Bug 277817 – REGRESSION(281440@main): Web content can become inaccessible after dynamic webpage changes

• Component: Accessibility
• Status: RESOLVED FIXED

[Tyler Wilcock]

In http://commits.webkit.org/281440@main, we prevented a bug where AXIsolatedTree::m_protectedFromDeletionIDs was cleared too early, which in turn caused objects to be incorrectly removed from AXIsolatedTree::m_nodeMap, eventually breaking the accessibility tree. We fixed this by moving:

m_pendingProtectedFromDeletionIDs.formUnion(std::exchange(m_protectedFromDeletionIDs, { }));

from queueRemovalsLocked to queueAppendsAndRemovals (which should happen once per tree-update cycle, making it the right time to clear this list).

This caused a new bug, however, in this sequence:

1. The main-thread starts a tree update via AXIsolatedTree::updateChildren
2. In doing so, we call queueRemovalsLocked, i.e. because an object lost a child (but that child isn't deleted, just has a different parent, so we mark it as protected on the main-thread in m_protectedFromDeletionIDs)
3. We call queueRemovalsLocked for that child and any others no longer children of the original object. After 281440@main, we no longer sync protected IDs in this function.
4. Before the full main-thread tree-update (AXIsolatedTree::updateChildren) is finished, resulting in a call to queueAppendsAndRemovals, the secondary thread processes a request from an AT, causing AXIsolatedTree::applyPendingChanges to run on the secondary thread (as expected to ensure we service the request with the most up-to-date information)
5. We delete the re-parented child, and any of its descendants, from the accessibility thread data structures because we failed to protect it
6. queueAppendsAndRemovals eventually finishes on the main-thread, and we sync the protected objects to the secondary thread, but at that point it's too late.
7. The accessibility tree is now broken, with random objects missing entirely.

[Radar WebKit Bug Importer]

<rdar://problem/133477773>

[Tyler Wilcock]

Pull request: https://github.com/WebKit/WebKit/pull/31907

[EWS]

Committed 282266@main (71ce3f1d011a): <https://commits.webkit.org/282266@main>

Reviewed commits have been landed. Closing PR #31907 and removing active labels.

[EWS]

Committed 280938.234@safari-7619-branch (42c3706a5669): <https://commits.webkit.org/280938.234@safari-7619-branch>

Reviewed commits have been landed. Closing PR #1601 and removing active labels.