fix(NODE-5225): eagerly clear MongoClient.topology in MongoClient.close()

src/mongo_client.ts

@@ -504,10 +504,14 @@ export class MongoClient extends TypedEventEmitter<MongoClientEvents> {
       return;
     }
 
+    // clear out references to old topology
+    const topology = this.topology;
+    this.topology = undefined;
+
     // 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 topologyDescription = topology.description;
     const serverDescriptions = Array.from(topologyDescription.servers.values());
     const servers = selector(topologyDescription, serverDescriptions);
     if (servers.length !== 0) {
@@ -522,10 +526,6 @@ export class MongoClient extends TypedEventEmitter<MongoClientEvents> {
       }
     }
 
-    // clear out references to old topology
-    const topology = this.topology;
-    this.topology = undefined;
-
     await new Promise<void>((resolve, reject) => {
       topology.close({ force }, error => {
         if (error) return reject(error);

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

@@ -385,6 +385,14 @@ describe('class MongoClient', function () {
     }
   });
 
+  it('can call close() concurrently', async function () {
+    const client = this.configuration.newClient();
+    await client.connect();
+    // Ensure topology is opened before trying to close
+    await client.db().command({ hello: 1 });
+    await Promise.all([client.close(), client.close()]);
+  });
+
   context('explict #connect()', () => {
     let client: MongoClient;
     beforeEach(function () {