io: add a QIOChannelNull equivalent to /dev/null

This is for code which needs a portable equivalent to a QIOChannelFile
connected to /dev/null.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>

include/io/channel-null.h

@@ -0,0 +1,55 @@
+/*
+ * QEMU I/O channels null driver
+ *
+ * Copyright (c) 2022 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 2.1 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_FILE_H
+#define QIO_CHANNEL_FILE_H
+
+#include "io/channel.h"
+#include "qom/object.h"
+
+#define TYPE_QIO_CHANNEL_NULL "qio-channel-null"
+OBJECT_DECLARE_SIMPLE_TYPE(QIOChannelNull, QIO_CHANNEL_NULL)
+
+
+/**
+ * QIOChannelNull:
+ *
+ * The QIOChannelNull object provides a channel implementation
+ * that discards all writes and returns EOF for all reads.
+ */
+
+struct QIOChannelNull {
+    QIOChannel parent;
+    bool closed;
+};
+
+
+/**
+ * qio_channel_null_new:
+ *
+ * Create a new IO channel object that discards all writes
+ * and returns EOF for all reads.
+ *
+ * Returns: the new channel object
+ */
+QIOChannelNull *
+qio_channel_null_new(void);
+
+#endif /* QIO_CHANNEL_NULL_H */

io/channel-null.c

@@ -0,0 +1,237 @@
+/*
+ * QEMU I/O channels null driver
+ *
+ * Copyright (c) 2022 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 2.1 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/>.
+ *
+ */
+
+#include "qemu/osdep.h"
+#include "io/channel-null.h"
+#include "io/channel-watch.h"
+#include "qapi/error.h"
+#include "trace.h"
+#include "qemu/iov.h"
+
+typedef struct QIOChannelNullSource QIOChannelNullSource;
+struct QIOChannelNullSource {
+    GSource parent;
+    QIOChannel *ioc;
+    GIOCondition condition;
+};
+
+
+QIOChannelNull *
+qio_channel_null_new(void)
+{
+    QIOChannelNull *ioc;
+
+    ioc = QIO_CHANNEL_NULL(object_new(TYPE_QIO_CHANNEL_NULL));
+
+    trace_qio_channel_null_new(ioc);
+
+    return ioc;
+}
+
+
+static void
+qio_channel_null_init(Object *obj)
+{
+    QIOChannelNull *ioc = QIO_CHANNEL_NULL(obj);
+    ioc->closed = false;
+}
+
+
+static ssize_t
+qio_channel_null_readv(QIOChannel *ioc,
+                       const struct iovec *iov,
+                       size_t niov,
+                       int **fds G_GNUC_UNUSED,
+                       size_t *nfds G_GNUC_UNUSED,
+                       Error **errp)
+{
+    QIOChannelNull *nioc = QIO_CHANNEL_NULL(ioc);
+
+    if (nioc->closed) {
+        error_setg_errno(errp, EINVAL,
+                         "Channel is closed");
+        return -1;
+    }
+
+    return 0;
+}
+
+
+static ssize_t
+qio_channel_null_writev(QIOChannel *ioc,
+                        const struct iovec *iov,
+                        size_t niov,
+                        int *fds G_GNUC_UNUSED,
+                        size_t nfds G_GNUC_UNUSED,
+                        int flags G_GNUC_UNUSED,
+                        Error **errp)
+{
+    QIOChannelNull *nioc = QIO_CHANNEL_NULL(ioc);
+
+    if (nioc->closed) {
+        error_setg_errno(errp, EINVAL,
+                         "Channel is closed");
+        return -1;
+    }
+
+    return iov_size(iov, niov);
+}
+
+
+static int
+qio_channel_null_set_blocking(QIOChannel *ioc G_GNUC_UNUSED,
+                              bool enabled G_GNUC_UNUSED,
+                              Error **errp G_GNUC_UNUSED)
+{
+    return 0;
+}
+
+
+static off_t
+qio_channel_null_seek(QIOChannel *ioc G_GNUC_UNUSED,
+                      off_t offset G_GNUC_UNUSED,
+                      int whence G_GNUC_UNUSED,
+                      Error **errp G_GNUC_UNUSED)
+{
+    return 0;
+}
+
+
+static int
+qio_channel_null_close(QIOChannel *ioc,
+                       Error **errp G_GNUC_UNUSED)
+{
+    QIOChannelNull *nioc = QIO_CHANNEL_NULL(ioc);
+
+    nioc->closed = true;
+    return 0;
+}
+
+
+static void
+qio_channel_null_set_aio_fd_handler(QIOChannel *ioc G_GNUC_UNUSED,
+                                    AioContext *ctx G_GNUC_UNUSED,
+                                    IOHandler *io_read G_GNUC_UNUSED,
+                                    IOHandler *io_write G_GNUC_UNUSED,
+                                    void *opaque G_GNUC_UNUSED)
+{
+}
+
+
+static gboolean
+qio_channel_null_source_prepare(GSource *source G_GNUC_UNUSED,
+                                gint *timeout)
+{
+    *timeout = -1;
+
+    return TRUE;
+}
+
+
+static gboolean
+qio_channel_null_source_check(GSource *source G_GNUC_UNUSED)
+{
+    return TRUE;
+}
+
+
+static gboolean
+qio_channel_null_source_dispatch(GSource *source,
+                                 GSourceFunc callback,
+                                 gpointer user_data)
+{
+    QIOChannelFunc func = (QIOChannelFunc)callback;
+    QIOChannelNullSource *ssource = (QIOChannelNullSource *)source;
+
+    return (*func)(ssource->ioc,
+                   ssource->condition,
+                   user_data);
+}
+
+
+static void
+qio_channel_null_source_finalize(GSource *source)
+{
+    QIOChannelNullSource *ssource = (QIOChannelNullSource *)source;
+
+    object_unref(OBJECT(ssource->ioc));
+}
+
+
+GSourceFuncs qio_channel_null_source_funcs = {
+    qio_channel_null_source_prepare,
+    qio_channel_null_source_check,
+    qio_channel_null_source_dispatch,
+    qio_channel_null_source_finalize
+};
+
+
+static GSource *
+qio_channel_null_create_watch(QIOChannel *ioc,
+                              GIOCondition condition)
+{
+    GSource *source;
+    QIOChannelNullSource *ssource;
+
+    source = g_source_new(&qio_channel_null_source_funcs,
+                          sizeof(QIOChannelNullSource));
+    ssource = (QIOChannelNullSource *)source;
+
+    ssource->ioc = ioc;
+    object_ref(OBJECT(ioc));
+
+    ssource->condition = condition;
+
+    return source;
+}
+
+
+static void
+qio_channel_null_class_init(ObjectClass *klass,
+                            void *class_data G_GNUC_UNUSED)
+{
+    QIOChannelClass *ioc_klass = QIO_CHANNEL_CLASS(klass);
+
+    ioc_klass->io_writev = qio_channel_null_writev;
+    ioc_klass->io_readv = qio_channel_null_readv;
+    ioc_klass->io_set_blocking = qio_channel_null_set_blocking;
+    ioc_klass->io_seek = qio_channel_null_seek;
+    ioc_klass->io_close = qio_channel_null_close;
+    ioc_klass->io_create_watch = qio_channel_null_create_watch;
+    ioc_klass->io_set_aio_fd_handler = qio_channel_null_set_aio_fd_handler;
+}
+
+
+static const TypeInfo qio_channel_null_info = {
+    .parent = TYPE_QIO_CHANNEL,
+    .name = TYPE_QIO_CHANNEL_NULL,
+    .instance_size = sizeof(QIOChannelNull),
+    .instance_init = qio_channel_null_init,
+    .class_init = qio_channel_null_class_init,
+};
+
+
+static void
+qio_channel_null_register_types(void)
+{
+    type_register_static(&qio_channel_null_info);
+}
+
+type_init(qio_channel_null_register_types);

