fix(NODE-5993): memory leak in the Connection class

src/cmap/connection.ts

@@ -1,6 +1,5 @@
 import { type Readable, Transform, type TransformCallback } from 'stream';
 import { clearTimeout, setTimeout } from 'timers';
-import { promisify } from 'util';
 
 import type { BSONSerializeOptions, Document, ObjectId } from '../bson';
 import type { AutoEncrypter } from '../client-side-encryption/auto_encrypter';
@@ -37,7 +36,7 @@ import {
   maxWireVersion,
   type MongoDBNamespace,
   now,
-  promiseWithResolvers,
+  once,
   uuidV4
 } from '../utils';
 import type { WriteConcern } from '../write_concern';
@@ -182,18 +181,18 @@ export class Connection extends TypedEventEmitter<ConnectionEvents> {
    * Once connection is established, command logging can log events (if enabled)
    */
   public established: boolean;
+  /** Indicates that the connection (including underlying TCP socket) has been closed. */
+  public closed = false;
 
   private lastUseTime: number;
   private clusterTime: Document | null = null;
+  private error: Error | null = null;
+  private dataEvents: AsyncGenerator<Buffer, void, void> | null = null;
 
   private readonly socketTimeoutMS: number;
   private readonly monitorCommands: boolean;
   private readonly socket: Stream;
-  private readonly controller: AbortController;
-  private readonly signal: AbortSignal;
   private readonly messageStream: Readable;
-  private readonly socketWrite: (buffer: Uint8Array) => Promise<void>;
-  private readonly aborted: Promise<never>;
 
   /** @event */
   static readonly COMMAND_STARTED = COMMAND_STARTED;
@@ -213,6 +212,7 @@ export class Connection extends TypedEventEmitter<ConnectionEvents> {
   constructor(stream: Stream, options: ConnectionOptions) {
     super();
 
+    this.socket = stream;
     this.id = options.id;
     this.address = streamIdentifier(stream, options);
     this.socketTimeoutMS = options.socketTimeoutMS ?? 0;
@@ -225,39 +225,12 @@ export class Connection extends TypedEventEmitter<ConnectionEvents> {
     this.generation = options.generation;
     this.lastUseTime = now();
 
-    this.socket = stream;
-
-    // TODO: Remove signal from connection layer
-    this.controller = new AbortController();
-    const { signal } = this.controller;
-    this.signal = signal;
-    const { promise: aborted, reject } = promiseWithResolvers<never>();
-    aborted.then(undefined, () => null); // Prevent unhandled rejection
-    this.signal.addEventListener(
-      'abort',
-      function onAbort() {
-        reject(signal.reason);
-      },
-      { once: true }
-    );
-    this.aborted = aborted;
-
     this.messageStream = this.socket
       .on('error', this.onError.bind(this))
       .pipe(new SizedMessageTransform({ connection: this }))
       .on('error', this.onError.bind(this));
     this.socket.on('close', this.onClose.bind(this));
     this.socket.on('timeout', this.onTimeout.bind(this));
-
-    const socketWrite = promisify(this.socket.write.bind(this.socket));
-    this.socketWrite = async buffer => {
-      return Promise.race([socketWrite(buffer), this.aborted]);
-    };
-  }
-
-  /** Indicates that the connection (including underlying TCP socket) has been closed. */
-  public get closed(): boolean {
-    return this.signal.aborted;
   }
 
   public get hello() {
@@ -308,7 +281,7 @@ export class Connection extends TypedEventEmitter<ConnectionEvents> {
     this.lastUseTime = now();
   }
 
-  public onError(error?: Error) {
+  public onError(error: Error) {
     this.cleanup(error);
   }
 
@@ -351,13 +324,15 @@ export class Connection extends TypedEventEmitter<ConnectionEvents> {
    *
    * This method does nothing if the connection is already closed.
    */
-  private cleanup(error?: Error): void {
+  private cleanup(error: Error): void {
     if (this.closed) {
       return;
     }
 
     this.socket.destroy();
-    this.controller.abort(error);
+    this.error = error;
+    this.dataEvents?.throw(error).then(undefined, () => null); // squash unhandled rejection
+    this.closed = true;
     this.emit(Connection.CLOSE);
   }
 
@@ -598,7 +573,7 @@ export class Connection extends TypedEventEmitter<ConnectionEvents> {
   }
 
   private throwIfAborted() {
-    this.signal.throwIfAborted();
+    if (this.error) throw this.error;
   }
 
