fix: transform wrapping

Author: nbbeeken

src/cursor/abstract_cursor.ts

@@ -533,7 +533,7 @@ export abstract class AbstractCursor<
     const oldTransform = this[kTransform] as (doc: TSchema) => TSchema; // TODO(NODE-3283): Improve transform typing
     if (oldTransform) {
       this[kTransform] = doc => {
-        return transform(oldTransform(doc));
+        return this.makeSafeTransform(transform)(oldTransform(doc));
       };
     } else {
       this[kTransform] = this.makeSafeTransform(transform);

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

@@ -153,6 +153,26 @@ describe('class AbstractCursor', function () {
       await client.close();
     });
 
+    it('wraps transform in result checking for each map call', async () => {
+      const control = { functionThatShouldReturnNull: 0 };
+      const makeCursor = () => {
+        const cursor = collection.find();
+        cursor
+          .map(doc => (control.functionThatShouldReturnNull === 0 ? null : doc))
+          .map(doc => (control.functionThatShouldReturnNull === 1 ? null : doc))
+          .map(doc => (control.functionThatShouldReturnNull === 2 ? null : doc));
+        return cursor;
+      };
+
+      for (const testFn of [0, 1, 2]) {
+        control.functionThatShouldReturnNull = testFn;
+        const error = await makeCursor()
+          .toArray()
+          .catch(error => error);
+        expect(error).to.be.instanceOf(MongoAPIError);
+      }
+    });
+
     context('toArray() with custom transforms', function () {
       for (const value of falseyValues) {
         it(`supports mapping to falsey value '${inspect(value)}'`, async function () {