io: add QIOTask class for async operations

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.

Some functions don't have the ability to run asynchronously
(eg getaddrinfo always blocks), so to facilitate their use,
the task class provides a mechanism to run a blocking
function in a thread, while triggering the completion
callback in the main event loop thread. This easily allows
any synchronous function to be made asynchronous, albeit
at the cost of spawning a thread.

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

The concept of QIOTask is inspired by the GAsyncResult
interface / GTask class in the GIO libraries. The min
version requirements on glib don't allow those to be
used from QEMU, so QIOTask provides a facsimilie which
can be easily switched to GTask in the future if the
min version is increased.

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

1 files changed, 256 insertions, 0 deletions

diff --git a/include/io/task.h b/include/io/task.h
new file mode 100644
index 0000000..2418714
--- /dev/null
+++ b/include/io/task.h
@@ -0,0 +1,256 @@
+/*
+ * 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"
+
+typedef struct QIOTask QIOTask;
+
+typedef void (*QIOTaskFunc)(Object *source,
+                            Error *err,
+                            gpointer opaque);
+
+typedef int (*QIOTaskWorker)(QIOTask *task,
+                             Error **errp,
+                             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:
+ *
+ * <example>
+ *   <title>Task callback function signature</title>
+ *   <programlisting>
+ *  void myobject_operation(QMyObject *obj,
+ *                          QIOTaskFunc *func,
+ *                          gpointer opaque,
+ *                          GDestroyNotify *notify);
+ *   </programlisting>
+ * </example>
+ *
+ * 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
+ *
+ * <example>
+ *   <title>Task callback function implementation</title>
+ *   <programlisting>
+ *    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,
+ *                         NULL);
+ *    }
+ *   </programlisting>
+ * </example>
+ *
+ * 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
+ *
+ * <example>
+ *   <title>Task timer function</title>
+ *   <programlisting>
+ *   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;
+ *   }
+ *   </programlisting>
+ * </example>
+ *
+ * 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.
+ *
+ * The QIOTask module can also be used to perform operations
+ * in a background thread context, while still reporting the
+ * results in the main event thread. This allows code which
+ * cannot easily be rewritten to be asychronous (such as DNS
+ * lookups) to be easily run non-blocking. Reporting the
+ * results in the main thread context means that the caller
+ * typically does not need to be concerned about thread
+ * safety wrt the QEMU global mutex.
+ *
+ * For example, the socket_listen() method will block the caller
+ * while DNS lookups take place if given a name, instead of IP
+ * address. The C library often do not provide a practical async
+ * DNS API, so the to get non-blocking DNS lookups in a portable
+ * manner requires use of a thread. So achieve a non-blocking
+ * socket listen using QIOTask would require:
+ *
+ * <example>
+ *    static int myobject_listen_worker(QIOTask *task,
+ *                                      Error **errp,
+ *                                      gpointer opaque)
+ *    {
+ *       QMyObject obj = QMY_OBJECT(qio_task_get_source(task));
+ *       SocketAddress *addr = opaque;
+ *
+ *       obj->fd = socket_listen(addr, errp);
+ *       if (obj->fd < 0) {
+ *          return -1;
+ *       }
+ *       return 0;
+ *    }
+ *
+ *    void myobject_listen_async(QMyObject *obj,
+ *                               SocketAddress *addr,
+ *                               QIOTaskFunc *func,
+ *                               gpointer opaque,
+ *                               GDestroyNotify *notify)
+ *    {
+ *      QIOTask *task;
+ *      SocketAddress *addrCopy;
+ *
+ *      qapi_copy_SocketAddress(&addrCopy, addr);
+ *      task = qio_task_new(OBJECT(obj), func, opaque, notify);
+ *
+ *      qio_task_run_in_thread(task, myobject_listen_worker,
+ *                             addrCopy,
+ *                             qapi_free_SocketAddress);
+ *    }
+ * </example>
+ *
+ * NB, The 'func' callback passed into myobject_listen_async
+ * will be invoked from the main event thread, despite the
+ * actual operation being performed in a different thread.
+ */
+
+/**
+ * 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 struct 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.
+ *
+ * The returned task will be released when one of
+ * qio_task_abort() or qio_task_complete() are invoked.
+ *
+ * Returns: the task struct
+ */
+QIOTask *qio_task_new(Object *source,
+                      QIOTaskFunc func,
+                      gpointer opaque,
+                      GDestroyNotify destroy);
+
+/**
+ * qio_task_run_in_thread:
+ * @task: the task struct
+ * @worker: the function to invoke in a thread
+ * @opaque: opaque data to pass to @worker
+ * @destroy: function to free @opaque
+ *
+ * Run a task in a background thread. If @worker
+ * returns 0 it will call qio_task_complete() in
+ * the main event thread context. If @worker
+ * returns -1 it will call qio_task_abort() in
+ * the main event thread context.
+ */
+void qio_task_run_in_thread(QIOTask *task,
+                            QIOTaskWorker worker,
+                            gpointer opaque,
+                            GDestroyNotify destroy);
+
+/**
+ * qio_task_complete:
+ * @task: the task struct
+ *
+ * Mark the operation as succesfully completed
+ * and free the memory for @task.
+ */
+void qio_task_complete(QIOTask *task);
+
+/**
+ * qio_task_abort:
+ * @task: the task struct
+ * @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. The @task will
+ * be freed when this call completes.
+ */
+void qio_task_abort(QIOTask *task,
+                    Error *err);
+
+
+/**
+ * qio_task_get_source:
+ * @task: the task struct
+ *
+ * Get the source object associated with the background
+ * task. This returns a new reference to the object,
+ * which the caller must released with object_unref()
+ * when no longer required.
+ *
+ * Returns: the source object
+ */
+Object *qio_task_get_source(QIOTask *task);
+
+#endif /* QIO_TASK_H__ */