fix: move session support check to operation layer

Session support check is now performed after server selection
this ensures the monitor was able to update the topology description

NODE-3100

src/mongo_client.ts

@@ -464,10 +464,6 @@ export class MongoClient extends EventEmitter {
       throw new MongoError('Must connect to a server before calling this method');
     }
 
-    if (!this.topology.hasSessionSupport()) {
-      throw new MongoError('Current topology does not support sessions');
-    }
-
     return this.topology.startSession(options, this.s.options);
   }
 

src/operations/execute_operation.ts

@@ -64,30 +64,31 @@ export function executeOperation<
   if (topology.shouldCheckForSessionSupport()) {
     return maybePromise(callback, cb => {
       topology.selectServer(ReadPreference.primaryPreferred, err => {
-        if (err) {
-          cb(err);
-          return;
-        }
+        if (err) return cb(err);
 
         executeOperation<T, TResult>(topology, operation, cb);
       });
     });
   }
 
-  // The driver sessions spec mandates that we implicitly create sessions for operations
-  // that are not explicitly provided with a session.
-  let session = operation.session;
-  let owner: symbol;
-  if (topology.hasSessionSupport()) {
-    if (session == null) {
-      owner = Symbol();
-      session = topology.startSession({ owner, explicit: false });
-    } else if (operation.session.hasEnded) {
-      throw new MongoError('Use of expired sessions is not permitted');
+  return maybePromise(callback, cb => {
+    // The driver sessions spec mandates that we implicitly create sessions for operations
+    // that are not explicitly provided with a session.
+    let session: ClientSession | undefined = operation.session;
+    let owner: symbol | undefined;
+    if (topology.hasSessionSupport()) {
+      if (session == null) {
+        owner = Symbol();
+        session = topology.startSession({ owner, explicit: false });
+      } else if (session.hasEnded) {
+        return cb(new MongoError('Use of expired sessions is not permitted'));
+      }
+    } else if (session) {
+      // If the user passed an explicit session and we are still, after server selection,
+      // trying to run against a topology that doesn't support sessions we error out.
+      return cb(new MongoError('Current topology does not support sessions'));
     }
-  }
 
-  return maybePromise(callback, cb => {
     try {
       executeWithServerSelection(topology, session, operation, (err, result) => {
         if (session && session.owner && session.owner === owner) {
@@ -113,7 +114,7 @@ function supportsRetryableReads(server: Server) {
 function executeWithServerSelection(
   topology: Topology,
   session: ClientSession,
-  operation: any,
+  operation: AbstractOperation,
   callback: Callback
 ) {
   const readPreference = operation.readPreference || ReadPreference.primary;

src/operations/operation.ts

@@ -38,7 +38,7 @@ const kSession = Symbol('session');
  * a specific aspect.
  * @internal
  */
-export abstract class AbstractOperation<T> {
+export abstract class AbstractOperation<TResult = any> {
   ns!: MongoDBNamespace;
   cmd!: Document;
   readPreference: ReadPreference;
@@ -48,6 +48,9 @@ export abstract class AbstractOperation<T> {
   // BSON serialization options
   bsonOptions?: BSONSerializeOptions;
 
+  // TODO: Each operation defines its own options, there should be better typing here
+  options: Document;
+
   [kSession]: ClientSession;
 
   constructor(options: OperationOptions = {}) {
@@ -61,9 +64,11 @@ export abstract class AbstractOperation<T> {
     if (options.session) {
       this[kSession] = options.session;
     }
+
+    this.options = options;
   }
 
-  abstract execute(server: Server, session: ClientSession, callback: Callback<T>): void;
+  abstract execute(server: Server, session: ClientSession, callback: Callback<TResult>): void;
 
   hasAspect(aspect: symbol): boolean {
     const ctor = this.constructor as OperationConstructor;

test/unit/sessions/client.test.js

@@ -14,9 +14,9 @@ describe('Sessions - client/unit', function () {
       });
     });
 
-    it('should throw an exception if sessions are not supported', {
+    it('should not throw a synchronous exception if sessions are not supported', {
       metadata: { requires: { topology: 'single' } },
-      test: function (done) {
+      test() {
         test.server.setMessageHandler(request => {
           var doc = request.document;
           if (doc.ismaster) {
@@ -27,13 +27,11 @@ describe('Sessions - client/unit', function () {
         });
 
         const client = this.configuration.newClient(`mongodb://${test.server.uri()}/test`);
-        client.connect(function (err, client) {
-          expect(err).to.not.exist;
-          expect(() => {
-            client.startSession();
-          }).to.throw(/Current topology does not support sessions/);
-
-          client.close(done);
+        return client.connect().then(() => {
+          expect(() => client.startSession()).to.not.throw(
+            'Current topology does not support sessions'
+          );
+          return client.close();
         });
       }
     });
@@ -93,15 +91,18 @@ describe('Sessions - client/unit', function () {
             return replicaSetMock.uri();
           })
           .then(uri => {
-            const client = this.configuration.newClient(uri);
-            return client.connect();
+            testClient = this.configuration.newClient(uri);
+            return testClient.connect();
           })
           .then(client => {
-            testClient = client;
-            expect(client.topology.s.description.logicalSessionTimeoutMinutes).to.not.exist;
-            expect(() => {
-              client.startSession();
-            }).to.throw(/Current topology does not support sessions/);
+            const session = client.startSession();
+            return client.db().collection('t').insertOne({ a: 1 }, { session });
+          })
+          .then(() => {
+            expect.fail('Expected an error to be thrown about not supporting sessions');
+          })
+          .catch(error => {
+            expect(error.message).to.equal('Current topology does not support sessions');
           })
           .finally(() => (testClient ? testClient.close() : null));
       }