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HTML and shared memory

by Anne van Kesteren in Browser API

You’d think that the HTML Standard would be pretty far removed from shared memory considerations, but as it happens HTML defines a parser for HTML which is intertwined with script execution, defines a way to instantiate new global objects through the iframe element, defines a way to instantiate new threads (and even processes, depending on the implementation) with workers, and all the various infrastructure pieces that go along with that. Finally, it also defines a message-based communication channel to communicate between those threads and processes.

That still doesn’t give us shared memory. For that, JavaScript needed to evolve and gain a new SharedArrayBuffer class: a sibling to ArrayBuffer, with the ability to be accessed from several threads at once. And on top of that we needed to do some work to make it play nicely with all the various globals the web platform provides and make sure it worked with the message-passing system (which you probably know as postMessage()), all while trying to avoid violating constraints that would make programming with SharedArrayBuffer objects a nightmare.

We ended up making several changes (and to make sure they all end up being interoperable we wrote accompanying tests):

As always, nothing is perfect and there are some gotchas without a good solution:

Although the above covers the integration of shared memory into the foundations of the web platform, there is still ongoing work on allowing specific APIs to accept and operate on shared memory. This requires changes to IDL to introduce a mechanism for safelisting APIs that can operate on SharedArrayBuffer objects, as well as updating specifications to use that new safelisting mechanism, and of course writing tests for these spec changes. This work is still ongoing, but at least now it can build on top of a solid foundation.

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