fix(cursor): cap parallel batchSize for populate at 5000

Fix #10449
Re: #9366

lib/cursor/QueryCursor.js

@@ -55,6 +55,9 @@ function QueryCursor(query, options) {
     if (this.options.batchSize) {
       this.options.cursor = options.cursor || {};
       this.options.cursor.batchSize = options.batchSize;
+
+      // Max out the number of documents we'll populate in parallel at 5000.
+      this.options._populateBatchSize = Math.min(this.options.batchSize, 5000);
     }
     model.collection.find(query._conditions, this.options, function(err, cursor) {
       if (_this._error) {
@@ -360,7 +363,7 @@ function _next(ctx, cb) {
         ctx.query._mongooseOptions);
       ctx._pop.__noPromise = true;
     }
-    if (ctx.query._mongooseOptions.populate && ctx.options.batchSize > 1) {
+    if (ctx.query._mongooseOptions.populate && ctx.options._populateBatchSize > 1) {
       if (ctx._batchDocs && ctx._batchDocs.length) {
         // Return a cached populated doc
         return _nextDoc(ctx, ctx._batchDocs.shift(), ctx._pop, callback);
@@ -418,7 +421,7 @@ function _onNext(error, doc) {
 
   this.ctx._batchDocs.push(doc);
 
-  if (this.ctx._batchDocs.length < this.ctx.options.batchSize) {
+  if (this.ctx._batchDocs.length < this.ctx.options._populateBatchSize) {
     this.ctx.cursor.next(_onNext.bind(this));
   } else {
     _populateBatch.call(this);