[ddc] clear constants properly on a hot restart

[vsmenon]

It looks like we're missing some state that should be cleared, but we had to revert

https://dart-review.googlesource.com/c/sdk/+/152581

due to issues in Flutter:

flutter/flutter#60286

[vsmenon]

fyi - @nshahan @Markzipan @jonahwilliams - can you add some context for when we revisit? Sounds like there may be an issue with enums and a potential concern with startup time.

[jonahwilliams]

Startup time for the first reload isn't a big concern, mostly it seemed like a symptom of something else going wrong. For reference the actual error being hit is:

══╡ EXCEPTION CAUGHT BY RENDERING LIBRARY ╞═════════════════════════════════════════════════════════
js_primitives.dart:30 The following assertion was thrown during performLayout():
js_primitives.dart:30 Assertion failed: "Unsupported TextAlign value TextAlign.start"
js_primitives.dart:30 
js_primitives.dart:30 The relevant error-causing widget was:
js_primitives.dart:30   Text-[<'title'>]
js_primitives.dart:30   file:///Users/jonahwilliams/Documents/flutter/examples/hello_world/lib/main.dart:11:9
js_primitives.dart:30 
js_primitives.dart:30 When the exception was thrown, this was the stack:
js_primitives.dart:30 dart-sdk/lib/_internal/js_dev_runtime/private/ddc_runtime/errors.dart 214:49  throw_
js_primitives.dart:30 lib/_engine/engine/text/paragraph.dart 1691:3                                 textAlignToCssValue
js_primitives.dart:30 lib/_engine/engine/text/paragraph.dart 1380:28                                _applyParagraphStyleToElement
js_primitives.dart:30 lib/_engine/engine/text/paragraph.dart 1012:5                                 new
js_primitives.dart:30 lib/ui/src/ui/text.dart 1493:21                                               new
js_primitives.dart:30 packages/flutter/src/painting/text_painter.dart 559:46                        layout
js_primitives.dart:30 packages/flutter/src/rendering/paragraph.dart 502:18                          [_layoutText]
js_primitives.dart:30 packages/flutter/src/rendering/paragraph.dart 525:5                           [_layoutTextWithConstraints]
js_primitives.dart:30 packages/flutter/src/rendering/paragraph.dart 597:5                           performLayout
js_primitives.dart:30 packages/flutter/src/rendering/object.dart 1769:7                             layout
js_primitives.dart:30 packages/flutter/src/rendering/shifted_box.dart 396:13                        performLayout
js_primitives.dart:30 packages/flutter/src/rendering/object.dart 1769:7                             layout
js_primitives.dart:30 packages/flutter/src/rendering/view.dart 169:48                               performLayout
js_primitives.dart:30 packages/flutter/src/rendering/object.dart 1632:7                             [_layoutWithoutResize]
js_primitives.dart:30 packages/flutter/src/rendering/object.dart 889:17                             flushLayout
js_primitives.dart:30 packages/flutter/src/rendering/binding.dart 404:19                            drawFrame
js_primitives.dart:30 packages/flutter/src/widgets/binding.dart 867:13                              drawFrame
js_primitives.dart:30 packages/flutter/src/rendering/binding.dart 286:5                             [_handlePersistentFrameCallback]
js_primitives.dart:30 packages/flutter/src/scheduler/binding.dart 1115:15                           [_invokeFrameCallback]
js_primitives.dart:30 packages/flutter/src/scheduler/binding.dart 1054:9                            handleDrawFrame
js_primitives.dart:30 packages/flutter/src/scheduler/binding.dart 863:7                             <fn>
js_primitives.dart:30 dart-sdk/lib/_internal/js_dev_runtime/private/isolate_helper.dart 48:19       internalCallback
js_primitives.dart:30 
js_primitives.dart:30 This RenderObject had the following child:
js_primitives.dart:30     text: TextSpan
js_primitives.dart:30 ══════════════════════════════════════════════════

The code that is throwing is a switch statement here: https://github.com/flutter/engine/blob/master/lib/web_ui/lib/src/engine/text/paragraph.dart#L1664 . The error seems to be cause by the switched over value no longer being equal (identical?) to any of the cases.

[sigmundch]

Here is a scenario that yields an inconsistent state with the original patch applied:

main.dart:

import 'a1.dart';
import 'a2.dart';

void main() {
  print(value1 == value2);
}

a1.dart:

class B {
  final int x;
  const B(this.x);
}

B get value1 = const B(2);

a2.dart:

import 'a1.dart';

int counterForForcingRecompile = 23;
B get value2 = const B(2);

Repro requires to load the app as is, modify the value of counterForFocrcingRecompile in a2.dart and then issue a hot restart, but keep a1.dart intact and value2 intact too. This will print true on the first load, but false after the hot restart.

What happens here is that DDC has 2 caches:

• a global cache to properly canonicalize constants in a program
• a module-level cache for constants used within a module

Today, the module-level cache is only cleared for modules that are reloaded, but not across the entire program. For example, the code above reloads 2 of 3 modules: a2 (modified) and main (depends on a2). Because a1 is not reloaded, the module-level cache of a1 becomes stale and inconsistent with the rest of the program. By keeping global cache unmodified, we shadowed these inconsistencies.

Going forward, we think we should clear the global cache and all module caches to maintain a consistent state across hot restarts.

[nshahan]

Fixed: https://dart-review.googlesource.com/c/sdk/+/347220

We suspect some module local caches were missed when clearing in the orignal change that got rolled back.