ImageData can't be structured-cloned by postMessage

[pshaughn]

This is tested in WPT workers/interfaces/DedicatedWorkerGlobalScope/postMessage/imagedata-cloned-canvas-in-array.html and structured-clone-imagedata.html; postMessage throws DataCloneError.

[pshaughn]

21715 is a prerequisite for doing this right.

[jdm]

Not necessarily; we could continue to hand-write serialization code in https://github.com/servo/servo/blob/master/components/script/dom/bindings/structuredclone.rs.

[pshaughn]

Yes, we could. The set of types that need structured-cloning is finite, and most of them are basically the same as an ArrayBuffer plus some constant-size fields.

[gterzian]

Since #21715 was opened, the situation has improved also quite a lot, if I say so myself. Although so far the "improvements" have been focused on supporting more "complicated" DOM objects, like Blob and Messageport.

So we do already have dom::bindings::serializable::Serializable, and I had a quick look at ImageData, I think implementing it for that would be mostly about gettting a Vec<u8> out of the Uint8ClampedArray stored as Heap<*mut JSobject>, and adding a bit of glue in structuredclone.rs, but it wouldn't be hugely manual with actually having to write the Vec<u8> using the mozjs structured clone API like was still done when #21715 was first opened.

So I don't think it would be very hard, I can help if someone wants to do it. If you look at Blob, it should be like that, but easier because you don't have to deal with the complications of Blob that can be seen in globalscope.rs.

[gterzian]

If you look at ImageData.as_slice, it would be essentially that followed by into_vec'ing it, then you would need to clone the height and width, and store the whole thing in the StructuredDataHolder, probably in a simple Vec since you don't need to identify the image data uniquely I think(like you needed with a blob).

So in script_traits::serializeable.rs, you could add something like:

#[derive(Debug, Deserialize, MallocSizeOf, Serialize)]
pub struct SerializedImageData {
    data: Vec<u8>,
    width: u32,
    height: u32,
}

Then in structuredclone.rs, add an Option<Vec<SerializedImageData>> to both variants of StructuredDataHolder, and write into/read from those in the implementation of dom::bindings::serializable::Serializable for ImageData.

[gterzian]

You would also need to add the Vec<SerializedImageData> to script_traits::StructuredSerializedData

[gterzian]

(and yes some of this is boilerplate that could be generated as suggested in https://github.com/servo/servo/issues/21715, the "hardest" part is the actual implementation of dom::bindings::serializable::Serializable, and even that should not be much harder than calling ImageData.as_slice)