From f06d1c75acd9e62c96829c105212854b507a93a9 Mon Sep 17 00:00:00 2001
From: Valeri Karpov <val@karpov.io>
Date: Wed, 4 Dec 2019 12:45:14 -0500
Subject: [PATCH] fix(cursor): wait until all `eachAsync()` functions finish
 before resolving the promise

Fix #8352
---
 lib/helpers/cursor/eachAsync.js | 18 +++++++++++++-----
 1 file changed, 13 insertions(+), 5 deletions(-)

diff --git a/lib/helpers/cursor/eachAsync.js b/lib/helpers/cursor/eachAsync.js
index 95a38dbf93a..029f203c9eb 100644
--- a/lib/helpers/cursor/eachAsync.js
+++ b/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);