Bug 314583 – [BFCache] CachedPage::restore() leaks cached frame state when focus restoration is skipped

• Component: WebCore Misc.
• Status: RESOLVED FIXED

[Basuke Suzuki]

CachedPage::restore() has an early-return at `if (!focusedOrMainFrame) return;`
that bypasses the `clear()` call at the end of the function. The cached frame
state (m_cachedMainFrame holding CachedFrames with documents/views) is never
released, even though the cached page's data has already been transferred to
the live page by `m_cachedMainFrame->open()`.

This causes ~CachedPage to call `m_cachedMainFrame->destroy()` later. After
the fix in bug 314238 (which destroys child CachedFrames before checking
m_document), destroy() now descends into child CachedFrames whose documents
have already been resumed by open() (state = NotInBackForwardCache). The
ASSERT at CachedFrame.cpp:291

    ASSERT(document->backForwardCacheState() == Document::InBackForwardCache);

then fires, and the subsequent destroy code path corrupts live document state
(detachViewsAndDocumentLoader, detachFromPage, removeAllEventListeners),
hanging the WebContent process.

The early-return path is reached when `page.focusController().focusedOrMainFrame()`
returns null, which happens for iframe-process BFCache restoration where the
page's main frame is a RemoteFrame and there is no LocalFrame to focus.

Fix: call `clear()` in the early-return branch to release the cached frame
state, since `open()` has already done its work and the cached storage is
no longer needed.

[Basuke Suzuki]

Tracked in rdar://176824162

[Basuke Suzuki]

<rdar://problem/176824162>

[Basuke Suzuki]

Pull request: https://github.com/WebKit/WebKit/pull/64708

[EWS]

Committed 313043@main (11ea56301070): <https://commits.webkit.org/313043@main>

Reviewed commits have been landed. Closing PR #64708 and removing active labels.