fix(NODE-5225): concurrent MongoClient.close() calls each attempt to close the client

src/mongo_client.ts

@@ -366,6 +366,8 @@ export class MongoClient extends TypedEventEmitter<MongoClientEvents> implements
   override readonly mongoLogger: MongoLogger | undefined;
   /** @internal */
   private connectionLock?: Promise<this>;
+  /** @internal */
+  private closeLock?: Promise<void>;
 
   /**
    * The consolidate, parsed, transformed and merged options.
@@ -638,6 +640,20 @@ export class MongoClient extends TypedEventEmitter<MongoClientEvents> implements
    * @param force - Force close, emitting no events
    */
   async close(force = false): Promise<void> {
+    if (this.closeLock) {
+      return await this.closeLock;
+    }
+
+    try {
+      this.closeLock = this._close(force);
+      await this.closeLock;
+    } finally {
+      // release
+      this.closeLock = undefined;
+    }
+  }
+
+  async _close(force = false): Promise<void> {
     // There's no way to set hasBeenClosed back to false
     Object.defineProperty(this.s, 'hasBeenClosed', {
       value: true,

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

@@ -722,6 +722,22 @@ describe('class MongoClient', function () {
         expect(client.s.sessionPool.sessions).to.have.lengthOf(1);
       });
     });
+
+    context('concurrent calls', () => {
+      context('when two concurrent calls to close() occur', () => {
+        it('no error is thrown', async function () {
+          expect(() => Promise.all([client.close(), client.close()])).to.not.throw;
+        });
+      });
+
+      context('when more than two concurrent calls to close() occur', () => {
+        it('no error is thrown', async function () {
+          expect(() =>
+            Promise.all([client.close(), client.close(), client.close(), client.close()])
+          ).to.not.throw;
+        });
+      });
+    });
   });
 
   context('when connecting', function () {

test/unit/mongo_client.test.ts

@@ -1223,4 +1223,22 @@ describe('MongoClient', function () {
       });
     });
   });
+
+  describe('closeLock', function () {
+    let client;
+
+    beforeEach(async function () {
+      client = new MongoClient('mongodb://blah');
+    });
+
+    it('when client.close is pending, client.closeLock is set', () => {
+      client.close();
+      expect(client.closeLock).to.be.instanceOf(Promise);
+    });
+
+    it('when client.close is not pending, client.closeLock is not set', async () => {
+      await client.close();
+      expect(client.closeLock).to.be.undefined;
+    });
+  });
 });