/* GIO - GLib Input, Output and Streaming Library

 * 

 * Copyright (C) 2006-2007 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/>.

 *

 * Author: Alexander Larsson <alexl@redhat.com>

 */

#include "config.h"

#include "glib.h"

#include <gioerror.h>

#include "glib-private.h"

#include "gcancellable.h"

#include "glibintl.h"

/**

 * SECTION:gcancellable

 * @short_description: Thread-safe Operation Cancellation Stack

 * @include: gio/gio.h

 *

 * GCancellable is a thread-safe operation cancellation stack used 

 * throughout GIO to allow for cancellation of synchronous and

 * asynchronous operations.

 */

enum {

  CANCELLED,

  LAST_SIGNAL

};

struct _GCancellablePrivate

{

  /* Atomic so that g_cancellable_is_cancelled does not require holding the mutex. */

  gboolean cancelled;

  /* Access to fields below is protected by cancellable_mutex. */

  guint cancelled_running : 1;

  guint cancelled_running_waiting : 1;

  guint fd_refcount;

  GWakeup *wakeup;

};

static guint signals[LAST_SIGNAL] = { 0 };

G_DEFINE_TYPE_WITH_PRIVATE (GCancellable, g_cancellable, G_TYPE_OBJECT)

static GPrivate current_cancellable;

static GMutex cancellable_mutex;

static GCond cancellable_cond;

static void

g_cancellable_finalize (GObject *object)

{

  GCancellable *cancellable = G_CANCELLABLE (object);

  if (cancellable->priv->wakeup)

    GLIB_PRIVATE_CALL (g_wakeup_free) (cancellable->priv->wakeup);

  G_OBJECT_CLASS (g_cancellable_parent_class)->finalize (object);

}

static void

g_cancellable_class_init (GCancellableClass *klass)

{

  GObjectClass *gobject_class = G_OBJECT_CLASS (klass);

  gobject_class->finalize = g_cancellable_finalize;

  /**

   * GCancellable::cancelled:

   * @cancellable: a #GCancellable.

   * 

   * Emitted when the operation has been cancelled.

   * 

   * Can be used by implementations of cancellable operations. If the

   * operation is cancelled from another thread, the signal will be

   * emitted in the thread that cancelled the operation, not the

   * thread that is running the operation.

   *

   * Note that disconnecting from this signal (or any signal) in a

   * multi-threaded program is prone to race conditions. For instance

   * it is possible that a signal handler may be invoked even after

   * a call to g_signal_handler_disconnect() for that handler has

   * already returned.

   * 

   * There is also a problem when cancellation happens right before

   * connecting to the signal. If this happens the signal will

   * unexpectedly not be emitted, and checking before connecting to

   * the signal leaves a race condition where this is still happening.

   *

   * In order to make it safe and easy to connect handlers there

   * are two helper functions: g_cancellable_connect() and

   * g_cancellable_disconnect() which protect against problems

   * like this.

   *

   * An example of how to us this:

   * |[<!-- language="C" -->

   *     // Make sure we don't do unnecessary work if already cancelled

   *     if (g_cancellable_set_error_if_cancelled (cancellable, error))

   *       return;

   *

   *     // Set up all the data needed to be able to handle cancellation

   *     // of the operation

   *     my_data = my_data_new (...);

   *

   *     id = 0;

   *     if (cancellable)

   *       id = g_cancellable_connect (cancellable,

   *                G_CALLBACK (cancelled_handler)

   *                data, NULL);

   *

   *     // cancellable operation here...

   *

   *     g_cancellable_disconnect (cancellable, id);

   *

   *     // cancelled_handler is never called after this, it is now safe

   *     // to free the data

   *     my_data_free (my_data);  

   * ]|

   *

   * Note that the cancelled signal is emitted in the thread that

   * the user cancelled from, which may be the main thread. So, the

   * cancellable signal should not do something that can block.

