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 2.1 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 typedef struct QIOTask QIOTask;
  25
  26 typedef void (*QIOTaskFunc)(QIOTask *task,
  27                             gpointer opaque);
  28
  29 typedef void (*QIOTaskWorker)(QIOTask *task,
  30                               gpointer opaque);
  31
  32 /**
  33  * QIOTask:
  34  *
  35  * The QIOTask object provides a simple mechanism for reporting
  36  * success / failure of long running background operations.
  37  *
  38  * A object on which the operation is to be performed could have
  39  * a public API which accepts a task callback:
  40  *
  41  * <example>
  42  *   <title>Task function signature</title>
  43  *   <programlisting>
  44  *  void myobject_operation(QMyObject *obj,
  45  *                          QIOTaskFunc *func,
  46  *                          gpointer opaque,
  47  *                          GDestroyNotify notify);
  48  *   </programlisting>
  49  * </example>
  50  *
  51  * The 'func' parameter is the callback to be invoked, and 'opaque'
  52  * is data to pass to it. The optional 'notify' function is used
  53  * to free 'opaque' when no longer needed.
  54  *
  55  * When the operation completes, the 'func' callback will be
  56  * invoked, allowing the calling code to determine the result
  57  * of the operation. An example QIOTaskFunc implementation may
  58  * look like
  59  *
  60  * <example>
  61  *   <title>Task callback implementation</title>
  62  *   <programlisting>
  63  *  static void myobject_operation_notify(QIOTask *task,
  64  *                                        gpointer opaque)
  65  *  {
  66  *      Error *err = NULL;
  67  *      if (qio_task_propagate_error(task, &err)) {
  68  *          ...deal with the failure...
  69  *          error_free(err);
  70  *      } else {
  71  *          QMyObject *src = QMY_OBJECT(qio_task_get_source(task));
  72  *          ...deal with the completion...
  73  *      }
  74  *  }
  75  *   </programlisting>
  76  * </example>
  77  *
  78  * Now, lets say the implementation of the method using the
  79  * task wants to set a timer to run once a second checking
  80  * for completion of some activity. It would do something
  81  * like
  82  *
  83  * <example>
  84  *   <title>Task function implementation</title>
  85  *   <programlisting>
  86  *    void myobject_operation(QMyObject *obj,
  87  *                            QIOTaskFunc *func,
  88  *                            gpointer opaque,
  89  *                            GDestroyNotify notify)
  90  *    {
  91  *      QIOTask *task;
  92  *
  93  *      task = qio_task_new(OBJECT(obj), func, opaque, notify);
  94  *
  95  *      g_timeout_add_full(G_PRIORITY_DEFAULT,
  96  *                         1000,
  97  *                         myobject_operation_timer,
  98  *                         task,
  99  *                         NULL);
 100  *    }
 101  *   </programlisting>
 102  * </example>
 103  *
 104  * It could equally have setup a watch on a file descriptor or
 105  * created a background thread, or something else entirely.
 106  * Notice that the source object is passed to the task, and
 107  * QIOTask will hold a reference on that. This ensure that
 108  * the QMyObject instance cannot be garbage collected while
 109  * the async task is still in progress.
 110  *
 111  * In this case, myobject_operation_timer will fire after
 112  * 3 secs and do
 113  *
 114  * <example>
 115  *   <title>Task timer function</title>
 116  *   <programlisting>
 117  *   gboolean myobject_operation_timer(gpointer opaque)
 118  *   {
 119  *      QIOTask *task = QIO_TASK(opaque);
 120  *      Error *err = NULL;
 121  *
 122  *      ...check something important...
 123  *       if (err) {
 124  *           qio_task_set_error(task, err);
 125  *           qio_task_complete(task);
 126  *           return FALSE;
 127  *       } else if (...work is completed ...) {
 128  *           qio_task_complete(task);
 129  *           return FALSE;
 130  *       }
 131  *       ...carry on polling ...
 132  *       return TRUE;
 133  *   }
 134  *   </programlisting>
 135  * </example>
 136  *
 137  * The 'qio_task_complete' call in this method will trigger
 138  * the callback func 'myobject_operation_notify' shown
 139  * earlier to deal with the results.
 140  *
 141  * Once this function returns false, object_unref will be called
 142  * automatically on the task causing it to be released and the
 143  * ref on QMyObject dropped too.
 144  *
 145  * The QIOTask module can also be used to perform operations
 146  * in a background thread context, while still reporting the
 147  * results in the main event thread. This allows code which
 148  * cannot easily be rewritten to be asychronous (such as DNS
 149  * lookups) to be easily run non-blocking. Reporting the
 150  * results in the main thread context means that the caller
 151  * typically does not need to be concerned about thread
 152  * safety wrt the QEMU global mutex.
 153  *
 154  * For example, the socket_listen() method will block the caller
 155  * while DNS lookups take place if given a name, instead of IP
 156  * address. The C library often do not provide a practical async
 157  * DNS API, so the to get non-blocking DNS lookups in a portable
 158  * manner requires use of a thread. So achieve a non-blocking
 159  * socket listen using QIOTask would require:
 160  *
 161  * <example>
 162  *    static void myobject_listen_worker(QIOTask *task,
 163  *                                       gpointer opaque)
 164  *    {
 165  *       QMyObject obj = QMY_OBJECT(qio_task_get_source(task));
 166  *       SocketAddress *addr = opaque;
 167  *       Error *err = NULL;
 168  *
 169  *       obj->fd = socket_listen(addr, &err);
 170  *
 171          qio_task_set_error(task, err);
 172  *    }
 173  *
 174  *    void myobject_listen_async(QMyObject *obj,
 175  *                               SocketAddress *addr,
 176  *                               QIOTaskFunc *func,
 177  *                               gpointer opaque,
 178  *                               GDestroyNotify notify)
 179  *    {
 180  *      QIOTask *task;
 181  *      SocketAddress *addrCopy;
 182  *
 183  *      addrCopy = QAPI_CLONE(SocketAddress, addr);
 184  *      task = qio_task_new(OBJECT(obj), func, opaque, notify);
 185  *
 186  *      qio_task_run_in_thread(task, myobject_listen_worker,
 187  *                             addrCopy,
 188  *                             qapi_free_SocketAddress);
 189  *    }
 190  * </example>
 191  *
 192  * NB, The 'func' callback passed into myobject_listen_async
 193  * will be invoked from the main event thread, despite the
 194  * actual operation being performed in a different thread.
