[Webkit-unassigned] [Bug 51351] New: UI process should respond to synchronous messages from the web process on a non-main thread by default

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=51351

           Summary: UI process should respond to synchronous messages from
                    the web process on a non-main thread by default
           Product: WebKit
           Version: 528+ (Nightly build)
          Platform: All
        OS/Version: All
            Status: NEW
          Keywords: NeedsRadar
          Severity: Normal
          Priority: P2
         Component: WebKit2
        AssignedTo: webkit-unassigned at lists.webkit.org
        ReportedBy: aroben at apple.com
                CC: andersca at apple.com, sam at webkit.org


The UI process currently responds to both synchronous and asynchronous messages from the web process on the main thread. Messages are received on the CoreIPC::Connection's work queue and then forwarded to the main thread for processing.

This can cause deadlocks on Windows when windowed plugins are involved. For example, take the case of a YouTube video embedded into some third party web page. Here's what happens:

Web process:
Clicking on the plugin opens a new window so that it can be navigated to the video's YouTube page. In order to open the new window, the web process sends a synchronous WebPageProxy::CreateNewPage message and blocks its main thread to wait for the reply.

UI process:
The UI process receives the WebPageProxy::CreateNewPage message, and ends up calling through to WKPageUIClient::createNewPage. The client app responds to this callback by creating a new window and selecting it. Selecting the new window causes the plugin to lose focus. As part of unfocusing the plugin, Windows sends the plugin a synchronous WM_KILLFOCUS message to it and blocks the main thread while waiting for the message to be handled.

Since the web process's main thread is still blocked waiting for the reply to WebPageProxy::CreateNewPage, it can't process the WM_KILLFOCUS message, so the two processes deadlock. (The web process is waiting for CreateNewPage to be handled, and the UI process is waiting for WM_KILLFOCUS to be handled.)

We should make the UI process respond to synchronous messages on a non-main thread by default. We'll want to continue making WK2 client callbacks (e.g., WKPageUIClient::createNewPage) on the main thread, but those need to happen asynchronously to avoid deadlocks.

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