include/io/task.h

   1 /*
   2  * QEMU I/O task
   3  *
   4  * Copyright (c) 2015 Red Hat, Inc.
   5  *
   6  * This library is free software; you can redistribute it and/or
   7  * modify it under the terms of the GNU Lesser General Public
   8  * License as published by the Free Software Foundation; either
   9  * version 2 of the License, or (at your option) any later version.
  10  *
  11  * This library is distributed in the hope that it will be useful,
  12  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  13  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  14  * Lesser General Public License for more details.
  15  *
  16  * You should have received a copy of the GNU Lesser General Public
  17  * License along with this library; if not, see <http://www.gnu.org/licenses/>.
  18  *
  19  */
  20
  21 #ifndef QIO_TASK_H
  22 #define QIO_TASK_H
  23
  24 #include "qemu-common.h"
  25 #include "qom/object.h"
  26
  27 typedef struct QIOTask QIOTask;
  28
  29 typedef void (*QIOTaskFunc)(Object *source,
  30                             Error *err,
  31                             gpointer opaque);
  32
  33 typedef int (*QIOTaskWorker)(QIOTask *task,
  34                              Error **errp,
  35                              gpointer opaque);
  36
  37 /**
  38  * QIOTask:
  39  *
  40  * The QIOTask object provides a simple mechanism for reporting
  41  * success / failure of long running background operations.
  42  *
  43  * A object on which the operation is to be performed could have
  44  * a public API which accepts a task callback:
  45  *
  46  * <example>
  47  *   <title>Task callback function signature</title>
  48  *   <programlisting>
  49  *  void myobject_operation(QMyObject *obj,
  50  *                          QIOTaskFunc *func,
  51  *                          gpointer opaque,
  52  *                          GDestroyNotify *notify);
  53  *   </programlisting>
  54  * </example>
  55  *
  56  * The 'func' parameter is the callback to be invoked, and 'opaque'
  57  * is data to pass to it. The optional 'notify' function is used
  58  * to free 'opaque' when no longer needed.
  59  *
  60  * Now, lets say the implementation of this method wants to set
  61  * a timer to run once a second checking for completion of some
  62  * activity. It would do something like
  63  *
  64  * <example>
  65  *   <title>Task callback function implementation</title>
  66  *   <programlisting>
  67  *    void myobject_operation(QMyObject *obj,
  68  *                            QIOTaskFunc *func,
  69  *                            gpointer opaque,
  70  *                            GDestroyNotify *notify)
  71  *    {
  72  *      QIOTask *task;
  73  *
  74  *      task = qio_task_new(OBJECT(obj), func, opaque, notify);
  75  *
  76  *      g_timeout_add_full(G_PRIORITY_DEFAULT,
  77  *                         1000,
  78  *                         myobject_operation_timer,
  79  *                         task,
  80  *                         NULL);
  81  *    }
  82  *   </programlisting>
  83  * </example>
  84  *
  85  * It could equally have setup a watch on a file descriptor or
  86  * created a background thread, or something else entirely.
  87  * Notice that the source object is passed to the task, and
  88  * QIOTask will hold a reference on that. This ensure that
  89  * the QMyObject instance cannot be garbage collected while
  90  * the async task is still in progress.
  91  *
  92  * In this case, myobject_operation_timer will fire after
  93  * 3 secs and do
  94  *
  95  * <example>
  96  *   <title>Task timer function</title>
  97  *   <programlisting>
  98  *   gboolean myobject_operation_timer(gpointer opaque)
  99  *   {
 100  *      QIOTask *task = QIO_TASK(opaque);
 101  *      Error *err;*
 102  *
 103  *      ...check something important...
 104  *       if (err) {
 105  *           qio_task_abort(task, err);
 106  *           error_free(task);
 107  *           return FALSE;
 108  *       } else if (...work is completed ...) {
 109  *           qio_task_complete(task);
 110  *           return FALSE;
 111  *       }
 112  *       ...carry on polling ...
 113  *       return TRUE;
 114  *   }
 115  *   </programlisting>
 116  * </example>
 117  *
 118  * Once this function returns false, object_unref will be called
 119  * automatically on the task causing it to be released and the
 120  * ref on QMyObject dropped too.
 121  *
 122  * The QIOTask module can also be used to perform operations
 123  * in a background thread context, while still reporting the
 124  * results in the main event thread. This allows code which
 125  * cannot easily be rewritten to be asychronous (such as DNS
 126  * lookups) to be easily run non-blocking. Reporting the
 127  * results in the main thread context means that the caller
 128  * typically does not need to be concerned about thread
 129  * safety wrt the QEMU global mutex.