 195  */
 196
 197 /**
 198  * qio_task_new:
 199  * @source: the object on which the operation is invoked
 200  * @func: the callback to invoke when the task completes
 201  * @opaque: opaque data to pass to @func when invoked
 202  * @destroy: optional callback to free @opaque
 203  *
 204  * Creates a new task struct to track completion of a
 205  * background operation running on the object @source.
 206  * When the operation completes or fails, the callback
 207  * @func will be invoked. The callback can access the
 208  * 'err' attribute in the task object to determine if
 209  * the operation was successful or not.
 210  *
 211  * The returned task will be released when qio_task_complete()
 212  * is invoked.
 213  *
 214  * Returns: the task struct
 215  */
 216 QIOTask *qio_task_new(Object *source,
 217                       QIOTaskFunc func,
 218                       gpointer opaque,
 219                       GDestroyNotify destroy);
 220
 221 /**
 222  * qio_task_run_in_thread:
 223  * @task: the task struct
 224  * @worker: the function to invoke in a thread
 225  * @opaque: opaque data to pass to @worker
 226  * @destroy: function to free @opaque
 227  * @context: the context to run the complete hook. If %NULL, the
 228  *           default context will be used.
 229  *
 230  * Run a task in a background thread. When @worker
 231  * returns it will call qio_task_complete() in
 232  * the thread that is running the main loop associated
 233  * with @context.
 234  */
 235 void qio_task_run_in_thread(QIOTask *task,
 236                             QIOTaskWorker worker,
 237                             gpointer opaque,
 238                             GDestroyNotify destroy,
 239                             GMainContext *context);
 240
 241
 242 /**
 243  * qio_task_wait_thread:
 244  * @task: the task struct
 245  *
 246  * Wait for completion of a task that was previously
 247  * invoked using qio_task_run_in_thread. This MUST
 248  * ONLY be invoked if the task has not already
 249  * completed, since after the completion callback
 250  * is invoked, @task will have been freed.
 251  *
 252  * To avoid racing with execution of the completion
 253  * callback provided with qio_task_new, this method
 254  * MUST ONLY be invoked from the thread that is
 255  * running the main loop associated with @context
 256  * parameter to qio_task_run_in_thread.
 257  *
 258  * When the thread has completed, the completion
 259  * callback provided to qio_task_new will be invoked.
 260  * When that callback returns @task will be freed,
 261  * so @task must not be referenced after this
 262  * method completes.
 263  */
 264 void qio_task_wait_thread(QIOTask *task);
 265
 266
 267 /**
 268  * qio_task_complete:
 269  * @task: the task struct
 270  *
 271  * Invoke the completion callback for @task and
 272  * then free its memory.
 273  */
 274 void qio_task_complete(QIOTask *task);
 275
 276
 277 /**
 278  * qio_task_set_error:
 279  * @task: the task struct
 280  * @err: pointer to the error, or NULL
 281  *
 282  * Associate an error with the task, which can later
 283  * be retrieved with the qio_task_propagate_error()
 284  * method. This method takes ownership of @err, so
 285  * it is not valid to access it after this call
 286  * completes. If @err is NULL this is a no-op. If
 287  * this is call multiple times, only the first
 288  * provided @err will be recorded, later ones will
 289  * be discarded and freed.
 290  */
 291 void qio_task_set_error(QIOTask *task,
 292                         Error *err);
 293
 294
 295 /**
 296  * qio_task_propagate_error:
 297  * @task: the task struct
 298  * @errp: pointer to a NULL-initialized error object
 299  *
 300  * Propagate the error associated with @task
 301  * into @errp.
 302  *
 303  * Returns: true if an error was propagated, false otherwise
 304  */
 305 bool qio_task_propagate_error(QIOTask *task,
 306                               Error **errp);
 307
 308
 309 /**
 310  * qio_task_set_result_pointer:
 311  * @task: the task struct
 312  * @result: pointer to the result data
 313  *
 314  * Associate an opaque result with the task,
 315  * which can later be retrieved with the
 316  * qio_task_get_result_pointer() method
 317  *
 318  */
 319 void qio_task_set_result_pointer(QIOTask *task,
 320                                  gpointer result,
 321                                  GDestroyNotify notify);
 322
 323
 324 /**
 325  * qio_task_get_result_pointer:
 326  * @task: the task struct
 327  *
 328  * Retrieve the opaque result data associated
 329  * with the task, if any.
 330  *
 331  * Returns: the task result, or NULL
 332  */
 333 gpointer qio_task_get_result_pointer(QIOTask *task);
 334
 335
 336 /**
 337  * qio_task_get_source:
 338  * @task: the task struct
 339  *
 340  * Get the source object associated with the background
 341  * task. The caller does not own a reference on the
 342  * returned Object, and so should call object_ref()
 343  * if it wants to keep the object pointer outside the
 344  * lifetime of the QIOTask object.
 345  *
 346  * Returns: the source object
 347  */
 348 Object *qio_task_get_source(QIOTask *task);
 349
 350 #endif /* QIO_TASK_H */