io/meson.build

@@ -3,6 +3,7 @@ io_ss.add(files(
   'channel-buffer.c',
   'channel-command.c',
   'channel-file.c',
+  'channel-null.c',
   'channel-socket.c',
   'channel-tls.c',
   'channel-util.c',

io/trace-events

@@ -10,6 +10,9 @@ qio_task_thread_result(void *task) "Task thread result task=%p"
 qio_task_thread_source_attach(void *task, void *source) "Task thread source attach task=%p source=%p"
 qio_task_thread_source_cancel(void *task, void *source) "Task thread source cancel task=%p source=%p"
 
+# channel-null.c
+qio_channel_null_new(void *ioc) "Null new ioc=%p"
+
 # channel-socket.c
 qio_channel_socket_new(void *ioc) "Socket new ioc=%p"
 qio_channel_socket_new_fd(void *ioc, int fd) "Socket new ioc=%p fd=%d"

tests/unit/meson.build

@@ -88,6 +88,7 @@ if have_block
     'test-io-channel-file': ['io-channel-helpers.c', io],
     'test-io-channel-command': ['io-channel-helpers.c', io],
     'test-io-channel-buffer': ['io-channel-helpers.c', io],
+    'test-io-channel-null': [io],
     'test-crypto-ivgen': [io],
     'test-crypto-afsplit': [io],
     'test-crypto-block': [io],