[Qemu-devel] Re: [PATCH 2/5] char: Introduce char_set/remove_fd_handlers()

[Gerd Hoffmann <kraxel@redhat.com>]

   Hi,

>> Moving the handlers to a separate struct is clearly a incremental
>> cleanup which can follow later.  Using enable/disable flags will
>> probably simplify the interfaces for the non-blocking mode and thus
>> simplify the whole patch series so I think this should be done now.
>
> Agree -- but it looks to be a big patch.  I have some initial work done,
> and hence am not converting anything other than unix/tcp backends.  The
> proposed interface here is local to this file, and just a couple of
> callers.  No big deal to change it once this is in.
>
> (The struct for that will look like:
>
> struct fd_handler {
> 	int fd;
>
> 	IOHandler *read;
> 	IOHandler *write;
> 	IOCanReadHandler *read_poll;
>
> 	bool read_enabled, write_enabled, read_poll_enabled;
>
> 	void (*set_read_handler)(IOHandler *read_handler);
> 	void (*set_write_handler)(IOHandler *write_handler);
> 	void (*set_readpoll_handler)(IOCanReadHandler *read_poll_handler);
> }

No, that isn't what I have in mind ...

I'm thinking more about this:

   (1) IOHandlerRecord gets variables to enable/disable the handlers
       (a bunch of bools, a bitmask, whatever).  They default to enabled
       to maintain backward compatibility.
   (2) One or more functions are added to enable/disable the handlers,
       something like qemu_fd_check_read(int fd, bool enable);
   (3) main_loop_wait() will check whenever the handler is actually
       enabled before adding it to the file handle set for the select()
       call.

This shouldn't be that big.  And with this initial stuff in place you 
can go forward with the non-blocking stuff.  No need to pass around the 
write handler or have callbacks just to enable/disable the checking for 
a writable fd, the non-blocking code can just call 
qemu_fd_check_write(fd, true/false) to do it.

Additional cleanups possible:

   (1) switch everybody who registers/unregisters handlers to the new
       enable/disable interface.
   (2) move the function pointers from IOHandlerRecord to a separate
       struct (say IOHandlerOps).

> Also: we also want to be able to select() on all fds so that we can
> detect disconnection events as they happen.  So we also need an array
> somewhere.)

I think you can't do that without switching from select() to poll().

cheers,
   Gerd