[Qemu-devel] [PATCH v1 RFC 25/34] io: add QIOTask class for async operations

["Daniel P. Berrange" <berrange@redhat.com>]

A number of I/O operations need to be performed asynchronously
to avoid blocking the main loop. The caller of such APIs need
to provide a callback to be invoked on completion/error and
need access to the error, if any. The small QIOTask provides
a simple framework for dealing with such probes. The API
docs inline provide an outline of how this is to be used.

In this series, the QIOTask class will be used for things like
the TLS handshake, the websockets handshake and TCP connect()
progress.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
---
 include/io/task.h | 168 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
 io/Makefile.objs  |   1 +
 io/task.c         |  84 +++++++++++++++++++++++++++
 3 files changed, 253 insertions(+)
 create mode 100644 include/io/task.h
 create mode 100644 io/task.c

diff --git a/include/io/task.h b/include/io/task.h
new file mode 100644
index 0000000..8f86902
--- /dev/null
+++ b/include/io/task.h
@@ -0,0 +1,168 @@
+/*
+ * QEMU I/O task
+ *
+ * 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_TASK_H__
+#define QIO_TASK_H__
+
+#include "qemu-common.h"
+#include "qapi/error.h"
+#include "qom/object.h"
+
+#define TYPE_QIO_TASK "qemu:io-task"
+#define QIO_TASK(obj)                                    \
+    OBJECT_CHECK(QIOTask, (obj), TYPE_QIO_TASK)
+#define QIO_TASK_CLASS(klass)                                   \
+    OBJECT_CLASS_CHECK(QIOTaskClass, klass, TYPE_QIO_TASK)
+#define QIO_TASK_GET_CLASS(obj)                         \
+    OBJECT_GET_CLASS(QIOTaskClass, obj, TYPE_QIO_TASK)
+
+typedef struct QIOTask QIOTask;
+typedef struct QIOTaskClass QIOTaskClass;
+
+typedef void (*QIOTaskFunc)(QIOTask *task,
+                            gpointer opaque);
+
+/**
+ * QIOTask:
+ *
+ * The QIOTask object provides a simple mechanism for reporting
+ * success / failure of long running background operations.
+ *
+ * A object on which the operation is to be performed could have
+ * a public API which accepts a task callback:
+ *
+ *  void myobject_operation(QMyObject *obj,
+ *                           QIOTaskFunc *func,
+ *                           gpointer opaque,
+ *                           GDestroyNotify *notify);
+ *
+ * The 'func' parameter is the callback to be invoked, and 'opaque'
+ * is data to pass to it. The optional 'notify' function is used
+ * to free 'opaque' when no longer needed.
+ *
+ * Now, lets say the implementation of this method wants to set
+ * a timer to run once a second checking for completion of some
+ * activity. It would do something like
+ *
+ *    void myobject_operation(QMyObject *obj,
+ *                            QIOTaskFunc *func,
+ *                            gpointer opaque,
+ *                            GDestroyNotify *notify)
+ *    {
+ *      QIOTask *task;
+ *
+ *      task = qio_task_new(OBJECT(obj), func, opaque, notify);
+ *
+ *      g_timeout_add_full(G_PRIORITY_DEFAULT,
+ *                         1000,
+ *                         myobject_operation_timer,
+ *                         task,
+ *                         (GDestroyNotify)object_unref);
+ *    }
+ *
+ * It could equally have setup a watch on a file descriptor or
+ * created a background thread, or something else entirely.
+ * Notice that the source object is passed to the task, and
+ * QIOTask will hold a reference on that. This ensure that
+ * the QMyObject instance cannot be garbage collected while
+ * the async task is still in progress.
+ *
+ * In this case, myobject_operation_timer will fire after
+ * 3 secs and do
+ *
+ *   gboolean myobject_operation_timer(gpointer opaque)
+ *   {
+ *      QIOTask *task = QIO_TASK(opaque);
+ *      Error *err;*
+ *
+ *      ...check something important...
+ *       if (err) {
