fix(NODE-4429): select server sync for endSessions during close (#3363)

Author: durran

src/mongo_client.ts

@@ -21,6 +21,7 @@ import { PromiseProvider } from './promise_provider';
 import type { ReadConcern, ReadConcernLevel, ReadConcernLike } from './read_concern';
 import { ReadPreference, ReadPreferenceMode } from './read_preference';
 import type { TagSet } from './sdam/server_description';
+import { readPreferenceServerSelector } from './sdam/server_selection';
 import type { SrvPoller } from './sdam/srv_polling';
 import type { Topology, TopologyEvents } from './sdam/topology';
 import { ClientSession, ClientSessionOptions, ServerSessionPool } from './sessions';
@@ -500,6 +501,19 @@ export class MongoClient extends TypedEventEmitter<MongoClientEvents> {
 
       Promise.all(activeSessionEnds)
         .then(() => {
+          if (this.topology == null) {
+            return;
+          }
+          // If we would attempt to select a server and get nothing back we short circuit
+          // to avoid the server selection timeout.
+          const selector = readPreferenceServerSelector(ReadPreference.primaryPreferred);
+          const topologyDescription = this.topology.description;
+          const serverDescriptions = Array.from(topologyDescription.servers.values());
+          const servers = selector(topologyDescription, serverDescriptions);
+          if (servers.length === 0) {
+            return;
+          }
+
           const endSessions = Array.from(this.s.sessionPool.sessions, ({ id }) => id);
           if (endSessions.length === 0) return;
           return this.db('admin')
@@ -511,7 +525,7 @@ export class MongoClient extends TypedEventEmitter<MongoClientEvents> {
         })
         .then(() => {
           if (this.topology == null) {
-            return callback();
+            return;
           }
           // clear out references to old topology
           const topology = this.topology;

test/integration/node-specific/mongo_client.test.ts

@@ -13,6 +13,7 @@ import {
 } from '../../../src';
 import { Connection } from '../../../src/cmap/connection';
 import { Db } from '../../../src/db';
+import { ServerDescription } from '../../../src/sdam/server_description';
 import { Topology } from '../../../src/sdam/topology';
 import { getTopology, isHello } from '../../../src/utils';
 import { runLater } from '../../tools/utils';
@@ -649,5 +650,26 @@ describe('class MongoClient', function () {
       expect(endEvents).to.have.lengthOf(1);
       expect(endEvents[0]).to.have.property('reply', undefined); // noReponse: true
     });
+
+    context('when server selection would return no servers', () => {
+      const serverDescription = new ServerDescription('a:1');
+
+      it('short circuits and does not end sessions', async () => {
+        const session = client.startSession(); // make a session to be ended
+        await client.db('test').command({ ping: 1 }, { session });
+        await session.endSession();
+
+        const startedEvents: CommandStartedEvent[] = [];
+        client.on('commandStarted', event => startedEvents.push(event));
+
+        const servers = new Map<string, ServerDescription>();
+        servers.set(serverDescription.address, serverDescription);
+        client.topology.description.servers = servers;
+        await client.close();
+
+        expect(startedEvents).to.be.empty;
+        expect(client.s.sessionPool.sessions).to.have.lengthOf(1);
+      });
+    });
   });
 });