Support signal option on iterators

Depends on Level/abstract-level#59.

binding.cc

@@ -937,6 +937,7 @@ struct Iterator final : public BaseIterator {
       first_(true),
       nexting_(false),
       isClosing_(false),
+      aborted_(false),
       ended_(false),
       ref_(NULL) {
   }
@@ -959,7 +960,7 @@ struct Iterator final : public BaseIterator {
     size_t bytesRead = 0;
     leveldb::Slice empty;
 
-    while (true) {
+    while (!aborted_) {
       if (!first_) Next();
       else first_ = false;
 
@@ -998,6 +999,7 @@ struct Iterator final : public BaseIterator {
   bool first_;
   bool nexting_;
   bool isClosing_;
+  bool aborted_;
   bool ended_;
   std::vector<Entry> cache_;
 
@@ -1805,6 +1807,16 @@ NAPI_METHOD(iterator_close) {
   return promise;
 }
 
+/**
+ * Aborts a NextWorker (if any, eventually).
+ */
+NAPI_METHOD(iterator_abort) {
+  NAPI_ARGV(1);
+  NAPI_ITERATOR_CONTEXT();
+  iterator->aborted_ = true;
+  NAPI_RETURN_UNDEFINED();
+}
+
 /**
  * Worker class for nexting an iterator.
  */
@@ -1828,6 +1840,15 @@ struct NextWorker final : public BaseWorker {
   }
 
   void HandleOKCallback (napi_env env, napi_deferred deferred) override {
+    if (iterator_->aborted_) {
+      napi_value err = CreateCodeError(env, "LEVEL_ABORTED", "Operation has been aborted");
+      napi_value name;
+      napi_create_string_utf8(env, "AbortError", NAPI_AUTO_LENGTH, &name);
+      napi_set_named_property(env, err, "name", name);
+      napi_reject_deferred(env, deferred, err);
+      return;
+    }
+
     size_t size = iterator_->cache_.size();
     napi_value jsArray;
     napi_create_array_with_length(env, size, &jsArray);
@@ -2154,6 +2175,7 @@ NAPI_INIT() {
   NAPI_EXPORT_FUNCTION(iterator_seek);
   NAPI_EXPORT_FUNCTION(iterator_close);
   NAPI_EXPORT_FUNCTION(iterator_nextv);
+  NAPI_EXPORT_FUNCTION(iterator_abort);
 
   NAPI_EXPORT_FUNCTION(batch_do);
   NAPI_EXPORT_FUNCTION(batch_init);

index.js

@@ -28,6 +28,9 @@ class ClassicLevel extends AbstractLevel {
       additionalMethods: {
         approximateSize: true,
         compactRange: true
+      },
+      signals: {
+        iterators: true
       }
     }, options)
 

iterator.js

@@ -7,6 +7,8 @@ const kContext = Symbol('context')
 const kCache = Symbol('cache')
 const kFirst = Symbol('first')
 const kPosition = Symbol('position')
+const kSignal = Symbol('signal')
+const kAbort = Symbol('abort')
 const empty = []
 
 // Does not implement _all() because the default implementation
@@ -22,6 +24,15 @@ class Iterator extends AbstractIterator {
     this[kFirst] = true
     this[kCache] = empty
     this[kPosition] = 0
+    this[kAbort] = this[kAbort].bind(this)
+
+    // TODO: consider exposing iterator.signal in abstract-level
+    if (options.signal != null) {
+      this[kSignal] = options.signal
+      this[kSignal].addEventListener('abort', this[kAbort], { once: true })
+    } else {
+      this[kSignal] = null
+    }
   }
 
   _seek (target, options) {
@@ -63,9 +74,20 @@ class Iterator extends AbstractIterator {
 
   async _close () {
     this[kCache] = empty
+
+    if (this[kSignal] !== null) {
+      this[kSignal].removeEventListener('abort', this[kAbort])
+      this[kSignal] = null
+    }
+
     return binding.iterator_close(this[kContext])
   }
 
+  [kAbort] () {
+    this[kSignal] = null
+    binding.iterator_abort(this[kContext])
+  }
+
   // Undocumented, exposed for tests only
   get cached () {
     return this[kCache].length - this[kPosition]