   */

  signals[CANCELLED] =

    g_signal_new (I_("cancelled"),

      G_TYPE_FROM_CLASS (gobject_class),

      G_SIGNAL_RUN_LAST,

      G_STRUCT_OFFSET (GCancellableClass, cancelled),

      NULL, NULL,

      NULL,

      G_TYPE_NONE, 0);

}

static void

g_cancellable_init (GCancellable *cancellable)

{

  cancellable->priv = g_cancellable_get_instance_private (cancellable);

}

/**

 * g_cancellable_new:

 * 

 * Creates a new #GCancellable object.

 *

 * Applications that want to start one or more operations

 * that should be cancellable should create a #GCancellable

 * and pass it to the operations.

 *

 * One #GCancellable can be used in multiple consecutive

 * operations or in multiple concurrent operations.

 *  

 * Returns: a #GCancellable.

 **/

GCancellable *

g_cancellable_new (void)

{

  return g_object_new (G_TYPE_CANCELLABLE, NULL);

}

/**

 * g_cancellable_push_current:

 * @cancellable: a #GCancellable object

 *

 * Pushes @cancellable onto the cancellable stack. The current

 * cancellable can then be received using g_cancellable_get_current().

 *

 * This is useful when implementing cancellable operations in

 * code that does not allow you to pass down the cancellable object.

 *

 * This is typically called automatically by e.g. #GFile operations,

 * so you rarely have to call this yourself.

 **/

void

g_cancellable_push_current (GCancellable *cancellable)

{

  GSList *l;

  g_return_if_fail (cancellable != NULL);

  l = g_private_get (&current_cancellable);

  l = g_slist_prepend (l, cancellable);

  g_private_set (&current_cancellable, l);

}

/**

 * g_cancellable_pop_current:

 * @cancellable: a #GCancellable object

 *

 * Pops @cancellable off the cancellable stack (verifying that @cancellable

 * is on the top of the stack).

 **/

void

g_cancellable_pop_current (GCancellable *cancellable)

{

  GSList *l;

  l = g_private_get (&current_cancellable);

  g_return_if_fail (l != NULL);

  g_return_if_fail (l->data == cancellable);

  l = g_slist_delete_link (l, l);

  g_private_set (&current_cancellable, l);

}

/**

 * g_cancellable_get_current:

 *

 * Gets the top cancellable from the stack.

 *

 * Returns: (nullable) (transfer none): a #GCancellable from the top

 * of the stack, or %NULL if the stack is empty.

 **/

GCancellable *

g_cancellable_get_current  (void)

{

  GSList *l;

  l = g_private_get (&current_cancellable);

  if (l == NULL)

    return NULL;

  return G_CANCELLABLE (l->data);

}

/**

 * g_cancellable_reset:

 * @cancellable: a #GCancellable object.

 * 

 * Resets @cancellable to its uncancelled state.

 *

 * If cancellable is currently in use by any cancellable operation

 * then the behavior of this function is undefined.

 *

 * Note that it is generally not a good idea to reuse an existing

 * cancellable for more operations after it has been cancelled once,

 * as this function might tempt you to do. The recommended practice

 * is to drop the reference to a cancellable after cancelling it,

 * and let it die with the outstanding async operations. You should

 * create a fresh cancellable for further async operations.

 **/

void 

g_cancellable_reset (GCancellable *cancellable)

{

  GCancellablePrivate *priv;

  g_return_if_fail (G_IS_CANCELLABLE (cancellable));

  g_mutex_lock (&cancellable_mutex);

  priv = cancellable->priv;

  while (priv->cancelled_running)

    {

      priv->cancelled_running_waiting = TRUE;

      g_cond_wait (&cancellable_cond, &cancellable_mutex);

    }

  if (g_atomic_int_get (&priv->cancelled))

    {

      if (priv->wakeup)

        GLIB_PRIVATE_CALL (g_wakeup_acknowledge) (priv->wakeup);

      g_atomic_int_set (&priv->cancelled, FALSE);

    }

  g_mutex_unlock (&cancellable_mutex);

}

/**

 * g_cancellable_is_cancelled:

 * @cancellable: (nullable): a #GCancellable or %NULL

 *

 * Checks if a cancellable job has been cancelled.

