feat(NODE-3881): require hello command + OP_MSG when 'loadBalanced=True'

src/cmap/connect.ts

@@ -214,7 +214,7 @@ export async function prepareHandshakeDocument(
   const { serverApi } = authContext.connection;
 
   const handshakeDoc: HandshakeDocument = {
-    [serverApi?.version ? 'hello' : LEGACY_HELLO_COMMAND]: 1,
+    [serverApi?.version || options.loadBalanced === true ? 'hello' : LEGACY_HELLO_COMMAND]: 1,
     helloOk: true,
     client: options.metadata,
     compression: compressors

src/cmap/connection.ts

@@ -487,7 +487,7 @@ export class Connection extends TypedEventEmitter<ConnectionEvents> {
     let cmd = { ...command };
 
     const readPreference = getReadPreference(options);
-    const shouldUseOpMsg = supportsOpMsg(this);
+    const shouldUseOpMsg = supportsOpMsg(this, { loadBalanced: cmd.loadBalanced });
     const session = options?.session;
 
     let clusterTime = this.clusterTime;
@@ -637,8 +637,17 @@ export function hasSessionSupport(conn: Connection): boolean {
   return description.logicalSessionTimeoutMinutes != null;
 }
 
-function supportsOpMsg(conn: Connection) {
+/** @internal */
+export function supportsOpMsg(conn: Connection, options: { loadBalanced?: boolean }) {
+  // If loadBalanced is true, then we MUST send the initial hello command using OP_MSG.
+  // Since this is the first message and hello/legacy hello hasn't been sent yet,
+  // conn.description will be null and we can't rely on the server check to determine if
+  // the server supports OP_MSG.
   const description = conn.description;
+  if (options.loadBalanced === true && description == null) {
+    return true;
+  }
+
   if (description == null) {
     return false;
   }

test/unit/cmap/connect.test.ts

@@ -242,6 +242,7 @@ describe('Connect Tests', function () {
           };
           const handshakeDocument = await prepareHandshakeDocument(authContext);
           expect(handshakeDocument).not.to.have.property('loadBalanced');
+          expect(handshakeDocument).to.have.property(LEGACY_HELLO_COMMAND, 1);
         });
       });
 
@@ -256,6 +257,7 @@ describe('Connect Tests', function () {
           };
           const handshakeDocument = await prepareHandshakeDocument(authContext);
           expect(handshakeDocument).not.to.have.property('loadBalanced');
+          expect(handshakeDocument).to.have.property(LEGACY_HELLO_COMMAND, 1);
         });
       });
 
@@ -270,6 +272,7 @@ describe('Connect Tests', function () {
           };
           const handshakeDocument = await prepareHandshakeDocument(authContext);
           expect(handshakeDocument).to.have.property('loadBalanced', true);
+          expect(handshakeDocument).to.have.property('hello', 1);
         });
       });
     });

test/unit/cmap/connection.test.ts

@@ -19,7 +19,8 @@ import {
   MongoNetworkTimeoutError,
   MongoRuntimeError,
   ns,
-  type OperationDescription
+  type OperationDescription,
+  supportsOpMsg
 } from '../../mongodb';
 import * as mock from '../../tools/mongodb-mock/index';
 import { generateOpMsgBuffer, getSymbolFrom } from '../../tools/utils';
@@ -894,6 +895,78 @@ describe('new Connection()', function () {
     });
   });
 
+  describe('.supportsOpMsg', function () {
+    let connection;
+
+    context('loadBalanced option is true', function () {
+      beforeEach(function () {
+        connection = {};
+      });
+
+      it('returns true', function () {
+        expect(supportsOpMsg(connection, { loadBalanced: true })).to.be.true;
+      });
+    });
+
+    context('loadBalanced is not requested and this is the first message', function () {
+      beforeEach(function () {
+        connection = {}; // The first message, therefore no description.
+      });
+
+      it('returns false', function () {
+        expect(supportsOpMsg(connection, {})).to.be.false;
+      });
+    });
+
+    context(
+      'the first message has been sent and maxWireVersion matches the compatible version',
+      function () {
+        beforeEach(function () {
+          connection = {
+            description: new InputStream(),
+            hello: { maxWireVersion: 6 }
+          };
+        });
+
+        it('returns true', function () {
+          expect(supportsOpMsg(connection, {})).to.be.true;
+        });
+      }
+    );
+
+    context(
+      'the first message has been sent and maxWireVersion is above the compatible version',
+      function () {
+        beforeEach(function () {
+          connection = {
+            description: new InputStream(),
+            hello: { maxWireVersion: 7 }
+          };
+        });
+
+        it('returns true', function () {
+          expect(supportsOpMsg(connection, {})).to.be.true;
+        });
+      }
+    );
+
+    context(
+      'the first message has been sent and maxWireVersion is below the compatible version',
+      function () {
+        beforeEach(function () {
+          connection = {
+            description: new InputStream(),
+            hello: { maxWireVersion: 5 }
+          };
+        });
+
+        it('returns false', function () {
+          expect(supportsOpMsg(connection, {})).to.be.false;
+        });
+      }
+    );
+  });
+
   describe('destroy()', () => {
     let connection: sinon.SinonSpiedInstance<Connection>;
     let clock: sinon.SinonFakeTimers;