   /**
@@ -621,7 +596,8 @@ export class Connection extends TypedEventEmitter<ConnectionEvents> {
 
     const buffer = Buffer.concat(await finalCommand.toBin());
 
-    return this.socketWrite(buffer);
+    if (this.socket.write(buffer)) return;
+    return once(this.socket, 'drain');
   }
 
   /**
@@ -634,13 +610,19 @@ export class Connection extends TypedEventEmitter<ConnectionEvents> {
    * Note that `for-await` loops call `return` automatically when the loop is exited.
    */
   private async *readMany(): AsyncGenerator<OpMsgResponse | OpQueryResponse> {
-    for await (const message of onData(this.messageStream, { signal: this.signal })) {
-      const response = await decompressResponse(message);
-      yield response;
+    try {
+      this.dataEvents = onData(this.messageStream);
+      for await (const message of this.dataEvents) {
+        const response = await decompressResponse(message);
+        yield response;
 
-      if (!response.moreToCome) {
-        return;
+        if (!response.moreToCome) {
+          return;
+        }
       }
+    } finally {
+      this.dataEvents = null;
+      this.throwIfAborted();
     }
   }
 }

src/cmap/wire_protocol/on_data.ts

@@ -16,11 +16,9 @@ type PendingPromises = Omit<
  * https://nodejs.org/api/events.html#eventsonemitter-eventname-options
  *
  * Returns an AsyncIterator that iterates each 'data' event emitted from emitter.
- * It will reject upon an error event or if the provided signal is aborted.
+ * It will reject upon an error event.
  */
-export function onData(emitter: EventEmitter, options: { signal: AbortSignal }) {
-  const signal = options.signal;
-
+export function onData(emitter: EventEmitter) {
   // Setup pending events and pending promise lists
   /**
    * When the caller has not yet called .next(), we store the
@@ -89,19 +87,8 @@ export function onData(emitter: EventEmitter, options: { signal: AbortSignal })
   emitter.on('data', eventHandler);
   emitter.on('error', errorHandler);
 
-  if (signal.aborted) {
-    // If the signal is aborted, set up the first .next() call to be a rejection
-    queueMicrotask(abortListener);
-  } else {
-    signal.addEventListener('abort', abortListener, { once: true });
-  }
-
   return iterator;
 
-  function abortListener() {
-    errorHandler(signal.reason);
-  }
-
   function eventHandler(value: Buffer) {
     const promise = unconsumedPromises.shift();
     if (promise != null) promise.resolve({ value, done: false });
@@ -119,7 +106,6 @@ export function onData(emitter: EventEmitter, options: { signal: AbortSignal })
     // Adding event handlers
     emitter.off('data', eventHandler);
     emitter.off('error', errorHandler);
-    signal.removeEventListener('abort', abortListener);
     finished = true;
     const doneResult = { value: undefined, done: finished } as const;
 

src/utils.ts

@@ -1,5 +1,6 @@
 import * as crypto from 'crypto';
 import type { SrvRecord } from 'dns';
+import { type EventEmitter } from 'events';
 import * as http from 'http';
 import { clearTimeout, setTimeout } from 'timers';
 import * as url from 'url';
@@ -1295,3 +1296,27 @@ export function promiseWithResolvers<T>() {
 }
 
 export const randomBytes = promisify(crypto.randomBytes);
+
+/**
+ * Replicates the events.once helper.
+ *
+ * Removes unused signal logic and It **only** supports 0 or 1 argument events.
+ *
+ * @param ee - An event emitter that may emit `ev`
+ * @param name - An event name to wait for
+ */
+export async function once<T>(ee: EventEmitter, name: string): Promise<T> {
+  const { promise, resolve, reject } = promiseWithResolvers<T>();
+  const onEvent = (data: T) => resolve(data);
+  const onError = (error: Error) => reject(error);
+
+  ee.once(name, onEvent).once('error', onError);
+  try {
+    const res = await promise;
+    ee.off('error', onError);
+    return res;
+  } catch (error) {
+    ee.off(name, onEvent);
+    throw error;
+  }
+}

test/integration/connection-monitoring-and-pooling/connection.test.ts

@@ -1,7 +1,11 @@
 import { expect } from 'chai';
+import { type EventEmitter, once } from 'events';
+import * as sinon from 'sinon';
+import { setTimeout } from 'timers';
 
 import {
   addContainerMetadata,
+  Binary,
   connect,
   Connection,
   type ConnectionOptions,
@@ -15,7 +19,9 @@ import {
   ServerHeartbeatStartedEvent,
   Topology
 } from '../../mongodb';
+import * as mock from '../../tools/mongodb-mock/index';
 import { skipBrokenAuthTestBeforeEachHook } from '../../tools/runner/hooks/configuration';
+import { getSymbolFrom, sleep } from '../../tools/utils';
 import { assert as test, setupDatabase } from '../shared';
 
 const commonConnectOptions = {
@@ -200,6 +206,84 @@ describe('Connection', function () {
       client.connect();
     });
 