 *

 * Returns: %TRUE if @cancellable is cancelled,

 * FALSE if called with %NULL or if item is not cancelled.

 **/

gboolean

g_cancellable_is_cancelled (GCancellable *cancellable)

{

  return cancellable != NULL && g_atomic_int_get (&cancellable->priv->cancelled);

}

/**

 * g_cancellable_set_error_if_cancelled:

 * @cancellable: (nullable): a #GCancellable or %NULL

 * @error: #GError to append error state to

 *

 * If the @cancellable is cancelled, sets the error to notify

 * that the operation was cancelled.

 *

 * Returns: %TRUE if @cancellable was cancelled, %FALSE if it was not

 */

gboolean

g_cancellable_set_error_if_cancelled (GCancellable  *cancellable,

                                      GError       **error)

{

  if (g_cancellable_is_cancelled (cancellable))

    {

      g_set_error_literal (error,

                           G_IO_ERROR,

                           G_IO_ERROR_CANCELLED,

                           _("Operation was cancelled"));

      return TRUE;

    }

  return FALSE;

}

/**

 * g_cancellable_get_fd:

 * @cancellable: a #GCancellable.

 * 

 * Gets the file descriptor for a cancellable job. This can be used to

 * implement cancellable operations on Unix systems. The returned fd will

 * turn readable when @cancellable is cancelled.

 *

 * You are not supposed to read from the fd yourself, just check for

 * readable status. Reading to unset the readable status is done

 * with g_cancellable_reset().

 * 

 * After a successful return from this function, you should use 

 * g_cancellable_release_fd() to free up resources allocated for 

 * the returned file descriptor.

 *

 * See also g_cancellable_make_pollfd().

 *

 * Returns: A valid file descriptor. `-1` if the file descriptor

 * is not supported, or on errors. 

 **/

int

g_cancellable_get_fd (GCancellable *cancellable)

{

  GPollFD pollfd;

#ifndef G_OS_WIN32

  gboolean retval G_GNUC_UNUSED  /* when compiling with G_DISABLE_ASSERT */;

#endif

  if (cancellable == NULL)

    return -1;

#ifdef G_OS_WIN32

  pollfd.fd = -1;

#else

  retval = g_cancellable_make_pollfd (cancellable, &pollfd);

  g_assert (retval);

#endif

  return pollfd.fd;

}

/**

 * g_cancellable_make_pollfd:

 * @cancellable: (nullable): a #GCancellable or %NULL

 * @pollfd: a pointer to a #GPollFD

 * 

 * Creates a #GPollFD corresponding to @cancellable; this can be passed

 * to g_poll() and used to poll for cancellation. This is useful both

 * for unix systems without a native poll and for portability to

 * windows.

 *

 * When this function returns %TRUE, you should use 

 * g_cancellable_release_fd() to free up resources allocated for the 

 * @pollfd. After a %FALSE return, do not call g_cancellable_release_fd().

 *

 * If this function returns %FALSE, either no @cancellable was given or

 * resource limits prevent this function from allocating the necessary 

 * structures for polling. (On Linux, you will likely have reached 

 * the maximum number of file descriptors.) The suggested way to handle

 * these cases is to ignore the @cancellable.

 *

 * You are not supposed to read from the fd yourself, just check for

 * readable status. Reading to unset the readable status is done

 * with g_cancellable_reset().

 *

 * Returns: %TRUE if @pollfd was successfully initialized, %FALSE on 

 *          failure to prepare the cancellable.

