chore: ensure command monitoring receives the same object instance

Author: nbbeeken

src/cmap/connection.ts

@@ -479,7 +479,21 @@ export class Connection extends TypedEventEmitter<ConnectionEvents> {
       );
     }
 
-    let document;
+    // If `documentsReturnedIn` not set or raw is not enabled, use input bson options
+    // Otherwise, support raw flag. Raw only works for cursors that hardcode firstBatch/nextBatch fields
+    const bsonOptions =
+      options.documentsReturnedIn == null || !options.raw
+        ? options
+        : {
+            ...options,
+            raw: false,
+            fieldsAsRaw: { [options.documentsReturnedIn]: true }
+          };
+
+    /** MongoDBResponse instance or subclass */
+    let document: MongoDBResponse | undefined = undefined;
+    /** Cached result of a toObject call */
+    let object: Document | undefined = undefined;
     try {
       this.throwIfAborted();
       for await (document of this.sendWire(message, options, responseType)) {
@@ -493,15 +507,12 @@ export class Connection extends TypedEventEmitter<ConnectionEvents> {
         }
 
         if (document.has('writeConcernError')) {
-          const objectWithWriteConcernError = document.toObject(options);
-          throw new MongoWriteConcernError(
-            objectWithWriteConcernError.writeConcernError,
-            objectWithWriteConcernError
-          );
+          object ??= document.toObject(bsonOptions);
+          throw new MongoWriteConcernError(object.writeConcernError, object);
         }
 
         if (document.isError) {
-          throw new MongoServerError(document.toObject(options));
+          throw new MongoServerError((object ??= document.toObject(bsonOptions)));
         }
 
         if (this.shouldEmitAndLogCommand) {
@@ -513,25 +524,15 @@ export class Connection extends TypedEventEmitter<ConnectionEvents> {
             new CommandSucceededEvent(
               this,
               message,
-              options.noResponse ? undefined : document.toObject(options),
+              options.noResponse ? undefined : (object ??= document.toObject(bsonOptions)),
               started,
               this.description.serverConnectionId
             )
           );
         }
 
         if (responseType == null) {
-          // If `documentsReturnedIn` not set or raw is not enabled, use input bson options
-          // Otherwise, support raw flag. Raw only works for cursors that hardcode firstBatch/nextBatch fields
-          const bsonOptions =
-            options.documentsReturnedIn == null || !options.raw
-              ? options
-              : {
-                  ...options,
-                  raw: false,
-                  fieldsAsRaw: { [options.documentsReturnedIn]: true }
-                };
-          yield document.toObject(bsonOptions);
+          yield (object ??= document.toObject(bsonOptions));
         } else {
           yield document;
         }
@@ -549,7 +550,7 @@ export class Connection extends TypedEventEmitter<ConnectionEvents> {
             new CommandSucceededEvent(
               this,
               message,
-              options.noResponse ? undefined : document?.toObject(options),
+              options.noResponse ? undefined : (object ??= document?.toObject(bsonOptions)),
               started,
               this.description.serverConnectionId
             )

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

@@ -14,6 +14,7 @@ import {
   makeClientMetadata,
   MongoClient,
   MongoClientAuthProviders,
+  MongoDBResponse,
   MongoServerError,
   ns,
   ServerHeartbeatStartedEvent,
@@ -101,6 +102,48 @@ describe('Connection', function () {
         }
       }
     });
+
+    afterEach(() => sinon.restore());
+
+    it('command monitoring event do not deserialize more than once', {
+      metadata: { requires: { apiVersion: false, topology: '!load-balanced' } },
+      test: async function () {
+        const connectOptions: ConnectionOptions = {
+          ...commonConnectOptions,
+          connectionType: Connection,
+          ...this.configuration.options,
+          monitorCommands: true,
+          metadata: makeClientMetadata({ driverInfo: {} }),
+          extendedMetadata: addContainerMetadata(makeClientMetadata({ driverInfo: {} }))
+        };
+
+        let conn;
+        try {
+          conn = await connect(connectOptions);
+
+          const toObjectSpy = sinon.spy(MongoDBResponse.prototype, 'toObject');
+
+          const events: any[] = [];
+          conn.on('commandStarted', event => events.push(event));
+          conn.on('commandSucceeded', event => events.push(event));
+          conn.on('commandFailed', event => events.push(event));
+
+          const hello = await conn.command(ns('admin.$cmd'), { ping: 1 });
+          expect(toObjectSpy).to.have.been.calledOnce;
+          expect(hello).to.have.property('ok', 1);
+          expect(events).to.have.lengthOf(2);
+
+          toObjectSpy.resetHistory();
+
+          const garbage = await conn.command(ns('admin.$cmd'), { garbage: 1 }).catch(e => e);
+          expect(toObjectSpy).to.have.been.calledOnce;
+          expect(garbage).to.have.property('ok', 0);
+          expect(events).to.have.lengthOf(4);
+        } finally {
+          conn?.destroy();
+        }
+      }
+    });
   });
 
   describe('Connection - functional', function () {