Fix for https://github.com/mongodb-partners/mongo-rocks/issues/108

src/rocks_record_store.cpp

@@ -1068,11 +1068,10 @@ namespace mongo {
 
         if (!startIterator.isNull()) {
             // This is a hack to speed up first/last record retrieval from the oplog
+            _oplogHackRestoreBeforeNext = true;
             _needFirstSeek = false;
             _lastLoc = startIterator;
-            iterator();
-            _skipNextAdvance = true;
-            _eof = false;
+            restore();
         }
     }
 
@@ -1130,6 +1129,7 @@ namespace mongo {
     }
 
     boost::optional<Record> RocksRecordStore::Cursor::next() {
+        _oplogHackRestoreBeforeNext = false;
         if (_eof) {
             return {};
         }
@@ -1159,6 +1159,7 @@ namespace mongo {
     }
 
     boost::optional<Record> RocksRecordStore::Cursor::seekExact(const RecordId& id) {
+        _oplogHackRestoreBeforeNext = false;
         _needFirstSeek = false;
         _skipNextAdvance = false;
         _iterator.reset();
@@ -1191,12 +1192,18 @@ namespace mongo {
             _currentSequenceNumber = ru->snapshot()->GetSequenceNumber();
         }
 
-        _skipNextAdvance = false;
-
-        if (_eof) return true;
-        if (_needFirstSeek) return true;
+        if (_eof || _needFirstSeek) {
+            _skipNextAdvance = false;
+            return true;
+        }
 
+        // _skipNextAdvance is reset by positionIterator()
+        // but in oplog first record retrieval hack case another logic is in use
         positionIterator();
+        if (_oplogHackRestoreBeforeNext) {
+            _skipNextAdvance = true;
+            _eof = false;
+        }
         // Return false if the collection is capped and we reached an EOF. Otherwise return true.
         return _cappedVisibilityManager && _eof ? false : true;
     }

src/rocks_record_store.h

@@ -254,6 +254,9 @@ namespace mongo {
             RecordId _lastLoc;
             std::unique_ptr<rocksdb::Iterator> _iterator;
             std::string _seekExactResult;
+            // true between oplog first record retrieval hack start and first
+            // positioning call (next(), seekExact())
+            bool _oplogHackRestoreBeforeNext = false;
             void positionIterator();
             rocksdb::Iterator* iterator();
         };