 * 

 * Since: 2.22

 **/

gboolean

g_cancellable_make_pollfd (GCancellable *cancellable, GPollFD *pollfd)

{

  g_return_val_if_fail (pollfd != NULL, FALSE);

  if (cancellable == NULL)

    return FALSE;

  g_return_val_if_fail (G_IS_CANCELLABLE (cancellable), FALSE);

  g_mutex_lock (&cancellable_mutex);

  cancellable->priv->fd_refcount++;

  if (cancellable->priv->wakeup == NULL)

    {

      cancellable->priv->wakeup = GLIB_PRIVATE_CALL (g_wakeup_new) ();

      if (g_atomic_int_get (&cancellable->priv->cancelled))

        GLIB_PRIVATE_CALL (g_wakeup_signal) (cancellable->priv->wakeup);

    }

  GLIB_PRIVATE_CALL (g_wakeup_get_pollfd) (cancellable->priv->wakeup, pollfd);

  g_mutex_unlock (&cancellable_mutex);

  return TRUE;

}

/**

 * g_cancellable_release_fd:

 * @cancellable: a #GCancellable

 *

 * Releases a resources previously allocated by g_cancellable_get_fd()

 * or g_cancellable_make_pollfd().

 *

 * For compatibility reasons with older releases, calling this function 

 * is not strictly required, the resources will be automatically freed

 * when the @cancellable is finalized. However, the @cancellable will

 * block scarce file descriptors until it is finalized if this function

 * is not called. This can cause the application to run out of file 

 * descriptors when many #GCancellables are used at the same time.

 * 

 * Since: 2.22

 **/

void

g_cancellable_release_fd (GCancellable *cancellable)

{

  GCancellablePrivate *priv;

  if (cancellable == NULL)

    return;

  g_return_if_fail (G_IS_CANCELLABLE (cancellable));

  priv = cancellable->priv;

  g_mutex_lock (&cancellable_mutex);

  g_assert (priv->fd_refcount > 0);

  priv->fd_refcount--;

  if (priv->fd_refcount == 0)

    {

      GLIB_PRIVATE_CALL (g_wakeup_free) (priv->wakeup);

      priv->wakeup = NULL;

    }

  g_mutex_unlock (&cancellable_mutex);

}

/**

 * g_cancellable_cancel:

 * @cancellable: (nullable): a #GCancellable object.

 * 

 * Will set @cancellable to cancelled, and will emit the

 * #GCancellable::cancelled signal. (However, see the warning about

 * race conditions in the documentation for that signal if you are

 * planning to connect to it.)

 *

 * This function is thread-safe. In other words, you can safely call

 * it from a thread other than the one running the operation that was

 * passed the @cancellable.

 *

 * If @cancellable is %NULL, this function returns immediately for convenience.

 *

 * The convention within GIO is that cancelling an asynchronous

 * operation causes it to complete asynchronously. That is, if you

 * cancel the operation from the same thread in which it is running,

 * then the operation's #GAsyncReadyCallback will not be invoked until

 * the application returns to the main loop.

 **/

void

g_cancellable_cancel (GCancellable *cancellable)

{

  GCancellablePrivate *priv;

  if (cancellable == NULL || g_cancellable_is_cancelled (cancellable))

    return;

  priv = cancellable->priv;

  g_mutex_lock (&cancellable_mutex);

  if (g_atomic_int_get (&priv->cancelled))

    {

      g_mutex_unlock (&cancellable_mutex);

      return;

    }

  g_atomic_int_set (&priv->cancelled, TRUE);

  priv->cancelled_running = TRUE;

  if (priv->wakeup)

    GLIB_PRIVATE_CALL (g_wakeup_signal) (priv->wakeup);

  g_mutex_unlock (&cancellable_mutex);

  g_object_ref (cancellable);

  g_signal_emit (cancellable, signals[CANCELLED], 0);

  g_mutex_lock (&cancellable_mutex);

  priv->cancelled_running = FALSE;

  if (priv->cancelled_running_waiting)

    g_cond_broadcast (&cancellable_cond);

  priv->cancelled_running_waiting = FALSE;

  g_mutex_unlock (&cancellable_mutex);

  g_object_unref (cancellable);

}

/**

 * g_cancellable_connect:

 * @cancellable: A #GCancellable.

 * @callback: The #GCallback to connect.

 * @data: Data to pass to @callback.

 * @data_destroy_func: (nullable): Free function for @data or %NULL.

 *

 * Convenience function to connect to the #GCancellable::cancelled

 * signal. Also handles the race condition that may happen

 * if the cancellable is cancelled right before connecting.

