Bug 313692 – [Site Isolation] postMessage of SharedArrayBuffer doesn't work in the same process

• Component: New Bugs
• Status: RESOLVED FIXED

[Anthony Tarbinian]

The following tests are failing with site isolation enabled:

imported/w3c/web-platform-tests/html/infrastructure/safe-passing-of-structured-data/messagechannel.any.html
imported/w3c/web-platform-tests/html/infrastructure/safe-passing-of-structured-data/messagechannel.any.serviceworker.html
imported/w3c/web-platform-tests/html/infrastructure/safe-passing-of-structured-data/messagechannel.any.sharedworker.html
imported/w3c/web-platform-tests/html/infrastructure/safe-passing-of-structured-data/messagechannel.any.worker.html
imported/w3c/web-platform-tests/html/infrastructure/safe-passing-of-structured-data/shared-array-buffers/window-messagechannel-success.https.html

These test failures involve when postMessage is called to pass a SharedArrayBuffer, 
to another context (i.e. another frame, window, worker). 
See the example JS below:

    const channel = new MessageChannel();
    const sab = new SharedArrayBuffer(16);
    channel.port1.postMessage(sab);

There is an optimization in WebKit to keep the MessagePorts
in a local HashMap (WebMessagePortChannelProvider::m_inProcessPortMessages)
to avoid performing unecessary IPC if the sender and receiver
are in the same process.
See https://commits.webkit.org/255948@main

This optimization caused issues with site isolation enabled
since it's possible for the target to be in a different process.
The optimization was undone in https://commits.webkit.org/295627@main
Without the optimization, all messages sent over a MessageChannel
are sent over IPC with site isolation enabled.

See the following code in WebMessagePortChannelProvider::postMessageToRemote
which falls back to IPC when the port is not found in m_inProcessPortMessages.
Remote here doesn't mean a different process, its just the receiver of postMessage.

    void WebMessagePortChannelProvider::postMessageToRemote(MessageWithMessagePorts&& message, const MessagePortIdentifier& remoteTarget)
    {
        auto iterator = m_inProcessPortMessages.find(remoteTarget);
        if (iterator != m_inProcessPortMessages.end()) {
            iterator->value.append(WTF::move(message));
            WebProcess::singleton().messagesAvailableForPort(remoteTarget);
            return;
        }

        for (auto& port : message.transferredPorts)
            messagePortSentToRemote(port.first);

        protect(networkProcessConnection())->send(Messages::NetworkConnectionToWebProcess::PostMessageToRemote { message, remoteTarget }, 0);
    }


However, this causes issues for messages which are not serializable over
IPC, such as SharedArrayBuffer which are marked as [NotSerialized]
https://searchfox.org/wubkat/source/Source/WebKit/Shared/WebCoreArgumentCoders.serialization.in#8013

This means that sending a SharedArrayBuffer will fail even when
the sender and receiver are in the same process and there is no 
need to cross IPC.

[Radar WebKit Bug Importer]

<rdar://problem/175890575>

[Anthony Tarbinian]

Pull request: https://github.com/WebKit/WebKit/pull/63933

[EWS]

Committed 312641@main (e2355e43e2ef): <https://commits.webkit.org/312641@main>

Reviewed commits have been landed. Closing PR #63933 and removing active labels.