fix(parallelCollectionScan): do not use implicit sessions on cursors

When we removed the session use from parallelCollectionScan,
we introduced an error where parallelCollectionScan cursors would
attempt to use implicit sessions on their cursors.

lib/cursor.js

@@ -143,10 +143,15 @@ function Cursor(bson, ns, cmd, options, topology, topologyOptions) {
     promiseLibrary: promiseLibrary,
     // Current doc
     currentDoc: null,
-    // Optional ClientSession
-    session: options.session
+    // explicitlyIgnoreSession
+    explicitlyIgnoreSession: options.explicitlyIgnoreSession
   };
 
+  // Optional ClientSession
+  if (!options.explicitlyIgnoreSession && options.session) {
+    this.s.session = options.session;
+  }
+
   // Translate correctly
   if (this.s.options.noCursorTimeout === true) {
     this.addCursorFlag('noCursorTimeout', true);
@@ -211,7 +216,7 @@ for (let name in CoreCursor.prototype) {
 }
 
 Cursor.prototype._initImplicitSession = function() {
-  if (!this.s.session && this.s.topology.hasSessionSupport()) {
+  if (!this.s.explicitlyIgnoreSession && !this.s.session && this.s.topology.hasSessionSupport()) {
     this.s.session = this.s.topology.startSession({ owner: this });
     this.cursorState.session = this.s.session;
   }

lib/operations/collection_ops.js

@@ -1021,6 +1021,8 @@ function parallelCollectionScan(coll, options, callback) {
         null
       );
 
+    options = Object.assign({ explicitlyIgnoreSession: true }, options);
+
     const cursors = [];
     // Add the raw back to the option
     if (raw) options.raw = raw;

test/functional/find_tests.js

@@ -2,6 +2,7 @@
 const test = require('./shared').assert;
 const setupDatabase = require('./shared').setupDatabase;
 const expect = require('chai').expect;
+const MongoClient = require('../../lib/mongo_client');
 
 describe('Find', function() {
   before(function() {
@@ -2624,6 +2625,59 @@ describe('Find', function() {
     }
   });
 
+  it('Should not use a session when using parallelCollectionScan', {
+    metadata: {
+      requires: {
+        mongodb: '>=3.6.0',
+        topology: ['single', 'replicaset', 'sharded', 'ssl', 'heap', 'wiredtiger']
+      }
+    },
+    test: function(done) {
+      const configuration = this.configuration;
+      const client = new MongoClient(configuration.url());
+
+      client.connect(function(err, client) {
+        var db = client.db(configuration.db);
+        var docs = [];
+
+        // Insert some documents
+        for (var i = 0; i < 1000; i++) {
+          docs.push({ a: i });
+        }
+
+        // Get the collection
+        var collection = db.collection('parallelCollectionScan_4');
+        // Insert 1000 documents in a batch
+        collection.insert(docs, function(err) {
+          expect(err).to.be.null;
+          var numCursors = 1;
+
+          // Execute parallelCollectionScan command
+          collection.parallelCollectionScan({ numCursors: numCursors }, function(err, cursors) {
+            expect(err).to.be.null;
+            expect(cursors)
+              .to.be.an('array')
+              .with.lengthOf(1);
+
+            const cursor = cursors[0];
+
+            expect(cursor).to.not.have.nested.property('s.session');
+            expect(cursor)
+              .to.have.nested.property('s.explicitlyIgnoreSession')
+              .that.equals(true);
+
+            cursor.toArray(err => {
+              expect(err).to.be.null;
+              expect(cursor).to.not.have.nested.property('s.session');
+
+              cursor.close().then(() => client.close().then(() => done()));
+            });
+          });
+        });
+      });
+    }
+  });
+
   it('Should correctly sort using text search on 2.6 or higher in find', {
     // Add a tag that our runner can trigger on
     // in this case we are setting that node needs to be higher than 0.10.X to run