Don't crop chain if height < offset

- Also make sure to re-attach earliest block after offset
if detached height == offset

src/wallet/wallet2.cpp

@@ -1734,6 +1734,19 @@ void wallet2::process_scan_txs(const tx_entry_data &sorted_txs_to_scan, const tx
       << sorted_txs_to_reprocess.tx_entries.size() << " txs");
 }
 //----------------------------------------------------------------------------------------------------
+void reattach_blockchain(hashchain &blockchain, wallet2::detached_blockchain_data &dbd)
+{
+  if (!dbd.detached_blockchain.empty())
+  {
+    LOG_PRINT_L0("Re-attaching " << dbd.detached_blockchain.size() << " blocks");
+    for (size_t i = 0; i < dbd.detached_blockchain.size(); ++i)
+      blockchain.push_back(dbd.detached_blockchain[i]);
+  }
+
+  THROW_WALLET_EXCEPTION_IF(blockchain.size() != dbd.original_chain_size,
+    error::wallet_internal_error, "Unexpected blockchain size after re-attaching");
+}
+//----------------------------------------------------------------------------------------------------
 void wallet2::scan_tx(const std::unordered_set<crypto::hash> &txids)
 {
   // Get the transactions from daemon in batches sorted lowest height to highest
@@ -1752,6 +1765,7 @@ void wallet2::scan_tx(const std::unordered_set<crypto::hash> &txids)
   // detect tx2. The wallet would need to scan tx1 first in that case.
   // TODO: handle this sweep case
   detached_blockchain_data dbd;
+  dbd.original_chain_size = m_blockchain.size();
   if (m_blockchain.size() > txs_to_scan.lowest_height)
   {
     LOG_PRINT_L0("Re-processing wallet's existing txs (if any) starting from height " << txs_to_scan.lowest_height);
@@ -1767,11 +1781,7 @@ void wallet2::scan_tx(const std::unordered_set<crypto::hash> &txids)
   tx_entry_data txs_to_reprocess = get_sorted_tx_entries(tx_hashes_to_reprocess);
 
   process_scan_txs(txs_to_scan, txs_to_reprocess, tx_hashes_to_reprocess, dbd);
-
-  if (!dbd.detached_blockchain.empty())
-    LOG_PRINT_L0("Re-attaching " << dbd.detached_blockchain.size() << " blocks");
-  for (size_t i = 0; i < dbd.detached_blockchain.size(); ++i)
-    m_blockchain.push_back(std::move(dbd.detached_blockchain[i]));
+  reattach_blockchain(m_blockchain, dbd);
 
   // If the highest scan_tx height > the wallet's known scan height, then the
   // wallet scanner should continue scanning from that tx's height and "pretend"
@@ -3910,13 +3920,16 @@ wallet2::detached_blockchain_data wallet2::detach_blockchain(uint64_t height, st
   MDEBUG(transfers_detached << " transfers detached / expected " << dbd.detached_tx_hashes.size());
   m_transfers.erase(it, m_transfers.end());
 
-  size_t blocks_detached = m_blockchain.size() - height;
-  for (size_t i = height; i < m_blockchain.size(); ++i)
-    if (i > m_blockchain.offset())
-      dbd.detached_blockchain.push_back(std::move(m_blockchain[i]));
-  size_t expected_blocks_detached = dbd.detached_blockchain.size() + (height < m_blockchain.offset() ? m_blockchain.offset() - height : 0);
-  MDEBUG(blocks_detached << " blocks detached / expected " << expected_blocks_detached);
-  m_blockchain.crop(height);
+  size_t blocks_detached = 0;
+  dbd.original_chain_size = m_blockchain.size();
+  if (height >= m_blockchain.offset())
+  {
+    for (size_t i = height; i < m_blockchain.size(); ++i)
+      dbd.detached_blockchain.push_back(m_blockchain[i]);
+    blocks_detached = m_blockchain.size() - height;
+    m_blockchain.crop(height);
+    MDEBUG(blocks_detached << " blocks detached / expected " << dbd.detached_blockchain.size());
+  }
 
   for (auto it = m_payments.begin(); it != m_payments.end(); )
   {

src/wallet/wallet2.h

@@ -819,6 +819,7 @@ namespace tools
     struct detached_blockchain_data
     {
       hashchain detached_blockchain;
+      size_t original_chain_size;
       std::unordered_set<crypto::hash> detached_tx_hashes;
       std::unordered_map<crypto::hash, std::vector<cryptonote::tx_destination_entry>> detached_confirmed_txs_dests;
     };