fix: silently ignore session with unacknowledged write

NODE-1341

lib/core/sessions.js

@@ -708,6 +708,11 @@ function applySession(session, command, options) {
     return new MongoError('Cannot use a session that has ended');
   }
 
+  // SPEC-1019: silently ignore explicit session with unacknowledged write for backwards compatibility
+  if (options && options.writeConcern && options.writeConcern.w === 0) {
+    return;
+  }
+
   const serverSession = session.serverSession;
   serverSession.lastUse = now();
   command.lsid = serverSession.id;

test/functional/sessions.test.js

@@ -1,4 +1,6 @@
 'use strict';
+
+const withMonitoredClient = require('./shared').withMonitoredClient;
 const expect = require('chai').expect;
 const setupDatabase = require('./shared').setupDatabase;
 const TestRunnerContext = require('./spec-runner').TestRunnerContext;
@@ -207,4 +209,25 @@ describe('Sessions', function() {
 
     generateTopologyTests(testSuites, testContext, testFilter);
   });
+  context('unacknowledged writes', () => {
+    it('should not include session for unacknowledged writes', {
+      metadata: { requires: { topology: 'single' } },
+      test: withMonitoredClient('insert', { clientOptions: { w: 0 } }, function(
+        client,
+        events,
+        done
+      ) {
+        client
+          .db('test')
+          .collection('foo')
+          .insertOne({ foo: 'bar' }, err => {
+            expect(err).to.not.exist;
+            const event = events[0];
+            expect(event.command.writeConcern.w).to.equal(0);
+            expect(event.command.lsid).to.equal(undefined);
+            done();
+          });
+      })
+    });
+  });
 });