io: add helper module for creating watches on FDs

A number of the channel implementations will require the
ability to create watches on file descriptors / sockets.
To avoid duplicating this code in each channel, provide a
helper API for dealing with file descriptor watches.

There are two watch implementations provided. The first
is useful for bi-directional file descriptors such as
sockets, regular files, character devices, etc. The
second works with a pair of unidirectional file descriptors
such as pipes.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>

include/io/channel-watch.h

@@ -0,0 +1,72 @@
+/*
+ * QEMU I/O channels watch helper APIs
+ *
+ * Copyright (c) 2015 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_WATCH_H__
+#define QIO_CHANNEL_WATCH_H__
+
+#include "io/channel.h"
+
+/*
+ * This module provides helper functions that will be needed by
+ * the various QIOChannel implementations, for creating watches
+ * on file descriptors / sockets
+ */
+
+/**
+ * qio_channel_create_fd_watch:
+ * @ioc: the channel object
+ * @fd: the file descriptor
+ * @condition: the I/O condition
+ *
+ * Create a new main loop source that is able to
+ * monitor the file descriptor @fd for the
+ * I/O conditions in @condition. This is able
+ * monitor block devices, character devices,
+ * sockets, pipes but not plain files.
+ *
+ * Returns: the new main loop source
+ */
+GSource *qio_channel_create_fd_watch(QIOChannel *ioc,
+                                     int fd,
+                                     GIOCondition condition);
+
+/**
+ * qio_channel_create_fd_pair_watch:
+ * @ioc: the channel object
+ * @fdread: the file descriptor for reading
+ * @fdwrite: the file descriptor for writing
+ * @condition: the I/O condition
+ *
+ * Create a new main loop source that is able to
+ * monitor the pair of file descriptors @fdread
+ * and @fdwrite for the I/O conditions in @condition.
+ * This is intended for monitoring unidirectional
+ * file descriptors such as pipes, where a pair
+ * of descriptors is required for bidirectional
+ * I/O
+ *
+ * Returns: the new main loop source
+ */
+GSource *qio_channel_create_fd_pair_watch(QIOChannel *ioc,
+                                          int fdread,
+                                          int fdwrite,
+                                          GIOCondition condition);
+
+#endif /* QIO_CHANNEL_WATCH_H__ */