validation: populate nChainTx value for assumedvalid chainstates

Use the expected AssumeutxoData in order to bootstrap nChainTx values
for assumedvalid blockindex entries in the snapshot chainstate. This
is necessary because nChainTx is normally built up from nTx values,
which are populated using blockdata which the snapshot chainstate
does not yet have.

src/node/blockstorage.cpp

@@ -378,13 +378,26 @@ CBlockIndex* BlockManager::InsertBlockIndex(const uint256& hash)
     return pindex;
 }
 
-bool BlockManager::LoadBlockIndex()
+bool BlockManager::LoadBlockIndex(const std::optional<uint256>& snapshot_blockhash)
 {
     if (!m_block_tree_db->LoadBlockIndexGuts(
             GetConsensus(), [this](const uint256& hash) EXCLUSIVE_LOCKS_REQUIRED(cs_main) { return this->InsertBlockIndex(hash); }, m_interrupt)) {
         return false;
     }
 
+    int snapshot_height = -1;
+    if (snapshot_blockhash) {
+        const AssumeutxoData au_data = *Assert(GetParams().AssumeutxoForBlockhash(*snapshot_blockhash));
+        snapshot_height = au_data.height;
+        CBlockIndex* base{LookupBlockIndex(*snapshot_blockhash)};
+
+        // Since nChainTx (responsible for estiamted progress) isn't persisted
+        // to disk, we must bootstrap the value for assumedvalid chainstates
+        // from the hardcoded assumeutxo chainparams.
+        base->nChainTx = au_data.nChainTx;
+        LogPrintf("[snapshot] set nChainTx=%d for %s\n", au_data.nChainTx, snapshot_blockhash->ToString());
+    }
+
     // Calculate nChainWork
     std::vector<CBlockIndex*> vSortedByHeight{GetAllBlockIndices()};
     std::sort(vSortedByHeight.begin(), vSortedByHeight.end(),
@@ -401,7 +414,11 @@ bool BlockManager::LoadBlockIndex()
         // Pruned nodes may have deleted the block.
         if (pindex->nTx > 0) {
             if (pindex->pprev) {
-                if (pindex->pprev->nChainTx > 0) {
+                if (snapshot_blockhash && pindex->nHeight == snapshot_height &&
+                        pindex->GetBlockHash() == *snapshot_blockhash) {
+                    // Should have been set above; don't disturb it with code below.
+                    Assert(pindex->nChainTx > 0);
+                } else if (pindex->pprev->nChainTx > 0) {
                     pindex->nChainTx = pindex->pprev->nChainTx + pindex->nTx;
                 } else {
                     pindex->nChainTx = 0;
@@ -444,9 +461,9 @@ bool BlockManager::WriteBlockIndexDB()
     return true;
 }
 
-bool BlockManager::LoadBlockIndexDB()
+bool BlockManager::LoadBlockIndexDB(const std::optional<uint256>& snapshot_blockhash)
 {
-    if (!LoadBlockIndex()) {
+    if (!LoadBlockIndex(snapshot_blockhash)) {
         return false;
     }
 

src/node/blockstorage.h

@@ -118,7 +118,7 @@ private:
      * per index entry (nStatus, nChainWork, nTimeMax, etc.) as well as peripheral
      * collections like m_dirty_blockindex.
      */
-    bool LoadBlockIndex()
+    bool LoadBlockIndex(const std::optional<uint256>& snapshot_blockhash)
         EXCLUSIVE_LOCKS_REQUIRED(cs_main);
 
     /** Return false if block file or undo file flushing fails. */
@@ -231,7 +231,8 @@ public:
     std::unique_ptr<BlockTreeDB> m_block_tree_db GUARDED_BY(::cs_main);
 
     bool WriteBlockIndexDB() EXCLUSIVE_LOCKS_REQUIRED(::cs_main);
-    bool LoadBlockIndexDB() EXCLUSIVE_LOCKS_REQUIRED(::cs_main);
+    bool LoadBlockIndexDB(const std::optional<uint256>& snapshot_blockhash)
+        EXCLUSIVE_LOCKS_REQUIRED(::cs_main);
 
     /**
      * Remove any pruned block & undo files that are still on disk.

src/validation.cpp

@@ -4542,7 +4542,7 @@ bool ChainstateManager::LoadBlockIndex()
     // Load block index from databases
     bool needs_init = fReindex;
     if (!fReindex) {
-        bool ret{m_blockman.LoadBlockIndexDB()};
+        bool ret{m_blockman.LoadBlockIndexDB(SnapshotBlockhash())};
         if (!ret) return false;
 
         m_blockman.ScanAndUnlinkAlreadyPrunedFiles();
@@ -4838,6 +4838,10 @@ void ChainstateManager::CheckBlockIndex()
     CBlockIndex* pindexFirstAssumeValid = nullptr; // Oldest ancestor of pindex which has BLOCK_ASSUMED_VALID
     while (pindex != nullptr) {
         nNodes++;
+        if (pindex->pprev && pindex->nTx > 0) {
+            // nChainTx should increase monotonically
+            assert(pindex->pprev->nChainTx <= pindex->nChainTx);
+        }
         if (pindexFirstAssumeValid == nullptr && pindex->nStatus & BLOCK_ASSUMED_VALID) pindexFirstAssumeValid = pindex;
         if (pindexFirstInvalid == nullptr && pindex->nStatus & BLOCK_FAILED_VALID) pindexFirstInvalid = pindex;
         if (pindexFirstMissing == nullptr && !(pindex->nStatus & BLOCK_HAVE_DATA)) {