fix(cursor): wait until all eachAsync() functions finish before resolving the promise

Fix #8352

Author: vkarpov15

lib/helpers/cursor/eachAsync.js

@@ -35,8 +35,9 @@ module.exports = function eachAsync(next, fn, options, callback) {
     }
   };
 
-  const iterate = function(callback) {
+  const iterate = function(finalCallback) {
     let drained = false;
+    let handleResultsInProgress = 0;
 
     let error = null;
     for (let i = 0; i < parallel; ++i) {
@@ -49,26 +50,33 @@ module.exports = function eachAsync(next, fn, options, callback) {
       }
 
       next(function(err, doc) {
-        if (drained || error) {
+        if (drained || error != null) {
           return done();
         }
         if (err != null) {
           error = err;
-          callback(err);
+          finalCallback(err);
           return done();
         }
         if (doc == null) {
           drained = true;
-          callback(null);
+          if (handleResultsInProgress <= 0) {
+            finalCallback(null);
+          }
           return done();
         }
 
         done();
 
+        ++handleResultsInProgress;
         handleNextResult(doc, function(err) {
+          --handleResultsInProgress;
           if (err != null) {
             error = err;
-            return callback(err);
+            return finalCallback(err);
+          }
+          if (drained && handleResultsInProgress <= 0) {
+            return finalCallback(null);
           }
 
           setTimeout(() => enqueue(fetch), 0);