+    context(
+      'when a large message is written to the socket',
+      { requires: { topology: 'single', auth: 'disabled' } },
+      () => {
+        let client, mockServer: import('../../tools/mongodb-mock/src/server').MockServer;
+
+        beforeEach(async function () {
+          mockServer = await mock.createServer();
+
+          mockServer
+            .addMessageHandler('insert', req => {
+              setTimeout(() => {
+                req.reply({ ok: 1 });
+              }, 800);
+            })
+            .addMessageHandler('hello', req => {
+              req.reply(Object.assign({}, mock.HELLO));
+            })
+            .addMessageHandler(LEGACY_HELLO_COMMAND, req => {
+              req.reply(Object.assign({}, mock.HELLO));
+            });
+
+          client = new MongoClient(`mongodb://${mockServer.uri()}`, {
+            minPoolSize: 1,
+            maxPoolSize: 1
+          });
+        });
+
+        afterEach(async function () {
+          await client.close();
+          mockServer.destroy();
+          sinon.restore();
+        });
+
+        it('waits for an async drain event because the write was buffered', async () => {
+          const connectionReady = once(client, 'connectionReady');
+          await client.connect();
+          await connectionReady;
+
+          // Get the only connection
+          const pool = [...client.topology.s.servers.values()][0].pool;
+
+          const connections = pool[getSymbolFrom(pool, 'connections')];
+          expect(connections).to.have.lengthOf(1);
+
+          const connection = connections.first();
+          const socket: EventEmitter = connection.socket;
+
+          // Spy on the socket event listeners
+          const addedListeners: string[] = [];
+          const removedListeners: string[] = [];
+          socket
+            .on('removeListener', name => removedListeners.push(name))
+            .on('newListener', name => addedListeners.push(name));
+
+          // Make server sockets block
+          for (const s of mockServer.sockets) s.pause();
+
+          const insert = client
+            .db('test')
+            .collection('test')
+            // Anything above 16Kb should work I think (10mb to be extra sure)
+            .insertOne({ a: new Binary(Buffer.alloc(10 * (2 ** 10) ** 2), 127) });
+
+          // Sleep a bit and unblock server sockets
+          await sleep(10);
+          for (const s of mockServer.sockets) s.resume();
+
+          // Let the operation finish
+          await insert;
+
+          // Ensure that we used the drain event for this write
+          expect(addedListeners).to.deep.equal(['drain', 'error']);
+          expect(removedListeners).to.deep.equal(['drain', 'error']);
+        });
+      }
+    );
+
     context('when connecting with a username and password', () => {
       let utilClient: MongoClient;
       let client: MongoClient;

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

@@ -1,5 +1,8 @@
+import * as v8 from 'node:v8';
+
 import { expect } from 'chai';
 
+import { sleep } from '../../tools/utils';
 import { runScript } from './resource_tracking_script_builder';
 
 /**
@@ -86,4 +89,34 @@ describe('Driver Resources', () => {
       });
     });
   });
+
+  context('when 100s of operations are executed and complete', () => {
+    beforeEach(function () {
+      if (this.currentTest && typeof v8.queryObjects !== 'function') {
+        this.currentTest.skipReason = 'Test requires v8.queryObjects API to count Promises';
+        this.currentTest?.skip();
+      }
+    });
+
+    let client;
+    beforeEach(async function () {
+      client = this.configuration.newClient();
+    });
+
+    afterEach(async function () {
+      await client.close();
+    });
+
+    it('does not leave behind additional promises', async () => {
+      const test = client.db('test').collection('test');
+      const promiseCountBefore = v8.queryObjects(Promise, { format: 'count' });
+      for (let i = 0; i < 100; i++) {
+        await test.findOne();
+      }
+      await sleep(10);
+      const promiseCountAfter = v8.queryObjects(Promise, { format: 'count' });
+
+      expect(promiseCountAfter).to.be.within(promiseCountBefore - 5, promiseCountBefore + 5);
+    });
+  });
 });