313692 2026-04-29 15:38:34 -0700 [Site Isolation] postMessage of SharedArrayBuffer doesn't work in the same process 2026-05-05 15:02:17 -0700 1 1 1 Unclassified WebKit New Bugs WebKit Nightly Build Unspecified Unspecified RESOLVED FIXED InRadar P1 Normal --- 1 a.tarbinian webkit-unassigned webkit-bug-importer oldest_to_newest 2206015 0 a.tarbinian 2026-04-29 15:38:34 -0700 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. 2206016 1 webkit-bug-importer 2026-04-29 15:38:39 -0700 <rdar://problem/175890575> 2206018 2 a.tarbinian 2026-04-29 15:56:34 -0700 Pull request: https://github.com/WebKit/WebKit/pull/63933 2207805 3 ews-feeder 2026-05-05 15:02:15 -0700 Committed 312641@main (e2355e43e2ef): <https://commits.webkit.org/312641@main> Reviewed commits have been landed. Closing PR #63933 and removing active labels.