Specific rebuild sequence throws 'Multiple widgets used the same GlobalKey' exception

[krisgiesing]

The test at https://gist.github.com/krisgiesing/afd4c3bd82458f8eeba847fe2cc7b08d appears to be legal. However, running it throws the following exception:

══╡ EXCEPTION CAUGHT BY WIDGETS LIBRARY ╞═══════════════════════════════════════════════════════════
The following assertion was thrown building _Stateful-[GlobalKey#bdeee 2](state:
_StatefulState#6790b):
Multiple widgets used the same GlobalKey [GlobalObjectKey String#77ca8].
The key [GlobalObjectKey String#77ca8] was used by multiple widgets. The parents of those widgets
were:
- _Stateful-[GlobalKey#e2abd 1](state: _StatefulState#5e00d)
- _Stateful-[GlobalKey#bdeee 2](state: _StatefulState#6790b)
A GlobalKey can only be specified on one widget at a time in the widget tree.

Widget creation tracking is currently disabled. Enabling it enables improved error messages. It can
be enabled by passing `--track-widget-creation` to `flutter run` or `flutter test`.

When the exception was thrown, this was the stack:
#0      GlobalKey._debugReserveFor.<anonymous closure> (package:flutter/src/widgets/framework.dart:189:11)
#1      GlobalKey._debugReserveFor (package:flutter/src/widgets/framework.dart:212:6)
#2      Element.updateChild.<anonymous closure> (package:flutter/src/widgets/framework.dart:2905:13)
#3      Element.updateChild (package:flutter/src/widgets/framework.dart:2909:6)
#4      ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4016:16)
#5      Element.rebuild (package:flutter/src/widgets/framework.dart:3793:5)
#6      StatefulElement.update (package:flutter/src/widgets/framework.dart:4181:5)
#7      Element.updateChild (package:flutter/src/widgets/framework.dart:2926:15)
#8      SingleChildRenderObjectElement.update (package:flutter/src/widgets/framework.dart:5210:14)
#9      Element.updateChild (package:flutter/src/widgets/framework.dart:2926:15)
#10     RenderObjectElement.updateChildren (package:flutter/src/widgets/framework.dart:4919:32)
#11     MultiChildRenderObjectElement.update (package:flutter/src/widgets/framework.dart:5319:17)
#12     Element.updateChild (package:flutter/src/widgets/framework.dart:2926:15)
#13     _LayoutBuilderElement._layout.<anonymous closure> (package:flutter/src/widgets/layout_builder.dart:115:18)
#14     BuildOwner.buildScope (package:flutter/src/widgets/framework.dart:2355:19)
#15     _LayoutBuilderElement._layout (package:flutter/src/widgets/layout_builder.dart:95:11)
#16     RenderObject.invokeLayoutCallback.<anonymous closure> (package:flutter/src/rendering/object.dart:1815:58)
#17     PipelineOwner._enableMutationsToDirtySubtrees (package:flutter/src/rendering/object.dart:875:15)
#18     RenderObject.invokeLayoutCallback (package:flutter/src/rendering/object.dart:1815:13)
#19     RenderConstrainedLayoutBuilder.layoutAndBuildChild (package:flutter/src/widgets/layout_builder.dart:173:5)
#20     _RenderLayoutBuilder.performLayout (package:flutter/src/widgets/layout_builder.dart:240:5)
#21     RenderObject._layoutWithoutResize (package:flutter/src/rendering/object.dart:1584:7)
#22     PipelineOwner.flushLayout (package:flutter/src/rendering/object.dart:844:18)
#23     AutomatedTestWidgetsFlutterBinding.drawFrame (package:flutter_test/src/binding.dart:974:23)
#24     RendererBinding._handlePersistentFrameCallback (package:flutter/src/rendering/binding.dart:279:5)
#25     SchedulerBinding._invokeFrameCallback (package:flutter/src/scheduler/binding.dart:1043:15)
#26     SchedulerBinding.handleDrawFrame (package:flutter/src/scheduler/binding.dart:982:9)
#27     AutomatedTestWidgetsFlutterBinding.pump.<anonymous closure> (package:flutter_test/src/binding.dart:869:9)
#30     TestAsyncUtils.guard (package:flutter_test/src/test_async_utils.dart:69:41)
#31     AutomatedTestWidgetsFlutterBinding.pump (package:flutter_test/src/binding.dart:856:27)
#32     WidgetTester.pumpWidget.<anonymous closure> (package:flutter_test/src/widget_tester.dart:320:22)
#35     TestAsyncUtils.guard (package:flutter_test/src/test_async_utils.dart:69:41)
#36     WidgetTester.pumpWidget (package:flutter_test/src/widget_tester.dart:317:27)
#37     main.<anonymous closure>.<anonymous closure> (file:///Users/kgiesing/Code/turquoise/flutter/packages/flutter/test/material/key_test.dart:62:20)
#51     AutomatedTestWidgetsFlutterBinding.runTest.<anonymous closure> (package:flutter_test/src/binding.dart:1063:17)
#53     AutomatedTestWidgetsFlutterBinding.runTest.<anonymous closure> (package:flutter_test/src/binding.dart:1051:35)
(elided 33 frames from class _FakeAsync, package dart:async, package dart:async-patch, and package stack_trace)

[krisgiesing]

@Hixie fyi

[krisgiesing]

[Transcribed from chat]

As far as I can tell these are the necessary factors:

1. a state object keyed by index is marked as needing rebuild
2. its child is keyed by contents
3. the map of index to contents changes before the state gets to rebuild
4. the entire tree also needs to rebuild
5. the entire tree is built using LayoutBuilder
6. the entire tree rebuilds using the new map of index to contents, changing the relationship between the index-keyed parent and the content-keyed child

This creates a situation where the two buildScope passes are using different ideas of what parents different keys are reserved for. It would all be resolved by the time the second buildScope would be done, but the exception is thrown first.

[mtcliatt]

Does the Flutter team have insight into what might be causing this issue?

[krisgiesing]

It would be great to have a Flutter team member take enough of a look at this to indicate whether it's a framework bug or something else. @Hixie suggested that perhaps the check needs to be done in a different part of the build sequence to account for LayoutBuilder; if that's true then I'm happy to look into moving it, but it seems like the issue needs a little bit more analysis on the Flutter side to know if that's the right thing to do.

[goderbauer]

It's on my radar to look at this soon, I just haven't found the time for it yet.