Re: [Qemu-devel] [RFC 3/5] nbd: Use aio_set_fd_handler2()

[Paolo Bonzini <pbonzini@redhat.com>]

Il 05/06/2014 10:12, Stefan Hajnoczi ha scritto:
> On Wed, Jun 04, 2014 at 08:02:06PM +0200, Paolo Bonzini wrote:
>> Il 04/06/2014 14:37, Stefan Hajnoczi ha scritto:
>>> Why is this design cleaner?  Because NBD code doesn't have to worry
>>> about fd handlers.  It uses straightforward coroutine send/recv for
>>> socket I/O inside nbd_read_req() and nbd_write_resp().  It's easy to see
>>> that only one coroutine receives from the socket and that only one
>>> coroutine writes to the socket.
>>
>> I don't understand how this would work without managing fd handlers.
>
> fd handlers still need to be managed, but not by NBD code.  They must be
> managed by coroutine recv/send utility functions.  In other words, fd
> handlers are used locally, not globally.
>
> def co_recv(fd, buf):
>     while True:
>         nbytes = recv(fd, buf, len(buf))
>         if nbytes == -1:
>             if errno == EINTR:
>                 continue
>             if errno == EAGAIN or errno == EWOULDBLOCK:
>                 aio_set_fd_read_handler(fd, co_recv_cb)
>                 qemu_coroutine_yield()
>                 aio_set_fd_read_handler(fd, NULL)
>                 continue
>         return nbytes
>
> The send function is similar.

I see what you mean now---this however is not how qemu_co_recv and 
qemu_co_send work.  NBD uses them, but they require the caller to set up 
the handlers manually.

Paolo