 *

 * @callback is called at most once, either directly at the

 * time of the connect if @cancellable is already cancelled,

 * or when @cancellable is cancelled in some thread.

 *

 * @data_destroy_func will be called when the handler is

 * disconnected, or immediately if the cancellable is already

 * cancelled.

 *

 * See #GCancellable::cancelled for details on how to use this.

 *

 * Since GLib 2.40, the lock protecting @cancellable is not held when

 * @callback is invoked.  This lifts a restriction in place for

 * earlier GLib versions which now makes it easier to write cleanup

 * code that unconditionally invokes e.g. g_cancellable_cancel().

 *

 * Returns: The id of the signal handler or 0 if @cancellable has already

 *          been cancelled.

 *

 * Since: 2.22

 */

gulong

g_cancellable_connect (GCancellable   *cancellable,

           GCallback       callback,

           gpointer        data,

           GDestroyNotify  data_destroy_func)

{

  gulong id;

  g_return_val_if_fail (G_IS_CANCELLABLE (cancellable), 0);

  g_mutex_lock (&cancellable_mutex);

  if (g_atomic_int_get (&cancellable->priv->cancelled))

    {

      void (*_callback) (GCancellable *cancellable,

                         gpointer      user_data);

      g_mutex_unlock (&cancellable_mutex);

      _callback = (void *)callback;

      id = 0;

      _callback (cancellable, data);

      if (data_destroy_func)

        data_destroy_func (data);

    }

  else

    {

      id = g_signal_connect_data (cancellable, "cancelled",

                                  callback, data,

                                  (GClosureNotify) data_destroy_func,

                                  0);

      g_mutex_unlock (&cancellable_mutex);

    }

  return id;

}

/**

 * g_cancellable_disconnect:

 * @cancellable: (nullable): A #GCancellable or %NULL.

 * @handler_id: Handler id of the handler to be disconnected, or `0`.

 *

 * Disconnects a handler from a cancellable instance similar to

 * g_signal_handler_disconnect().  Additionally, in the event that a

 * signal handler is currently running, this call will block until the

 * handler has finished.  Calling this function from a

 * #GCancellable::cancelled signal handler will therefore result in a

 * deadlock.

 *

 * This avoids a race condition where a thread cancels at the

 * same time as the cancellable operation is finished and the

 * signal handler is removed. See #GCancellable::cancelled for

 * details on how to use this.

 *

 * If @cancellable is %NULL or @handler_id is `0` this function does

 * nothing.

 *

 * Since: 2.22

 */

void

g_cancellable_disconnect (GCancellable  *cancellable,

        gulong         handler_id)

{

  GCancellablePrivate *priv;

  if (handler_id == 0 ||  cancellable == NULL)

    return;

  g_mutex_lock (&cancellable_mutex);

  priv = cancellable->priv;

  while (priv->cancelled_running)

    {

      priv->cancelled_running_waiting = TRUE;

      g_cond_wait (&cancellable_cond, &cancellable_mutex);

    }

  g_signal_handler_disconnect (cancellable, handler_id);

  g_mutex_unlock (&cancellable_mutex);

}

typedef struct {

  GSource       source;

  GCancellable *cancellable;

  gulong        cancelled_handler;

  /* Protected by cancellable_mutex: */

  gboolean      resurrected_during_cancellation;

} GCancellableSource;

/*

 * The reference count of the GSource might be 0 at this point but it is not

 * finalized yet and its dispose function did not run yet, or otherwise we

 * would have disconnected the signal handler already and due to the signal

 * emission lock it would be impossible to call the signal handler at that

 * point. That is: at this point we either have a fully valid GSource, or

 * it's not disposed or finalized yet and we can still resurrect it as needed.

 *

 * As such we first ensure that we have a strong reference to the GSource in

 * here before calling any other GSource API.

 */

static void

cancellable_source_cancelled (GCancellable *cancellable,

            gpointer      user_data)

{

  GSource *source = user_data;

  GCancellableSource *cancellable_source = (GCancellableSource *) source;

  g_mutex_lock (&cancellable_mutex);

  /* Drop the reference added in cancellable_source_dispose(); see the comment there.

