Reslove Blob to an existing ArrayBuffer

[Zhang-Junzhi]

Currently, Blobs can only be resloved with a newly-created ArrayBuffer.

Sometimes, it would be much more efficient if a Blob can be directly resolved to an existing ArrayBuffer(If the size of ArrayBuffer is sufficient). For example, a content of a very large file can be directly read to the ArrayBuffer of a WASM memory. Without this feature, we need to first call File.ArrayBuffer to resolve the large content to a newly-created ArrayBuffer, and then copy it to the WASM memory.

[annevk]

Where is reslove defined?

What would the exact semantics of this be?

[Zhang-Junzhi]

Sorry, to clarify the definition:

Reslove here means returning a Promise that resolves with the contents of the blob as binary data contained in the ArrayBuffer, like Blob.ArrayBuffer, FileReader.readAsArrayBuffer

[annevk]

Would there be a single write for all bytes?

[Zhang-Junzhi]

AFAIK, currently there's no such way of resolving Blob to an existing ArrayBuffer(Or I am happy to be wrong).

If I use WebAssembly, and I want to read the content of a file, I need to first call File.ArrayBuffer to resolve the large content to a newly-created ArrayBuffer, and then copy it to the WASM memory. This means double writes.

[annevk]

I understand, I was basically asking if the writing would be done similarly to https://encoding.spec.whatwg.org/#dom-textencoder-encodeinto. So unless you're multi-threaded and use SharedArrayBuffer, you cannot observe a partially filled buffer.

Also, if we're doing this we should probably do it by changing the Body mixin in Fetch.

[Zhang-Junzhi]

I understand, I was basically asking if the writing would be done similarly to https://encoding.spec.whatwg.org/#dom-textencoder-encodeinto. So unless you're multi-threaded and use SharedArrayBuffer, you cannot observe a partially filled buffer.

Also, if we're doing this we should probably do it by changing the Body mixin in Fetch.

I am not sure which specific way to resolve the issue, but just raised the efficiency issue in my use case.

[Zhang-Junzhi]

I understand, I was basically asking if the writing would be done similarly to https://encoding.spec.whatwg.org/#dom-textencoder-encodeinto. So unless you're multi-threaded and use SharedArrayBuffer, you cannot observe a partially filled buffer.

Did you mean that I can use SharedArrayBuffer together with encodeInto via multi-threads, so WASM can work early while still coping buffer to WASM memory, as a workaround to reduce latency?

But consider if I am reading a 100MB file which is not of streaming format, that means unless the whole 100MB content of the file has been ready in WSAM memory, partically ready content doesn't have much value. In that case, SharedArrayBuffer + encodeInto still doesn't help much.

[mkruisselbrink]

If I understand what is being requested correctly, I believe you can mostly do that already. I.e. to read a Blob into a script-supplied (pre-allocated) ArrayBuffer, you can call Blob.stream() to get a ReadableStream, and then use a ReadableStreamBYOBReader to read from that stream into a script-supplied array buffer.

[Zhang-Junzhi]

Good method! Thanks for your reply. That is a specific guide to achieve my purpose(Though it seems none of the major browsers has implemented ReadableStreamBYOBReader yet.

[Zhang-Junzhi]

One more issue to put this topic further:

If WASM runs in a worker thread, since File blobs can only be used in the window thread. two copies still seems unavoidable, because WASM memory buffer cannot be detached and transferred to the window thread, and File objects in the window thread cannot be used in the worker thread.

[annevk]

You can message a File object to a worker. If that's not sufficient for some reason it might be best to open a separate issue for that as it seems different from reading a blob into an existing buffer.

[Zhang-Junzhi]

You can message a File object to a worker.

Oops, didn't know of that. Thanks for pointing out.

After checking the definition of File, and structuredserializeinternal in the HTML spec, I realised I had misunderstood File.

If that's not sufficient for some reason it might be best to open a separate issue for that as it seems different from reading a blob into an existing buffer.

Since it's still connected to the topic "read Blob into an allocated ArrayBuffer", just a special case of it. So I decided to post it in the same issue.

[jimmywarting]

fyi a ReadableStream is also transferable with postMessages

try {
  const readable = new ReadableStream()
  const mc = new MessageChannel()
  mc.port1.postMessage(readable, [readable])
  // tested support for transferable readable streams
} catch(err) {}

but it only works in chrome right now i think...

[annevk]

Let's dupe this into #83.