fix(shell-api): improve AbstractCursor iteration performance MONGOSH-1688 (#1920)

- Delegate cursor iteration to the driver where possible
- Do not call the wrapped `.tryNext()` method from inside other
  shell API methods, and instead only call an internal unwrapped
  version of it (similar to `db.runCommand()` vs `db._runCommand()`

This results in a 60% improvement of runtime in local testing on the
`db_cursor_iteration_plainvm` benchmark.

Author: addaleax

packages/shell-api/src/abstract-cursor.ts

@@ -83,22 +83,38 @@ export abstract class AbstractCursor<
 
   @returnsPromise
   async tryNext(): Promise<Document | null> {
+    return this._tryNext();
+  }
+
+  async _tryNext(): Promise<Document | null> {
     let result = await this._cursor.tryNext();
     if (result !== null && this._transform !== null) {
       result = await this._transform(result);
     }
     return result;
   }
 
+  _canDelegateIterationToUnderlyingCursor(): boolean {
+    return this._transform === null;
+  }
+
   get [Symbol.for('@@mongosh.syntheticAsyncIterable')]() {
     return true;
   }
 
   async *[Symbol.asyncIterator]() {
+    if (
+      this._cursor[Symbol.asyncIterator] &&
+      this._canDelegateIterationToUnderlyingCursor()
+    ) {
+      yield* this._cursor;
+      return;
+    }
+
     let doc;
     // !== null should suffice, but some stubs in our tests return 'undefined'
     // eslint-disable-next-line eqeqeq
-    while ((doc = await this.tryNext()) != null) {
+    while ((doc = await this._tryNext()) != null) {
       yield doc;
     }
   }
@@ -114,14 +130,23 @@ export abstract class AbstractCursor<
   @returnsPromise
   async itcount(): Promise<number> {
     let count = 0;
-    while (await this.tryNext()) {
+    while (await this._tryNext()) {
       count++;
     }
     return count;
   }
 
   @returnsPromise
   async toArray(): Promise<Document[]> {
+    // toArray is always defined for driver cursors, but not necessarily
+    // in tests
+    if (
+      typeof this._cursor.toArray === 'function' &&
+      this._canDelegateIterationToUnderlyingCursor()
+    ) {
+      return await this._cursor.toArray();
+    }
+
     const result = [];
     for await (const doc of this) {
       result.push(doc);

packages/shell-api/src/collection.spec.ts

@@ -209,8 +209,7 @@ describe('Collection', function () {
 
       it('returns an AggregationCursor that wraps the service provider one', async function () {
         const toArrayResult = [{ foo: 'bar' }];
-        serviceProviderCursor.tryNext.onFirstCall().resolves({ foo: 'bar' });
-        serviceProviderCursor.tryNext.onSecondCall().resolves(null);
+        serviceProviderCursor.toArray.resolves(toArrayResult);
         serviceProvider.aggregate.returns(serviceProviderCursor);
 
         const cursor = await collection.aggregate([