HTML General Review: Cross-Document Messaging

[travisleithead]

Hello TAG！

This issue is part of the TAG's larger effort to review the HTML spec in its entirety--please see the original issue https://github.com/w3ctag/design-reviews/issues/174 for a summary of all the break-out issues.

The "Sections" are all the sections of the WHATWG HTML spec that should be reviewed as part of this issue. Where the spec section has associated Web Platform Tests, the specific WPT path is noted. While the primary focus of the review is the specification text, it can be helpful to review the related tests to help clarify algorithms or see interoperability conformance issues (or find issues with the tests).

The "Features" are just a sample of what you will encounter as part of this spec section, it's not meant to be exhaustive.

Here are some example suggestions for what to look for during the review, but don't limit to only these suggestions!

• Look for any APIs that could be 'modernized' according to current design practices.
• Look for things that could require permissions that aren't modelled in the permissions API at the moment.
• Look for areas of the platform that contain UA 'magic' (aren't possible for JavaScript programs to emulate due to missing primitives in the platform). These are candidates for future Extensible Web archeology.
• Look for areas of the spec that describe "wishful thinking" (e.g., that describe a feature that is implemented by no one). Such features should at least have implementor commitments, or they might be candidates for removal from the spec.
• Look for cryptic and hard-to-follow algorithms that could be improved with extra explanatory text or improved prose. E.g., sometimes adding a "developer note" (green box) can add the needed clarity to understand the intent/purpose or outcome of a complex concept.
• Look for concepts that are meant to be used together, but where this is not spelled out or explained clearly

Sections	WPT path	Features
9.1, 9.4, 9.5	webmessaging	MessageChannel, BroadcastChannel

Please provide feedback as (please select one):

• [ ] open issues in our Github repo for each point of feedback
• [ ] open a single issue in our Github repo for the entire review
• [x] leave review feedback as a comment in this issue and @-notify [github usernames]

[travisleithead]

initMessageEvent sadly cannot be removed for web compat I assume?

Edit: per Chrome status, usage has been dropping over the years to the point that use of this API is almost non-existent. https://www.chromestatus.com/metrics/feature/popularity#InitMessageEvent Maybe we can in fact remove it!

[travisleithead]

User agents are also encouraged to consider rate-limiting message traffic between different origins, to protect naïve sites from denial-of-service attacks.

Is this rate-limiting actually done in any UA's? Seems like general good advice to UA, but if none do it, perhaps it's not considered a major threat.

[travisleithead]

The message infrastructure (structured serialize) has no way of allowing for customization of serialization, unlike JSON, with it's toJSON check to enable an object to serialize an alternative representation. For many objects, we think there is likely some kind of custom toStructuredSerialize that should have a correlated reviver function that is related to the specific object (e.g., not a generic reviver function), which could be mapped via tag or Symbol to the graph. This feels like a significant missing part of the infrastructure.

Also, cloning is synchronous today, because the intermediate cloned structure is not visible to the web platform. For many applications, having an asynchronous structured clone process would be preferable, but raises questions as to what the representation of the intermediate object should be. Perhaps it could be an opaque object like an ImageBitmap or CanvasProxy.

[slightlyoff]

The lack of an onclose event in the MessageChannel interface presents a persistent usability hazard.

Previously offered arguments about GC observability have not withstood the passage of time. The addition of Web Audio Worklets, Shared Array Buffers, and Atomics have created fine-grained locking systems which were previously not available. Recently renewed work on Weak References highlights the ways in which observability across turns is not a threat. Read consistency is only required within the same turn, and it is easy to satisfy this requirement by adding one-turn latency in dispatching close notifications. This should have no adverse performance impact.

Please add port closed events!

[slightlyoff]

/cc @kinu @mattto @wanderview

[travisleithead]

9.5.1.2. Ports as the basis of an object-capability model on the web

This got the juices flowing. Alex pointed me to comlink, which does almost exactly what I was thinking--actually realize a capability API on top of a MessagePort object. It would be really nice to see this baked into the platform.

[travisleithead]

//cc @aliams for workers stuff.

[travisleithead]

Re: https://github.com/w3ctag/design-reviews/issues/269#issuecomment-407584290, section 9.5.4. is terrible advice regarding the polling system for detecting closed message ports. Close event would be the right way to rectify this..