If the request contains a "request_id", copy it back into the
response. There is no interpretation of the request_id value by mpv; the
only purpose is to make it easier on the requester by providing an
ability to match up responses with requests.
Because the IPC mechanism sends events continously, it's possible for
the response to a request to arrive several events after the request was
made. This can make it very difficult on the requester to determine
which response goes to which request.
Some rationale for the documented/suggested behavior:
It's not really clear what to do with invalid UTF-8, since JSON simply
can't transport this information. Maybe you could transfer such strings
as byte arrays, but that would be very verbose and inconvenient, and
would pose the problem that it's hard to distinguish between strings
encoded in this way and actual arrays.
There are many other ways how this could be handled. For example, you
could replace invalid sequences with '?'. Or you could do it like
Python, and use certain reserved unicode codepoints to "tunnel" through
invalid bytes.
Which of these works really depends on the application. And since this
can be done entirely on the byte level (invalid UTF-8 sequences can
appear only in strings in our case), it's best to leave this to the
receiver.
This is not realy obvious, so I assume this is a helpful hint.
Although the usefulness of such an approach is probably influenced by
the fact that the player might send events that arrive in the short
window while the socket is connected.
It's kind of obvious, since the protocol by design has to allow you to
read (loadfile) and write (screenshot_to) random files, but better
make it explicit so that nobody accidentally does something insecure.
