[FFmpeg-devel] [RFC] Event loop

[Mark Thompson]

On 18/02/2021 15:39, Nicolas George wrote:
> Mark Thompson (12021-02-17):
>> I'm not sure you can avoid writing Windows code for this to work at
>> all, because I don't think poll() as we have now is sufficient to get
>> the functionality you want.
> 
> I'm sure I can avoid writing Windows code, because, as I have already
> pointed, our current implementation already work using poll(). If it
> currently works, I have no intention of breaking it, just reorganizing
> it to make it work better and make easier to extend.

If the existing remote-socket-only poll is sufficent (so no local events), then I think I have misunderstood your original definition of the event loop because I was expecting that other threads would be able to interact with it.

Would it be possible to add an example what you intend API use to actually look like to your design summary?  (In loose pseudocode.)

>> I also worry that codifying something in this form is going to exclude
>> the possibility of getting a better result on Windows later, because
>> that wouldn't involve file descriptors or internals calling default
>> BSD send()/recv().
> 
> You have got it backwards: it will make the possibility of getting a
> better result on Windows later easier, because it will make everything
> modular.
> 
> In fact, libev and libuv already implement code specific to Windows, so
> we get them for free as soon as we implement support for one of these
> libraries.

libev does not implement enough Windows support to be used generally, though it is sufficient if you know the event loop will never want to interact with anything which is not a socket (it hacks up self-pipes by connecting TCP sockets to random ports on localhost).

I believe that libuv and libevent both implement it properly (I have no experience with libuv, but Lynne seems to be advocating for it), so I would strongly recommend starting your implementation using one of them in order to ensure that you don't end up tied to the Unix model in an unhelpful way.  (Note in particular that this means essentially everything dealing with sockets has to be delegated to the event library so that it can deal with asynchronous calls internally, especially send()/recv() calls.)

Thanks,

- Mark