They're useless, and I have no idea what they're actually supposed to do
(wrt. pending input processing changes).
Also remove their implicit uses from the IPC handlers.
The default security descriptor for named pipes in Windows allows the
pipe to be opened for read access by the Everyone group and Anonymous
account, as well as low-integrity processes (like web browser renderer
processes.) This does not allow commands to be ran, but it does allow
events to be received.
I don't think any sensitive data is exposed by events, but that may not
always be the case and Lua plugins might change this, since they can
broadcast their own events with script-message. To be safe, this commit
sets a custom security descriptor on the named pipe which only allows
access from processes running under the same user account with an
integrity level greater than or equal to the one used by mpv.
This implements the JSON IPC protocol with named pipes, which are
probably the closest Windows equivalent to Unix domain sockets in terms
of functionality. Like with Unix sockets, this will allow mpv to listen
for IPC connections and handle multiple IPC clients at once. A few cross
platform libraries and frameworks (Qt, node.js) use named pipes for IPC
on Windows and Unix sockets on Linux and Unix, so hopefully this will
ease the creation of portable JSON IPC clients.
Unlike the Unix implementation, this doesn't share code with
--input-file, meaning --input-file on Windows won't understand JSON
commands (yet.) Sharing code and removing the separate implementation in
pipe-win32.c is definitely a possible future improvement.
