add prose test for handshake construction changes

Author: baileympearson

src/cmap/connect.ts

@@ -222,6 +222,7 @@ export interface HandshakeDocument extends Document {
   compression: string[];
   saslSupportedMechs?: string;
   loadBalanced?: boolean;
+  backpressure: true;
 }
 
 /**
@@ -239,6 +240,7 @@ export async function prepareHandshakeDocument(
 
   const handshakeDoc: HandshakeDocument = {
     [serverApi?.version || options.loadBalanced === true ? 'hello' : LEGACY_HELLO_COMMAND]: 1,
+    backpressure: true,
     helloOk: true,
     client: clientMetadata,
     compression: compressors

test/integration/mongodb-handshake/mongodb-handshake.prose.test.ts

@@ -1,7 +1,14 @@
 import { expect } from 'chai';
 import * as sinon from 'sinon';
 
-import { type ClientMetadata, type DriverInfo, Int32, MongoClient } from '../../../src';
+import {
+  type ClientMetadata,
+  type Document,
+  type DriverInfo,
+  type HandshakeDocument,
+  Int32,
+  MongoClient
+} from '../../../src';
 import { Connection } from '../../../src/cmap/connection';
 import { getFAASEnv, isDriverInfoEqual } from '../../../src/cmap/handshake/client_metadata';
 import { LEGACY_HELLO_COMMAND } from '../../../src/constants';
@@ -939,3 +946,38 @@ describe('Client Metadata Update Prose Tests', function () {
     }
   });
 });
+
+// TODO: add prose test descriptions here to align the test with the spec.
+describe.only('Backpressure Metadata', function () {
+  let client: MongoClient;
+  let spy: sinon.SinonSpy<Parameters<typeof Connection.prototype.command>>;
+
+  beforeEach(async function () {
+    client = this.configuration.newClient();
+    spy = sinon.spy(Connection.prototype, 'command');
+    await client.connect();
+
+    // run an operation to force a connection establishment,
+    // if we're testing noauth load balanced mode.
+    await client.db('foo').collection('bar').insertOne({ name: 'bumpy' });
+  });
+
+  afterEach(async function () {
+    sinon.restore();
+    await client?.close();
+  });
+
+  it('includes backpressure in the handshake document', function () {
+    const isHello = (cmd: Document): cmd is HandshakeDocument =>
+      `hello` in cmd || LEGACY_HELLO_COMMAND in cmd;
+
+    const hellos = spy.args.map(([_ns, command, _options]) => command).filter(isHello);
+
+    expect(hellos.length).to.be.greaterThan(0);
+
+    expect(
+      hellos.every(hello => hello.backpressure === true),
+      `some handshake documents did not specify backpressure: true`
+    );
+  });
+});