cannot post an Error, Map or Set between worker threads

[Fenzland]

// index.js

const worker= new Worker( './worker.js', { name:'worker', type:'module', }, );

worker.onmessage= ( { data, }, )=> {
	console.assert( data.error instanceof Error, );
	console.assert( data.error.hasOwnProperty( 'stack', ), );
	console.assert( data.map instanceof Map, );
	console.assert( data.map.has( 'foo', ), );
	console.assert( data.map.get( 'foo', ) === 'bar', );
	console.assert( data.set instanceof Set, );
	console.assert( data.set.has( 'foo', ), );
};
worker.postMessage( {
	error: new Error(),
	map: new Map( [ [ 'foo', 'bar', ], ], ),
	set: new Set( [ 'foo', ], ),
}, );

// worker.js

globalThis.onmessage= ( { data, }, )=> {
	console.assert( data.error instanceof Error, );
	console.assert( data.error.hasOwnProperty( 'stack', ), );
	console.assert( data.map instanceof Map, );
	console.assert( data.map.has( 'foo', ), );
	console.assert( data.map.get( 'foo', ) === 'bar', );
	console.assert( data.set instanceof Set, );
	console.assert( data.set.has( 'foo', ), );
	
	globalThis.postMessage( {
		error: new Error(),
		map: new Map( [ [ 'foo', 'bar', ], ], ),
		set: new Set( [ 'foo', ], ),
	}, );
	
	globalThis.close();
};

In Chrome, all the assert successful. But in deno (until 0.23.0), they fail. After post between threads, errors, maps and sets become empty objects.

[kitsonk]

Hmmmmm... it seems that the structure clone on postMessage shouldn't be handling Error or Function objects. It feels like the browser must be filtering them out and cloning them in some other way: https://developer.mozilla.org/en-US/docs/Web/API/Web_Workers_API/Structured_clone_algorithm

I will admit I don't know how we implemented cloning in Deno for workers...

[kevinkassimo]

@kitsonk we currently stringify data as JSON into some buffer that is passed directly to the Rust side and pushed as a message through a channel, where the receiving end polls and parses the JSON as data.

This is indeed not fully complying with structured clone requirements (e.g. transferables). But for Errors I would expect us to simply throw an error when attempted.

[Fenzland]

JSON.stringify not only lose errors, also miss maps and sets. They are both pass correctly in browsers too.

[kevinkassimo]

@Fenzland I agree we will need to fix this. /cc @bartlomieju for worker stuff

[kitsonk]

There is the value serialiser in V8 that does the structured cloning: https://v8docs.nodesource.com/node-7.10/df/d48/classv8_1_1_value_serializer.html

I am sure there must be APIs too for transferring as well, I just haven't found them yet.

[bartlomieju]

@kitsonk https://denolib.github.io/v8-docs/classv8_1_1ValueDeserializer.html ?

[kitsonk]

Well that and: https://denolib.github.io/v8-docs/classv8_1_1ValueSerializer.html so post should serialize the message and onmessage should be called with the deserialized message I think. I am not so hot at the V8 APIs though.

[bartlomieju]

Makes sense. This issue (or more general workers) are in my backlog, but I don't know when I'll get to them. If someone wants to take a stab at it I'll be happy to provide support

[kevinkassimo]

@bartlomieju Oh I almost forgot I still have that v8 docs up... it has not been updated for a while. I mostly just refer to https://cs.chromium.org/chromium/src/v8/include/v8.h , which is well updated and, since it is using Google's CodeSearch, easy to look up references and usages than on Github.

[kitsonk]

@bartlomieju we should close in favour of #3557 as that solution would solve all of this properly.