fix(query): clone PopulateOptions when setting _localModel to avoid s…

…tate leaking between subpopulate instances

Fix #15026

lib/query.js

@@ -4775,12 +4775,13 @@ Query.prototype._castUpdate = function _castUpdate(obj) {
  */
 
 Query.prototype.populate = function() {
+  const args = Array.from(arguments);
   // Bail when given no truthy arguments
-  if (!Array.from(arguments).some(Boolean)) {
+  if (!args.some(Boolean)) {
     return this;
   }
 
-  const res = utils.populate.apply(null, arguments);
+  const res = utils.populate.apply(null, args);
 
   // Propagate readConcern and readPreference and lean from parent query,
   // unless one already specified

lib/queryHelpers.js

@@ -4,6 +4,7 @@
  * Module dependencies
  */
 
+const PopulateOptions = require('./options/populateOptions');
 const checkEmbeddedDiscriminatorKeyProjection =
   require('./helpers/discriminator/checkEmbeddedDiscriminatorKeyProjection');
 const get = require('./helpers/get');
@@ -13,32 +14,6 @@ const isDefiningProjection = require('./helpers/projection/isDefiningProjection'
 const clone = require('./helpers/clone');
 const isPathSelectedInclusive = require('./helpers/projection/isPathSelectedInclusive');
 
-/**
- * Prepare a set of path options for query population.
- *
- * @param {Query} query
- * @param {Object} options
- * @return {Array}
- */
-
-exports.preparePopulationOptions = function preparePopulationOptions(query, options) {
-  const _populate = query.options.populate;
-  const pop = Object.keys(_populate).reduce((vals, key) => vals.concat([_populate[key]]), []);
-
-  // lean options should trickle through all queries
-  if (options.lean != null) {
-    pop
-      .filter(p => (p && p.options && p.options.lean) == null)
-      .forEach(makeLean(options.lean));
-  }
-
-  pop.forEach(opts => {
-    opts._localModel = query.model;
-  });
-
-  return pop;
-};
-
 /**
  * Prepare a set of path options for query population. This is the MongooseQuery
  * version
@@ -74,7 +49,16 @@ exports.preparePopulationOptionsMQ = function preparePopulationOptionsMQ(query,
 
   const projection = query._fieldsForExec();
   for (let i = 0; i < pop.length; ++i) {
-    pop[i] = { ...pop[i], _queryProjection: projection, _localModel: query.model };
+    if (pop[i] instanceof PopulateOptions) {
+      pop[i] = new PopulateOptions({
+        ...pop[i],
+        _queryProjection: projection,
+        _localModel: query.model
+      });
+    } else {
+      pop[i]._queryProjection = projection;
+      pop[i]._localModel = query.model;
+    }
   }
 
   return pop;