Avoid iterator_nextv() call if end was reached

binding.cc

@@ -98,6 +98,11 @@ static std::map<std::string, LevelDbHandle> db_handles;
     }                                                                   \
   }
 
+/**
+ * Bit fields.
+ */
+#define STATE_ENDED 1
+
 /*********************************************************************
  * Helpers.
  ********************************************************************/
@@ -926,7 +931,8 @@ struct Iterator final : public BaseIterator {
             const bool fillCache,
             const Encoding keyEncoding,
             const Encoding valueEncoding,
-            const uint32_t highWaterMarkBytes)
+            const uint32_t highWaterMarkBytes,
+            unsigned char* state)
     : BaseIterator(database, reverse, lt, lte, gt, gte, limit, fillCache),
       id_(id),
       keys_(keys),
@@ -939,6 +945,7 @@ struct Iterator final : public BaseIterator {
       isClosing_(false),
       aborted_(false),
       ended_(false),
+      state_(state),
       ref_(NULL) {
   }
 
@@ -1001,6 +1008,7 @@ struct Iterator final : public BaseIterator {
   bool isClosing_;
   bool aborted_;
   bool ended_;
+  unsigned char* state_;
   std::vector<Entry> cache_;
 
 private:
@@ -1707,10 +1715,15 @@ static void FinalizeIterator (napi_env env, void* data, void* hint) {
  * Create an iterator.
  */
 NAPI_METHOD(iterator_init) {
-  NAPI_ARGV(2);
+  NAPI_ARGV(3);
   NAPI_DB_CONTEXT();
 
-  napi_value options = argv[1];
+  unsigned char* state = 0;
+  size_t stateLength;
+  NAPI_STATUS_THROWS(napi_get_typedarray_info(env, argv[1], NULL, &stateLength, (void**)&state, NULL, NULL));
+  assert(stateLength == 1);
+
+  napi_value options = argv[2];
   const bool reverse = BooleanProperty(env, options, "reverse", false);
   const bool keys = BooleanProperty(env, options, "keys", true);
   const bool values = BooleanProperty(env, options, "values", true);
@@ -1728,7 +1741,8 @@ NAPI_METHOD(iterator_init) {
   const uint32_t id = database->currentIteratorId_++;
   Iterator* iterator = new Iterator(database, id, reverse, keys,
                                     values, limit, lt, lte, gt, gte, fillCache,
-                                    keyEncoding, valueEncoding, highWaterMarkBytes);
+                                    keyEncoding, valueEncoding, highWaterMarkBytes,
+                                    state);
   napi_value result;
 
   NAPI_STATUS_THROWS(napi_create_external(env, iterator,
@@ -1862,6 +1876,11 @@ struct NextWorker final : public BaseWorker {
       napi_set_element(env, jsArray, idx, element);
     }
 
+    // TODO: use state_ internally too, replacing ended_?
+    if (iterator_->ended_) {
+      *iterator_->state_ |= STATE_ENDED;
+    }
+
     napi_resolve_deferred(env, deferred, jsArray);
   }
 

iterator.js

@@ -7,10 +7,14 @@ const kContext = Symbol('context')
 const kCache = Symbol('cache')
 const kFirst = Symbol('first')
 const kPosition = Symbol('position')
+const kState = Symbol('state')
 const kSignal = Symbol('signal')
 const kAbort = Symbol('abort')
 const empty = []
 
+// Bit fields
+const STATE_ENDED = 1
+
 // Does not implement _all() because the default implementation
 // of abstract-level falls back to nextv(1000) and using all()
 // on more entries than that probably isn't a realistic use case,
@@ -20,7 +24,8 @@ class Iterator extends AbstractIterator {
   constructor (db, context, options) {
     super(db, options)
 
-    this[kContext] = binding.iterator_init(context, options)
+    this[kState] = new Uint8Array(1)
+    this[kContext] = binding.iterator_init(context, this[kState], options)
     this[kFirst] = true
     this[kCache] = empty
     this[kPosition] = 0
@@ -38,17 +43,22 @@ class Iterator extends AbstractIterator {
   _seek (target, options) {
     this[kFirst] = true
     this[kCache] = empty
+    this[kState][0] &= ~STATE_ENDED // Unset
     this[kPosition] = 0
 
     binding.iterator_seek(this[kContext], target)
   }
 
-  // TODO (v2): document/benchmark that we now always need a last call
   async _next () {
     if (this[kPosition] < this[kCache].length) {
       return this[kCache][this[kPosition]++]
     }
 
+    // Avoid iterator_nextv() call if end was already reached
+    if ((this[kState][0] & STATE_ENDED) !== 0) {
+      return undefined
+    }
+
     if (this[kFirst]) {
       // It's common to only want one entry initially or after a seek()
       this[kFirst] = false
@@ -69,6 +79,12 @@ class Iterator extends AbstractIterator {
   // TODO (v2): read from cache first?
   async _nextv (size, options) {
     this[kFirst] = false
+
+    // Avoid iterator_nextv() call if end was already reached
+    if ((this[kState][0] & STATE_ENDED) !== 0) {
+      return []
+    }
+
     return binding.iterator_nextv(this[kContext], size)
   }