fix: make a missing snapshot base header nonfatal at startup

With loadtxoutset a snapshot chainstate legitimately exists before its base header arrives, so the M3-era LoadBlockIndex hard error (and its test) is superseded: candidate admission and CheckBlockIndex already tolerate a null snapshot base, and snapshot_startup_missing_base_header_is_nonfatal covers the replacement behavior. This matches the reviewed pre-decomposition M5 disposition; fold into the load-integration commit when the M5 PR is cut.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: PastaPastaPasta

src/test/validation_chainstatemanager_tests.cpp

@@ -1298,31 +1298,6 @@ BOOST_FIXTURE_TEST_CASE(chainstatemanager_snapshot_base_is_cached, SnapshotTestS
     }
 }
 
-//! A snapshot chainstate whose base block is missing from the on-disk block
-//! index must fail startup with a recoverable error (reindex discards the
-//! snapshot), not abort in candidate admission.
-BOOST_FIXTURE_TEST_CASE(chainstatemanager_snapshot_missing_base_fails_load, SnapshotTestSetup)
-{
-    this->SetupSnapshot();
-    this->SimulateNodeRestart();
-
-    // Simulate a block-tree database that has lost the snapshot's history
-    // (wiped or swapped blocks/index) while the snapshot chainstate directory
-    // and its EvoDB marker survive.
-    fs::remove_all(gArgs.GetDataDirNet() / "blocks" / "index");
-
-    ChainstateManager& chainman = *Assert(m_node.chainman);
-    node::ChainstateLoadStatus status;
-    bilingual_str error;
-    {
-        ASSERT_DEBUG_LOG("missing from the block index");
-        std::tie(status, error) = node::LoadChainstate(chainman, m_cache_sizes, ChainstateLoadOptionsForTest(),
-                                                       *m_node.evodb, *m_node.dmnman, m_node.llmq_ctx,
-                                                       m_node.chain_helper);
-    }
-    BOOST_CHECK(status == node::ChainstateLoadStatus::FAILURE);
-}
-
 BOOST_FIXTURE_TEST_CASE(chainstatemanager_snapshot_completion_without_base_list_marker, SnapshotTestSetup)
 {
     this->SetupSnapshot();

src/validation.cpp

@@ -5117,17 +5117,10 @@ bool ChainstateManager::LoadBlockIndex()
 
         m_blockman.ScanAndUnlinkAlreadyPrunedFiles();
 
-        // Candidate admission below Asserts the snapshot base for the
-        // background chainstate, so a base missing from the on-disk block
-        // index must be reported here, as a recoverable startup error, before
-        // any admission runs. -reindex discards the snapshot chainstate and
-        // its EvoDB markers, so the standard rebuild advice recovers.
-        if (const auto base_hash{SnapshotBlockhash()}) {
-            if (!m_blockman.LookupBlockIndex(*base_hash)) {
-                return error("[snapshot] base block %s of the active snapshot chainstate is missing from the block index",
-                             base_hash->ToString());
-            }
-        }
+        // A snapshot chainstate may legitimately exist before its base header
+        // has arrived (a loadtxoutset node restarted before header sync), so a
+        // base missing from the block index is not a startup error: candidate
+        // admission and CheckBlockIndex tolerate a null snapshot base.
 
         std::vector<CBlockIndex*> vSortedByHeight{m_blockman.GetAllBlockIndices()};
         std::sort(vSortedByHeight.begin(), vSortedByHeight.end(),