Re: [PATCH v19 08/20] io: add qio_channel_readv_full_all_eof & qio_channel_readv_full_all helpers

[Daniel P . Berrangé]

On Thu, Jan 14, 2021 at 12:55:58PM -0500, Jag Raman wrote:
> 
> 
> > On Jan 14, 2021, at 11:27 AM, Stefan Hajnoczi <stefanha@redhat.com> wrote:
> > 
> > On Thu, Jan 14, 2021 at 10:40:03AM -0500, Jagannathan Raman wrote:
> >> +int qio_channel_readv_full_all(QIOChannel *ioc,
> >> +                               const struct iovec *iov,
> >> +                               size_t niov,
> >> +                               int **fds, size_t *nfds,
> >> +                               Error **errp)
> >> {
> >> -    int ret = qio_channel_readv_all_eof(ioc, iov, niov, errp);
> >> +    int ret = qio_channel_readv_full_all_eof(ioc, iov, niov, fds, nfds, 
> >> errp);
> >> 
> >>     if (ret == 0) {
> >> -        ret = -1;
> >>         error_setg(errp,
> >>                    "Unexpected end-of-file before all bytes were read");
> > 
> > qio_channel_readv_full_all_eof() can read file descriptors but no data
> > and return 0.
> > 
> > Here that case is converted into an error and the file descriptors
> > aren't closed, freed, and fds/nfds isn't cleared.
> 
> That’s a valid point. I’m wondering if the fix for this case should be in
> qio_channel_readv_full_all_eof(), instead of here.
> 
> qio_channel_readv_full_all_eof() should probably return error (-1) if the
> amount of data read does not match iov_size(). If the caller is only expecting
> to read fds, and not any data, it would indicate that by setting iov to NULL
> and/or setting niov=0. If the caller is setting these parameters, it means it 
> is
> expecting data.Does that sound good?

The API spec for the existing _eof() methods says:

 * The function will wait for all requested data
 * to be read, yielding from the current coroutine
 * if required.
 *
 * If end-of-file occurs before any data is read,
 * no error is reported; otherwise, if it occurs
 * before all requested data has been read, an error
 * will be reported.


IOW, return '0' is *only* valid if we've not read anything. I consider
file descriptors to be something.

IOW, qio_channel_readv_full_all_eof must only return 0, if it didn't
read any data and also didn't receive any file descriptors. So yeah,
we must return -1 in the scenario Stefan describes

Regards,
Daniel
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