+    //! Add new chainstate.
+    Chainstate& AddChainstate(std::unique_ptr<Chainstate> chainstate) EXCLUSIVE_LOCKS_REQUIRED(::cs_main);
+
     void ResetChainstates() EXCLUSIVE_LOCKS_REQUIRED(::cs_main);
 
     //! Remove the snapshot-based chainstate and all on-disk artifacts.
     //! Used when reindex{-chainstate} is called during snapshot use.
     [[nodiscard]] bool DeleteSnapshotChainstate() EXCLUSIVE_LOCKS_REQUIRED(::cs_main);
 
-    //! Switch the active chainstate to one based on a UTXO snapshot that was loaded
-    //! previously.
-    Chainstate& ActivateExistingSnapshot(uint256 base_blockhash) EXCLUSIVE_LOCKS_REQUIRED(::cs_main);
-
     //! If we have validated a snapshot chain during this runtime, copy its
     //! chainstate directory over to the main `chainstate` location, completing
     //! validation of the snapshot.