io: return 0 for EOF in TLS session read after shutdown

GNUTLS takes a paranoid approach when seeing 0 bytes returned by the
underlying OS read() function. It will consider this an error and
return GNUTLS_E_PREMATURE_TERMINATION instead of propagating the 0
return value. It expects apps to arrange for clean termination at
the protocol level and not rely on seeing EOF from a read call to
detect shutdown. This is to harden apps against a malicious 3rd party
causing termination of the sockets layer.

This is unhelpful for the QEMU NBD code which does have a clean
protocol level shutdown, but still relies on seeing 0 from the I/O
channel read in the coroutine handling incoming replies.

The upshot is that when using a plain NBD connection shutdown is
silent, but when using TLS, the client spams the console with

  Cannot read from TLS channel: Broken pipe

The NBD connection has, however, called qio_channel_shutdown()
at this point to indicate that it is done with I/O. This gives
the opportunity to optimize the code such that when the channel
has been shutdown in the read direction, the error code
GNUTLS_E_PREMATURE_TERMINATION gets turned into a '0' return
instead of an error.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20181119134228.11031-1-berrange@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>

1 files changed, 3 insertions, 3 deletions

diff --git a/include/io/channel.h b/include/io/channel.h
index e8cdadb0b0..da2f138200 100644
--- a/include/io/channel.h
+++ b/include/io/channel.h
@@ -51,9 +51,9 @@ enum QIOChannelFeature {
 typedef enum QIOChannelShutdown QIOChannelShutdown;
 
 enum QIOChannelShutdown {
-    QIO_CHANNEL_SHUTDOWN_BOTH,
-    QIO_CHANNEL_SHUTDOWN_READ,
-    QIO_CHANNEL_SHUTDOWN_WRITE,
+    QIO_CHANNEL_SHUTDOWN_READ = 1,
+    QIO_CHANNEL_SHUTDOWN_WRITE = 2,
+    QIO_CHANNEL_SHUTDOWN_BOTH = 3,
 };
 
 typedef gboolean (*QIOChannelFunc)(QIOChannel *ioc,