 130  *
 131  * For example, the socket_listen() method will block the caller
 132  * while DNS lookups take place if given a name, instead of IP
 133  * address. The C library often do not provide a practical async
 134  * DNS API, so the to get non-blocking DNS lookups in a portable
 135  * manner requires use of a thread. So achieve a non-blocking
 136  * socket listen using QIOTask would require:
 137  *
 138  * <example>
 139  *    static int myobject_listen_worker(QIOTask *task,
 140  *                                      Error **errp,
 141  *                                      gpointer opaque)
 142  *    {
 143  *       QMyObject obj = QMY_OBJECT(qio_task_get_source(task));
 144  *       SocketAddress *addr = opaque;
 145  *
 146  *       obj->fd = socket_listen(addr, errp);
 147  *       if (obj->fd < 0) {
 148  *          return -1;
 149  *       }
 150  *       return 0;
 151  *    }
 152  *
 153  *    void myobject_listen_async(QMyObject *obj,
 154  *                               SocketAddress *addr,
 155  *                               QIOTaskFunc *func,
 156  *                               gpointer opaque,
 157  *                               GDestroyNotify *notify)
 158  *    {
 159  *      QIOTask *task;
 160  *      SocketAddress *addrCopy;
 161  *
 162  *      addrCopy = QAPI_CLONE(SocketAddress, addr);
 163  *      task = qio_task_new(OBJECT(obj), func, opaque, notify);
 164  *
 165  *      qio_task_run_in_thread(task, myobject_listen_worker,
 166  *                             addrCopy,
 167  *                             qapi_free_SocketAddress);
 168  *    }
 169  * </example>
 170  *
 171  * NB, The 'func' callback passed into myobject_listen_async
 172  * will be invoked from the main event thread, despite the
 173  * actual operation being performed in a different thread.
 174  */
 175
 176 /**
 177  * qio_task_new:
 178  * @source: the object on which the operation is invoked
 179  * @func: the callback to invoke when the task completes
 180  * @opaque: opaque data to pass to @func when invoked
 181  * @destroy: optional callback to free @opaque
 182  *
 183  * Creates a new task struct to track completion of a
 184  * background operation running on the object @source.
 185  * When the operation completes or fails, the callback
 186  * @func will be invoked. The callback can access the
 187  * 'err' attribute in the task object to determine if
 188  * the operation was successful or not.
 189  *
 190  * The returned task will be released when one of
 191  * qio_task_abort() or qio_task_complete() are invoked.
 192  *
 193  * Returns: the task struct
 194  */
 195 QIOTask *qio_task_new(Object *source,
 196                       QIOTaskFunc func,
 197                       gpointer opaque,
 198                       GDestroyNotify destroy);
 199
 200 /**
 201  * qio_task_run_in_thread:
 202  * @task: the task struct
 203  * @worker: the function to invoke in a thread
 204  * @opaque: opaque data to pass to @worker
 205  * @destroy: function to free @opaque
 206  *
 207  * Run a task in a background thread. If @worker
 208  * returns 0 it will call qio_task_complete() in
 209  * the main event thread context. If @worker
 210  * returns -1 it will call qio_task_abort() in
 211  * the main event thread context.
 212  */
 213 void qio_task_run_in_thread(QIOTask *task,
 214                             QIOTaskWorker worker,
 215                             gpointer opaque,
 216                             GDestroyNotify destroy);
 217
 218 /**
 219  * qio_task_complete:
 220  * @task: the task struct
 221  *
 222  * Mark the operation as successfully completed
 223  * and free the memory for @task.
 224  */
 225 void qio_task_complete(QIOTask *task);
 226
 227 /**
 228  * qio_task_abort:
 229  * @task: the task struct
 230  * @err: the error to record for the operation
 231  *
 232  * Mark the operation as failed, with @err providing
 233  * details about the failure. The @err may be freed
 234  * afer the function returns, as the notification
 235  * callback is invoked synchronously. The @task will
 236  * be freed when this call completes.
 237  */
 238 void qio_task_abort(QIOTask *task,
 239                     Error *err);
 240
 241
 242 /**
 243  * qio_task_get_source:
 244  * @task: the task struct
 245  *
 246  * Get the source object associated with the background
 247  * task. This returns a new reference to the object,
 248  * which the caller must released with object_unref()
 249  * when no longer required.
 250  *
 251  * Returns: the source object
 252  */
 253 Object *qio_task_get_source(QIOTask *task);
 254
 255 #endif /* QIO_TASK_H */