fix(NODE-5901): propagate errors to transformed stream in cursor (#3985)

Co-authored-by: Durran Jordan <[email protected]>

src/cursor/abstract_cursor.ts

@@ -337,7 +337,7 @@ export abstract class AbstractCursor<
       const transform = options.transform;
       const readable = new ReadableCursorStream(this);
 
-      return readable.pipe(
+      const transformedStream = readable.pipe(
         new Transform({
           objectMode: true,
           highWaterMark: 1,
@@ -351,6 +351,12 @@ export abstract class AbstractCursor<
           }
         })
       );
+
+      // Bubble errors to transformed stream, because otherwise no way
+      // to handle this error.
+      readable.on('error', err => transformedStream.emit('error', err));
+
+      return transformedStream;
     }
 
     return new ReadableCursorStream(this);

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

@@ -1,9 +1,16 @@
 import { expect } from 'chai';
 import { once } from 'events';
 import * as sinon from 'sinon';
+import { Transform } from 'stream';
 import { inspect } from 'util';
 
-import { type Collection, type FindCursor, MongoAPIError, type MongoClient } from '../../mongodb';
+import {
+  type Collection,
+  type FindCursor,
+  MongoAPIError,
+  type MongoClient,
+  MongoServerError
+} from '../../mongodb';
 
 describe('class AbstractCursor', function () {
   describe('regression tests NODE-5372', function () {
@@ -233,4 +240,39 @@ describe('class AbstractCursor', function () {
       });
     });
   });
+
+  describe('transform stream error handling', function () {
+    let client: MongoClient;
+    let collection: Collection;
+    const docs = [{ count: 0 }];
+    beforeEach(async function () {
+      client = this.configuration.newClient();
+
+      collection = client.db('abstract_cursor_integration').collection('test');
+
+      await collection.insertMany(docs);
+    });
+
+    afterEach(async function () {
+      await collection.deleteMany({});
+      await client.close();
+    });
+
+    it('propagates errors to transform stream', async function () {
+      const transform = new Transform({
+        transform(data, encoding, callback) {
+          callback(null, data);
+        }
+      });
+
+      // MongoServerError: unknown operator: $bar
+      const stream = collection.find({ foo: { $bar: 25 } }).stream({ transform });
+
+      const error: Error | null = await new Promise(resolve => {
+        stream.on('error', error => resolve(error));
+        stream.on('end', () => resolve(null));
+      });
+      expect(error).to.be.instanceof(MongoServerError);
+    });
+  });
 });