Add more asserts to track down the bug

src/wallet/wallet.cpp

@@ -675,6 +675,8 @@ void CWallet::IncrementNoteWitnesses(const CBlockIndex* pindex,
                     for (std::pair<const uint256, CWalletTx>& wtxItem : mapWallet) {
                         for (mapNoteData_t::value_type& item : wtxItem.second.mapNoteData) {
                             CNoteData* nd = &(item.second);
+                            // Check the validity of the cache
+                            assert(nWitnessCacheSize >= nd->witnesses.size());
                             if (nd->witnesses.size() > 0) {
                                 nd->witnesses.front().append(note_commitment);
                             }
@@ -695,6 +697,13 @@ void CWallet::IncrementNoteWitnesses(const CBlockIndex* pindex,
         if (nWitnessCacheSize < WITNESS_CACHE_SIZE) {
             nWitnessCacheSize += 1;
         }
+        for (std::pair<const uint256, CWalletTx>& wtxItem : mapWallet) {
+            for (mapNoteData_t::value_type& item : wtxItem.second.mapNoteData) {
+                CNoteData* nd = &(item.second);
+                // Check the validity of the cache
+                assert(nWitnessCacheSize >= nd->witnesses.size());
+            }
+        }
         if (fFileBacked) {
             CWalletDB walletdb(strWalletFile);
             WriteWitnessCache(walletdb);
@@ -709,12 +718,21 @@ void CWallet::DecrementNoteWitnesses()
         for (std::pair<const uint256, CWalletTx>& wtxItem : mapWallet) {
             for (mapNoteData_t::value_type& item : wtxItem.second.mapNoteData) {
                 CNoteData* nd = &(item.second);
+                // Check the validity of the cache
+                assert(nWitnessCacheSize >= nd->witnesses.size());
                 if (nd->witnesses.size() > 0) {
                     nd->witnesses.pop_front();
                 }
             }
         }
         nWitnessCacheSize -= 1;
+        for (std::pair<const uint256, CWalletTx>& wtxItem : mapWallet) {
+            for (mapNoteData_t::value_type& item : wtxItem.second.mapNoteData) {
+                CNoteData* nd = &(item.second);
+                // Check the validity of the cache
+                assert(nWitnessCacheSize >= nd->witnesses.size());
+            }
+        }
         // TODO: If nWitnessCache is zero, we need to regenerate the caches (#1302)
         assert(nWitnessCacheSize > 0);
         if (fFileBacked) {