fix and tests

src/cmap/connection.ts

@@ -237,6 +237,8 @@ export class Connection extends TypedEventEmitter<ConnectionEvents> {
       .on('error', this.onError.bind(this));
     this.socket.on('close', this.onClose.bind(this));
     this.socket.on('timeout', this.onTimeout.bind(this));
+
+    this.messageStream.pause();
   }
 
   public get hello() {
@@ -651,6 +653,7 @@ export class Connection extends TypedEventEmitter<ConnectionEvents> {
   private async *readMany(): AsyncGenerator<OpMsgResponse | OpReply> {
     try {
       this.dataEvents = onData(this.messageStream);
+      this.messageStream.resume();
       for await (const message of this.dataEvents) {
         const response = await decompressResponse(message);
         yield response;
@@ -661,6 +664,7 @@ export class Connection extends TypedEventEmitter<ConnectionEvents> {
       }
     } finally {
       this.dataEvents = null;
+      this.messageStream.pause();
       this.throwIfAborted();
     }
   }

test/unit/cmap/connection.test.ts

@@ -1,11 +1,15 @@
+import { Socket } from 'node:net';
+
 import { expect } from 'chai';
 import * as sinon from 'sinon';
+import { setTimeout } from 'timers/promises';
 
 import {
   connect,
   Connection,
   isHello,
   MongoClientAuthProviders,
+  MongoDBCollectionNamespace,
   MongoNetworkTimeoutError,
   ns
 } from '../../mongodb';
@@ -142,4 +146,181 @@ describe('new Connection()', function () {
     expect(beforeHandshakeSymbol).to.be.a('symbol');
     expect(error).to.have.property(beforeHandshakeSymbol, true);
   });
+
+  describe('NODE-6370: regression test', function () {
+    class MockSocket extends Socket {
+      override write(_data: string | Buffer) {
+        return false;
+      }
+    }
+
+    let socket: MockSocket;
+    let connection: Connection;
+
+    this.timeout(10_000);
+
+    beforeEach(function () {
+      socket = new MockSocket();
+      connection = new Connection(socket, {});
+    });
+
+    const validResponse = Buffer.from(
+      'a30000002a0800004b010000dd07000000000000008e000000016f6b00000000000000f03f0324636c757374657254696d65005800000011636c757374657254696d65001c00000093f6f266037369676e61747572650033000000056861736800140000000072d8d6eab4e0703d2d50846e2db7adb5d2733cc4126b65794964000200000026f6f2660000116f7065726174696f6e54696d65001c00000093f6f26600',
+      'hex'
+    );
+
+    const chunks = [validResponse.slice(0, 10), validResponse.slice(10)];
+
+    describe('when data is emitted before drain', function () {
+      describe('first command', function () {
+        describe('when there is no delay between data and drain', function () {
+          it('does not hang', async function () {
+            const result$ = connection.command(
+              MongoDBCollectionNamespace.fromString('foo.bar'),
+              { ping: 1 },
+              {}
+            );
+            // there is an await in writeCommand, we must move the event loop forward just enough
+            // so that we reach the `await drain`.  Otherwise, we'll emit both data and drain before
+            // listeners are attached.
+            await setTimeout(0);
+
+            socket.emit('data', validResponse);
+            socket.emit('drain');
+
+            await result$;
+          });
+        });
+
+        describe('when there is a delay between data and drain', function () {
+          it('does not hang', async function () {
+            const result$ = connection.command(
+              MongoDBCollectionNamespace.fromString('foo.bar'),
+              { ping: 1 },
+              {}
+            );
+
+            // there is an await in writeCommand, we must move the event loop forward just enough
+            // so that we reach the `await drain`.  Otherwise, we'll emit both data and drain before
+            // listeners are attached.
+            await setTimeout(0);
+            socket.emit('data', validResponse);
+
+            await setTimeout(10);
+
+            socket.emit('drain');
+            await result$;
+          });
+        });
+
+        describe('when the data comes in multiple chunks', function () {
+          it('does not hang', async function () {
+            const result$ = connection.command(
+              MongoDBCollectionNamespace.fromString('foo.bar'),
+              { ping: 1 },
+              {}
+            );
+
+            // there is an await in writeCommand, we must move the event loop forward just enough
+            // so that we reach the `await drain`.  Otherwise, we'll emit both data and drain before
+            // listeners are attached.
+            await setTimeout(0);
+            socket.emit('data', chunks[0]);
+
+            await setTimeout(10);
+            socket.emit('drain');
+
+            socket.emit('data', chunks[1]);
+
+            await result$;
+          });
+        });
+      });
+
+      describe('not first command', function () {
+        beforeEach(async function () {
+          const result$ = connection.command(
+            MongoDBCollectionNamespace.fromString('foo.bar'),
+            { ping: 1 },
+            {}
+          );
+
+          // there is an await in writeCommand, we must move the event loop forward just enough
+          // so that we reach the `await drain`.  Otherwise, we'll emit both data and drain before
+          // listeners are attached.
+          await setTimeout(0);
+          socket.emit('drain');
+          socket.emit('data', validResponse);
+
+          await result$;
+        });
+
+        describe('when there is no delay between data and drain', function () {
+          it('does not hang', async function () {
+            const result$ = connection.command(
+              MongoDBCollectionNamespace.fromString('foo.bar'),
+              { ping: 1 },
+              {}
+            );
+
+            // there is an await in writeCommand, we must move the event loop forward just enough
+            // so that we reach the `await drain`.  Otherwise, we'll emit both data and drain before
+            // listeners are attached.
+            await setTimeout(0);
+            socket.emit('data', validResponse);
+
+            // await setTimeout(0);
+            // await setTimeout(10);
+            socket.emit('drain');
+            await result$;
+          });
+        });
+
+        describe('when there is a delay between data and drain', function () {
+          it('does not hang', async function () {
+            const result$ = connection.command(
+              MongoDBCollectionNamespace.fromString('foo.bar'),
+              { ping: 1 },
+              {}
+            );
+
+            // there is an await in writeCommand, we must move the event loop forward just enough
+            // so that we reach the `await drain`.  Otherwise, we'll emit both data and drain before
+            // listeners are attached.
+            await setTimeout(0);
+            socket.emit('data', validResponse);
+
+            await setTimeout(10);
+            // await setTimeout(10);
+            socket.emit('drain');
+            await result$;
+          });
+        });
+
+        describe('when the data comes in multiple chunks', function () {
+          it('does not hang', async function () {
+            const result$ = connection.command(
+              MongoDBCollectionNamespace.fromString('foo.bar'),
+              { ping: 1 },
+              {}
+            );
+
+            // there is an await in writeCommand, we must move the event loop forward just enough
+            // so that we reach the `await drain`.  Otherwise, we'll emit both data and drain before
+            // listeners are attached.
+            await setTimeout(0);
+
+            socket.emit('data', chunks[0]);
+
+            await setTimeout(10);
+
+            socket.emit('drain');
+
+            socket.emit('data', chunks[1]);
+            await result$;
+          });
+        });
+      });
+    });
+  });
 });