1864360 - [wpt-sync] Sync PR 43102 - Reland "[A11y] Do not allow asymmetrical parent-child relations in tree"

Status: RESOLVED FIXED

(Testing :: web-platform-tests, task, P4)

Description

[Web Platform Test Sync Bot [:wpt-sync] (Matrix: #interop:mozilla.org)]

Sync web-platform-tests PR 43102 into mozilla-central (this bug is closed when the sync is complete).

PR: https://github.com/web-platform-tests/wpt/pull/43102
Details from upstream follow.

Aaron Leventhal <[e-mail]> wrote:

Reland "[A11y] Do not allow asymmetrical parent-child relations in tree"

This is a reland of commit 0953e5a0ef7748af2c932568ec9d9bbdbc480f11

Original change's description:

[A11y] Do not allow asymmetrical parent-child relations in tree

Design doc:
https://docs.google.com/document/d/1PnjhlWqTtivmsUVTF0kqMAUIsO9a_eTp32pRY4KJh_U/edit#heading=h.wlgzk7gh4m76

Fix failures brought to light by the Eager AX Tree Updates project and related checks, so that serialization operates on a truly frozen, complete AX Tree, and does not lead to changes to the underlying data at unsafe times. After landing this CL and any follow-ups, the goal is to have a vastly more stable, predictable Blink accessibility engine.

The current implementation can lead to processing the tree when there
are "holes" in the data structure, such as missing parents or children,
which can lead to crashes, checks and unpredictability.

1. Avoid holes in the first place: do not call AXObject::ClearChildren() immediately when calling AXObject::SetNeedsToUpdateChildren(), so that missing parent and missing child holes are not created for unchanged siblings. These holes tended to sit around and cause problems when downstream operations did not expect them. Only call ClearChildren() right before rebuilding the children, in UpdateChildrenIfNecessary().
2. More complete repairs: Instead of only repairing missing parents, when a parentless ensure a complete subtree structure around the child, starting with the included parent. The new method to accomplish this is called AXObjectCacheImpl::RepairIncludedParentsChildren(). It makes sure that every parent up to the included parent has a complete set of children.
3. More timely repairs: instead of waiting for a hole to be discovered while tree walking, eagerly ensure that objects up to their included parent have repairs, whenever an object is being retrieved for deferred event processing, or when an object is being created in the middle of a tree. If the necessary parents do not exist, and thus the AXObject itself is not viable, make sure any stale AXObjects in that subtree are eagerly removed, because they are also not viable.
4. More complete tree structure checks in AXObjectCacheImpl::CheckTreeIsUpdated(), which ensure symmetrical included parent-child relationships are complete. This helps guarantee that no lazy computations try to alter the tree while it’s being serialized (in a frozen state). The new checks do not pass without the other code changes, which are in service of the new checks.

Future work: attempt to avoid any tree repairs and guarantee completeness in more places, providing even more predictability, for example, by handling CSS display changes similarly to role changes.

Fixed: 1422755,1483877,1482591,1481940,1480442,1488246,1486249,1484029,1353205,1480627,1480429,1494849,1493953,1484394,1489027,1491163
Change-Id: Ied4258680ffe4099caaf4c5e614c59c70c61a013
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4873421
Reviewed-by: Chris Harrelson \<[e-mail]>
Commit-Queue: Aaron Leventhal \<[e-mail]>
Cr-Commit-Position: refs/heads/main@{#1223164}

Change-Id: I96fcc71601842ea7b38de70ee24f77df1f011f24
Cq-Do-Not-Cancel-Tryjobs: true
Reviewed-on: https://chromium-review.googlesource.com/5019353
WPT-Export-Revision: 5a87f6f56f9feb7703d9b5631633f5190be85210

Comment 1

[Web Platform Test Sync Bot [:wpt-sync] (Matrix: #interop:mozilla.org)]

Pushed to try (stability) https://treeherder.mozilla.org/#/jobs?repo=try&revision=26f1af2fb3818782cee679389904354c35195d91

Comment 2

[Pulsebot]

Pushed by [e-mail]:
https://hg.mozilla.org/integration/autoland/rev/1fb1a86ff6d4
[wpt PR 43102] - Reland "[A11y] Do not allow asymmetrical parent-child relations in tree", a=testonly

Comment 3

[Web Platform Test Sync Bot [:wpt-sync] (Matrix: #interop:mozilla.org)]

Test result changes from PR not available.

Comment 4

[Pulsebot]

Pushed by [e-mail]:
https://hg.mozilla.org/integration/autoland/rev/078e4a1b40e6
[wpt PR 43102] - Reland "[A11y] Do not allow asymmetrical parent-child relations in tree", a=testonly

Comment 5

[Cosmin Sabou [:CosminS]]

https://hg.mozilla.org/mozilla-central/rev/078e4a1b40e6