io: introduce helper for creating channels from file descriptors

Depending on what object a file descriptor refers to a different
type of IO channel will be needed - either a QIOChannelFile or
a QIOChannelSocket. Introduce a qio_channel_new_fd() method
which will return the appropriate channel implementation.

Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>

include/io/channel-util.h

@@ -0,0 +1,52 @@
+/*
+ * QEMU I/O channels utility APIs
+ *
+ * Copyright (c) 2016 Red Hat, Inc.
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, see <http://www.gnu.org/licenses/>.
+ *
+ */
+
+#ifndef QIO_CHANNEL_UTIL_H__
+#define QIO_CHANNEL_UTIL_H__
+
+#include "io/channel.h"
+
+/*
+ * This module provides helper functions that are useful when dealing
+ * with QIOChannel objects
+ */
+
+
+/**
+ * qio_channel_new_fd:
+ * @fd: the file descriptor
+ * @errp: pointer to a NULL-initialized error object
+ *
+ * Create a channel for performing I/O on the file
+ * descriptor @fd. The particular subclass of QIOChannel
+ * that is returned will depend on what underlying object
+ * the file descriptor is associated with. It may be either
+ * a QIOChannelSocket or a QIOChannelFile instance. Upon
+ * success, the returned QIOChannel instance will own
+ * the @fd file descriptor, and take responsibility for
+ * closing it when no longer required. On failure, the
+ * caller is responsible for closing @fd.
+ *
+ * Returns: the channel object, or NULL on error
+ */
+QIOChannel *qio_channel_new_fd(int fd,
+                               Error **errp);
+
+#endif /* QIO_CHANNEL_UTIL_H__ */