Re: [PATCH v3 1/2] io: Add support for MSG_PEEK for socket channel

[Peter Xu]

On Tue, Nov 22, 2022 at 09:01:59PM +0530, manish.mishra wrote:
> 
> On 22/11/22 8:19 pm, Daniel P. Berrangé wrote:
> > On Tue, Nov 22, 2022 at 09:41:01AM -0500, Peter Xu wrote:
> > > On Tue, Nov 22, 2022 at 02:38:53PM +0530, manish.mishra wrote:
> > > > On 22/11/22 2:30 pm, Daniel P. Berrangé wrote:
> > > > > On Sat, Nov 19, 2022 at 09:36:14AM +0000, manish.mishra wrote:
> > > > > > MSG_PEEK reads from the peek of channel, The data is treated as
> > > > > > unread and the next read shall still return this data. This
> > > > > > support is currently added only for socket class. Extra parameter
> > > > > > 'flags' is added to io_readv calls to pass extra read flags like
> > > > > > MSG_PEEK.
> > > > > > 
> > > > > > Suggested-by: Daniel P. Berrangé <berrange@redhat.com
> > > > > > Signed-off-by: manish.mishra<manish.mishra@nutanix.com>
> > > > > > ---
> > > > > >    chardev/char-socket.c               |  4 +-
> > > > > >    include/io/channel.h                | 83 
> > > > > > +++++++++++++++++++++++++++++
> > > > > >    io/channel-buffer.c                 |  1 +
> > > > > >    io/channel-command.c                |  1 +
> > > > > >    io/channel-file.c                   |  1 +
> > > > > >    io/channel-null.c                   |  1 +
> > > > > >    io/channel-socket.c                 | 16 +++++-
> > > > > >    io/channel-tls.c                    |  1 +
> > > > > >    io/channel-websock.c                |  1 +
> > > > > >    io/channel.c                        | 73 
> > > > > > +++++++++++++++++++++++--
> > > > > >    migration/channel-block.c           |  1 +
> > > > > >    scsi/qemu-pr-helper.c               |  2 +-
> > > > > >    tests/qtest/tpm-emu.c               |  2 +-
> > > > > >    tests/unit/test-io-channel-socket.c |  1 +
> > > > > >    util/vhost-user-server.c            |  2 +-
> > > > > >    15 files changed, 179 insertions(+), 11 deletions(-)
> > > > > 
> > > > > > diff --git a/io/channel-socket.c b/io/channel-socket.c
> > > > > > index b76dca9cc1..a06b24766d 100644
> > > > > > --- a/io/channel-socket.c
> > > > > > +++ b/io/channel-socket.c
> > > > > > @@ -406,6 +406,8 @@ qio_channel_socket_accept(QIOChannelSocket *ioc,
> > > > > >        }
> > > > > >    #endif /* WIN32 */
> > > > > > +    qio_channel_set_feature(QIO_CHANNEL(cioc), 
> > > > > > QIO_CHANNEL_FEATURE_READ_MSG_PEEK);
> > > > > > +
> > > > > This covers the incoming server side socket.
> > > > > 
> > > > > This also needs to be set in outgoing client side socket in
> > > > > qio_channel_socket_connect_async
> > > > 
> > > > Yes sorry, i considered only current use-case, but as it is generic one 
> > > > both should be there. Thanks will update it.
> > > > 
> > > > > > @@ -705,7 +718,6 @@ static ssize_t 
> > > > > > qio_channel_socket_writev(QIOChannel *ioc,
> > > > > >    }
> > > > > >    #endif /* WIN32 */
> > > > > > -
> > > > > >    #ifdef QEMU_MSG_ZEROCOPY
> > > > > >    static int qio_channel_socket_flush(QIOChannel *ioc,
> > > > > >                                        Error **errp)
> > > > > Please remove this unrelated whitespace change.
> > > > > 
> > > > > 
> > > > > > @@ -109,6 +117,37 @@ int qio_channel_readv_all_eof(QIOChannel *ioc,
> > > > > >        return qio_channel_readv_full_all_eof(ioc, iov, niov, NULL, 
> > > > > > NULL, errp);
> > > > > >    }
> > > > > > +int qio_channel_readv_peek_all_eof(QIOChannel *ioc,
> > > > > > +                                   const struct iovec *iov,
> > > > > > +                                   size_t niov,
> > > > > > +                                   Error **errp)
> > > > > > +{
> > > > > > +   ssize_t len = 0;
> > > > > > +   ssize_t total = iov_size(iov, niov);
> > > > > > +
> > > > > > +   while (len < total) {
> > > > > > +       len = qio_channel_readv_full(ioc, iov, niov, NULL,
> > > > > > +                                    NULL, 
> > > > > > QIO_CHANNEL_READ_FLAG_MSG_PEEK, errp);
> > > > > > +
> > > > > > +       if (len == QIO_CHANNEL_ERR_BLOCK) {
> > > > > > +            if (qemu_in_coroutine()) {
> > > > > > +                qio_channel_yield(ioc, G_IO_IN);
> > > > > > +            } else {
> > > > > > +                qio_channel_wait(ioc, G_IO_IN);
> > > > > > +            }
> > > > > > +            continue;
> > > > > > +       }
> > > > > > +       if (len == 0) {
> > > > > > +           return 0;
> > > > > > +       }
> > > > > > +       if (len < 0) {
> > > > > > +           return -1;
> > > > > > +       }
> > > > > > +   }
> > > > > This will busy wait burning CPU where there is a read > 0 and < total.
> > > > > 
> > > > Daniel, i could use MSG_WAITALL too if that works but then we will lose 
> > > > opportunity to yield. Or if you have some other idea.
> > > How easy would this happen?
> > > 
> > > Another alternative is we could just return the partial len to caller then
> > > we fallback to the original channel orders if it happens.  And then if it
> > > mostly will never happen it'll behave merely the same as what we want.
> > Well we're trying to deal with a bug where the slow and/or unreliable
> > network causes channels to arrive in unexpected order. Given we know
> > we're having network trouble, I wouldn't want to make more assumptions
> > about things happening correctly.
> > 
> > 
> > With regards,
> > Daniel
> 
> 
> Peter, I have seen MSG_PEEK used in combination with MSG_WAITALL, but looks 
> like even though chances are less it can still return partial data even with 
> multiple retries for signal case, so is not full proof.
> 
> *MSG_WAITALL *(since Linux 2.2)
>               This flag requests that the operation block until the full
>               request is satisfied.  However, the call may still return
>               less data than requested if a signal is caught, an error
>               or disconnect occurs, or the next data to be received is
>               of a different type than that returned.  This flag has no
>               effect for datagram sockets.
> 
> Actual read ahead will be little hackish, so just confirming we all are in 
> agreement to do actual read ahead and i can send patch? :)

Yet another option is the caller handles partial PEEK and then we can sleep
in the migration code before another PEEK attempt until it reaches the full
length.

Even with that explicit sleep code IMHO it is cleaner than the read-header
flag plus things like !tls check just to avoid the handshake dead lock
itself (and if to go with this route we'd better also have a full document
on why !tls, aka, how the dead lock can happen).

Would that work?

-- 
Peter Xu