[test/ffi] Test struct workaround for external fields

Make sure the individual getter/setter workaround for structs in NNBD
works. That way structs can be migrated without having to opt in to
the proposed --enable-experiment=external-abstract-variables for
external fields.

Note that this requires dart:ffi users to migrate twice for NNBD, once
to the workaround, and later a cleanup to external fields.

Change-Id: I2a9020ff7df3ebf08e04f71183f9297d87acb1b1
Cq-Include-Trybots:dart/try:vm-ffi-android-debug-arm-try,vm-ffi-android-debug-arm64-try,vm-precomp-ffi-qemu-linux-release-arm-try
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/153777
Reviewed-by: Erik Ernst <eernst@google.com>
Commit-Queue: Daco Harkes <dacoharkes@google.com>

tests/ffi/coordinate_nnbd_workaround.dart

@@ -0,0 +1,29 @@
+// Copyright (c) 2020, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+library FfiTest;
+
+import 'dart:ffi';
+import "package:ffi/ffi.dart";
+
+/// Sample struct for dart:ffi library.
+class Coordinate extends Struct {
+  @Double()
+  external double get x;
+  external set x(double v);
+
+  @Double()
+  external double get y;
+  external set y(double v);
+
+  external Pointer<Coordinate> get next;
+  external set next(Pointer<Coordinate> v);
+
+  factory Coordinate.allocate(double x, double y, Pointer<Coordinate> next) {
+    return allocate<Coordinate>().ref
+      ..x = x
+      ..y = y
+      ..next = next;
+  }
+}

tests/ffi/structs_nnbd_workaround_test.dart

@@ -0,0 +1,100 @@
+// Copyright (c) 2020, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+//
+// Dart test program for testing dart:ffi struct pointers.
+//
+// VMOptions=--deterministic --optimization-counter-threshold=50
+
+import 'dart:ffi';
+
+import "package:expect/expect.dart";
+import "package:ffi/ffi.dart";
+
+import 'coordinate_nnbd_workaround.dart';
+
+void main() {
+  for (int i = 0; i < 100; i++) {
+    testStructAllocate();
+    testStructFromAddress();
+    testStructWithNulls();
+    testTypeTest();
+    testUtf8();
+  }
+}
+
+/// allocates each coordinate separately in c memory
+void testStructAllocate() {
+  Pointer<Coordinate> c1 = Coordinate.allocate(10.0, 10.0, nullptr).addressOf;
+  Pointer<Coordinate> c2 = Coordinate.allocate(20.0, 20.0, c1).addressOf;
+  Pointer<Coordinate> c3 = Coordinate.allocate(30.0, 30.0, c2).addressOf;
+  c1.ref.next = c3;
+
+  Coordinate currentCoordinate = c1.ref;
+  Expect.equals(10.0, currentCoordinate.x);
+  currentCoordinate = currentCoordinate.next.ref;
+  Expect.equals(30.0, currentCoordinate.x);
+  currentCoordinate = currentCoordinate.next.ref;
+  Expect.equals(20.0, currentCoordinate.x);
+  currentCoordinate = currentCoordinate.next.ref;
+  Expect.equals(10.0, currentCoordinate.x);
+
+  free(c1);
+  free(c2);
+  free(c3);
+}
+
+/// allocates coordinates consecutively in c memory
+void testStructFromAddress() {
+  Pointer<Coordinate> c1 = allocate(count: 3);
+  Pointer<Coordinate> c2 = c1.elementAt(1);
+  Pointer<Coordinate> c3 = c1.elementAt(2);
+  c1.ref
+    ..x = 10.0
+    ..y = 10.0
+    ..next = c3;
+  c2.ref
+    ..x = 20.0
+    ..y = 20.0
+    ..next = c1;
+  c3.ref
+    ..x = 30.0
+    ..y = 30.0
+    ..next = c2;
+
+  Coordinate currentCoordinate = c1.ref;
+  Expect.equals(10.0, currentCoordinate.x);
+  currentCoordinate = currentCoordinate.next.ref;
+  Expect.equals(30.0, currentCoordinate.x);
+  currentCoordinate = currentCoordinate.next.ref;
+  Expect.equals(20.0, currentCoordinate.x);
+  currentCoordinate = currentCoordinate.next.ref;
+  Expect.equals(10.0, currentCoordinate.x);
+
+  free(c1);
+}
+
+void testStructWithNulls() {
+  Pointer<Coordinate> coordinate =
+      Coordinate.allocate(10.0, 10.0, nullptr).addressOf;
+  Expect.equals(coordinate.ref.next, nullptr);
+  coordinate.ref.next = coordinate;
+  Expect.notEquals(coordinate.ref.next, nullptr);
+  coordinate.ref.next = nullptr;
+  Expect.equals(coordinate.ref.next, nullptr);
+  free(coordinate);
+}
+
+void testTypeTest() {
+  Coordinate c = Coordinate.allocate(10, 10, nullptr);
+  Expect.isTrue(c is Struct);
+  Expect.isTrue(c.addressOf is Pointer<Coordinate>);
+  free(c.addressOf);
+}
+
+void testUtf8() {
+  final String test = 'Hasta Mañana';
+  final Pointer<Utf8> medium = Utf8.toUtf8(test);
+  Expect.equals(test, Utf8.fromUtf8(medium));
+  free(medium);
+}