+ *           qio_task_abort(task, err);
+ *           error_free(task);
+ *           return FALSE;
+ *       } else if (...work is completed ...) {
+ *           qio_task_complete(task);
+ *           return FALSE;
+ *       }
+ *       ...carry on polling ...
+ *       return TRUE;
+ *   }
+ *
+ * Once this function returns false, object_unref will be called
+ * automatically on the task causing it to be released and the
+ * ref on QMyObject dropped too.
+ */
+
+struct QIOTask {
+    Object parent;
+    Object *source;
+    QIOTaskFunc func;
+    gpointer opaque;
+    GDestroyNotify destroy;
+    Error *err;
+};
+
+struct QIOTaskClass {
+    ObjectClass parent;
+};
+
+/**
+ * qio_task_new:
+ * @source: the object on which the operation is invoked
+ * @func: the callback to invoke when the task completes
+ * @opaque: opaque data to pass to @func when invoked
+ * @destroy: optional callback to free @opaque
+ *
+ * Creates a new task object to track completion of a
+ * background operation running on the object @source.
+ * When the operation completes or fails, the callback
+ * @func will be invoked. The callback can access the
+ * 'err' attribute in the task object to determine if
+ * the operation was successful or not.
+ *
+ * Returns: the task object
+ */
+QIOTask *qio_task_new(Object *source,
+                      QIOTaskFunc func,
+                      gpointer opaque,
+                      GDestroyNotify destroy);
+
+/**
+ * qio_task_complete:
+ * @task: the task object
+ *
+ * Mark the operation as succesfully completed
+ */
+void qio_task_complete(QIOTask *task);
+
+/**
+ * qio_task_abort:
+ * @task: the task object
+ * @err: the error to record for the operation
+ *
+ * Mark the operation as failed, with @err providing
+ * details about the failure. The @err may be freed
+ * afer the function returns, as the notification
+ * callback is invoked synchronously.
+ */
+void qio_task_abort(QIOTask *task,
+                    Error *err);
+
+#endif /* QIO_TASK_H__ */
diff --git a/io/Makefile.objs b/io/Makefile.objs
index eec1b43..b9973ac 100644
--- a/io/Makefile.objs
+++ b/io/Makefile.objs
@@ -1,3 +1,4 @@
+util-obj-y += task.o
 util-obj-y += channel.o
 util-obj-y += channel-unix.o
 util-obj-y += channel-socket.o
diff --git a/io/task.c b/io/task.c
new file mode 100644
index 0000000..453f474
--- /dev/null
+++ b/io/task.c
@@ -0,0 +1,84 @@
+/*
+ * QEMU I/O task
+ *
+ * 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/>.
+ *
+ */
+
+#include "io/task.h"
+
+QIOTask *qio_task_new(Object *source,
+                      QIOTaskFunc func,
+                      gpointer opaque,
+                      GDestroyNotify destroy)
+{
+    QIOTask *task;
+
+    task = QIO_TASK(object_new(TYPE_QIO_TASK));
+
+    task->source = source;
+    object_ref(source);
+    task->func = func;
+    task->opaque = opaque;
+    task->destroy = destroy;
+
+    return task;
+}
+
+void qio_task_complete(QIOTask *task)
+{
+    task->func(task, task->opaque);
+}
+
+void qio_task_abort(QIOTask *task,
+                    Error *err)
+{
+    task->err = err;
+    task->func(task, task->opaque);
+    task->err = NULL;
+}
+
+static void qio_task_initfn(Object *obj G_GNUC_UNUSED)
+{
+    /* nada */
+}
+
+static void qio_task_finalize(Object *obj)
+{
+    QIOTask *task = QIO_TASK(obj);
+
+    object_unref(task->source);
+    if (task->destroy) {
+        task->destroy(task->opaque);
+    }
+}
+
+
+static const TypeInfo qio_task_info = {
+    .parent = TYPE_OBJECT,
+    .name = TYPE_QIO_TASK,
+    .instance_size = sizeof(QIOTask),
+    .instance_init = qio_task_initfn,
+    .instance_finalize = qio_task_finalize,
+    .class_size = sizeof(QIOTaskClass),
+};
+
+static void qio_task_register_types(void)
+{
+    type_register_static(&qio_task_info);
+}
+
+type_init(qio_task_register_types);
-- 
2.1.0