Add stdin stream support.

[PaulKinlan]

Many applications stream STDIN to allow ffmpeg to encode from a data source that is not a file, or a data source that will never end (i.e, livestreaming a web cam).

Right now the lib can't support it because stdin read in WASM needs to be blocking (which freezes the thread) and this means the event loop is frozen too so that data that would have been sent via postMessage can't be processed.

Describe the solution you'd like
I added stdin buffer support to a fork of the other ffmpeg.js that allows STDIN to be read inside a worker. PaulKinlan/ffmpeg.js#1

It uses SharedArrayBuffers to communicate between the window and the worker, so this limit's it to Chrome on Desktop at the moment. It does however allow ffmpeg to block on stdin and then recieve data from the window asynchronously.

[jeromewu]

This is an interested one, you can check the latest version (^0.8.1) to see if we can do it after the major refectoring.
I will take a look when available, thanks.

[PaulKinlan]

Yep. i can certainly have a look.

[ROBERT-MCDOWELL]

Just following up about this important feature. Is the last git can let us experiment it?

[deepesh-agarwal]

This feature would be very helpful.

[Banou26]

Is there an ETA on that major refactoring ?

after the major refectoring

If the refactor is not gonna happen(since i have not seen any commits on this repo & your fork of FFMPEG for months), I'm really interested in the stdin stream support and would like to work on it if possible, but i have very small knowledge of any native/wasm tools that this project uses, so if you could tell me where to start/what to look for i'd really appreciate it.

[creativefctr]

@jeromewu any updates on this?

[jeromewu]

@creativefctr sadly, the answer is no and I would say it is hard to achieve considering the limitation we have right now.

[bradisbell]

Hello @jeromewu. The limitation you mentioned... is that lack of SharedArrayBuffer support in browsers? It seems that SharedArrayBuffer is available again. Is it now possible to add STDIN support? Or, is there some other limitation?

[jeromewu]

Hmm, it is not SharedArrayBuffer that introduces this limitation. It is more like the current state of WebAssembly cannot handle stdin, so I cannot find a way to support stdin at the moment.

[Banou26]

Not sure if most people wanna get into this but you can pretty easily make a small LibAV C++ API compiled to WASM that deals with only computing when necessary to stream data in and out of WASM-land, this is obviously not as simple as this library's plug and play API but if you really want it you can manage to do smth with a little bit of work, I have almost no prior experience with C++ but still managed to make a transmuxer work with streaming in WASM.