Re: [Qemu-block] [PATCH 05/25] nbd: Avoid generic -EINVAL

[Max Reitz]

On 2015-03-16 at 10:42, Paolo Bonzini wrote:
> On 16/03/2015 14:51, Max Reitz wrote:
> > > Propagating the return value from write_sync is uglier, but it is even
> > > better in terms of returned value.
> > We can only return -errno values, but write_sync() may do partial writes
> > so it may return non-negative values which still indicate an error. So
> > we'd have to check whether the return value is negative, if it is,
> > return that, if it isn't but if it's still below what we wanted to
> > write, return a fixed error (such as -EIO). I'd rather just return -EIO
> > and be done with it, but if you really want me to, I can of course do it
> > differently.
> nbd_wr_sync doesn't do that, it always returns negative errno for a partial
> error:

qemu_send() might do a partial send, returning a value smaller than len. 
nbd_wr_sync() will try iterating until everything has been sent, but if 
send() returns 0, the loop is aborted and a value smaller than len may 
be returned.

Maybe send() never returns 0, in which case nbd_wr_sync() will actually 
return either len or -errno, but this isn't clear from the structure of 
nbd_wr_sync(). If you really want to pass the value returned from 
nbd_wr_sync(), I'd rather restructure that function so that it always 
returns either len or -errno.

Max

>          if (len < 0) {
>              err = socket_error();
>
>              /* recoverable error */
>              if (err == EINTR || (offset > 0 && (err == EAGAIN || err == 
> EWOULDBLOCK))) {
>                  continue;
>              }
>
>              /* unrecoverable error */
>              return -err;
>          }
>
> The precise error can be useful to distinguish a network error from something
> else.  I'm just in doubt about partial reads; those can return a positive error,
> in which case you can return ESHUTDOWN (in read_sync).
>
> Paolo