   * The reference must be dropped after unlocking @cancellable_mutex since

   * it could be the final reference, and the dispose function takes

   * @cancellable_mutex. */

  if (cancellable_source->resurrected_during_cancellation)

    {

      cancellable_source->resurrected_during_cancellation = FALSE;

      g_mutex_unlock (&cancellable_mutex);

      g_source_unref (source);

      return;

    }

  g_source_ref (source);

  g_mutex_unlock (&cancellable_mutex);

  g_source_set_ready_time (source, 0);

  g_source_unref (source);

}

static gboolean

cancellable_source_dispatch (GSource     *source,

           GSourceFunc  callback,

           gpointer     user_data)

{

  GCancellableSourceFunc func = (GCancellableSourceFunc)callback;

  GCancellableSource *cancellable_source = (GCancellableSource *)source;

  g_source_set_ready_time (source, -1);

  return (*func) (cancellable_source->cancellable, user_data);

}

static void

cancellable_source_dispose (GSource *source)

{

  GCancellableSource *cancellable_source = (GCancellableSource *)source;

  g_mutex_lock (&cancellable_mutex);

  if (cancellable_source->cancellable)

    {

      if (cancellable_source->cancellable->priv->cancelled_running)

        {

          /* There can be a race here: if thread A has called

           * g_cancellable_cancel() and has got as far as committing to call

           * cancellable_source_cancelled(), then thread B drops the final

           * ref on the GCancellableSource before g_source_ref() is called in

           * cancellable_source_cancelled(), then cancellable_source_dispose()

           * will run through and the GCancellableSource will be finalised

           * before cancellable_source_cancelled() gets to g_source_ref(). It

           * will then be left in a state where it’s committed to using a

           * dangling GCancellableSource pointer.

           *

           * Eliminate that race by resurrecting the #GSource temporarily, and

           * then dropping that reference in cancellable_source_cancelled(),

           * which should be guaranteed to fire because we’re inside a

           * @cancelled_running block.

           */

          g_source_ref (source);

          cancellable_source->resurrected_during_cancellation = TRUE;

        }

      g_clear_signal_handler (&cancellable_source->cancelled_handler,

                              cancellable_source->cancellable);

      g_clear_object (&cancellable_source->cancellable);

    }

  g_mutex_unlock (&cancellable_mutex);

}

static gboolean

cancellable_source_closure_callback (GCancellable *cancellable,

             gpointer      data)

{

  GClosure *closure = data;

  GValue params = G_VALUE_INIT;

  GValue result_value = G_VALUE_INIT;

  gboolean result;

  g_value_init (&result_value, G_TYPE_BOOLEAN);

  g_value_init (&params, G_TYPE_CANCELLABLE);

  g_value_set_object (&params, cancellable);

  g_closure_invoke (closure, &result_value, 1, &params, NULL);

  result = g_value_get_boolean (&result_value);

  g_value_unset (&result_value);

  g_value_unset (&params);

  return result;

}

static GSourceFuncs cancellable_source_funcs =

{

  NULL,

  NULL,

  cancellable_source_dispatch,

  NULL,

  (GSourceFunc)cancellable_source_closure_callback,

  NULL,

};

/**

 * g_cancellable_source_new:

 * @cancellable: (nullable): a #GCancellable, or %NULL

 *

 * Creates a source that triggers if @cancellable is cancelled and

 * calls its callback of type #GCancellableSourceFunc. This is

 * primarily useful for attaching to another (non-cancellable) source

 * with g_source_add_child_source() to add cancellability to it.

 *

 * For convenience, you can call this with a %NULL #GCancellable,

 * in which case the source will never trigger.

 *

 * The new #GSource will hold a reference to the #GCancellable.

 *

 * Returns: (transfer full): the new #GSource.

 *

 * Since: 2.28

 */

GSource *

g_cancellable_source_new (GCancellable *cancellable)

{

  GSource *source;

  GCancellableSource *cancellable_source;

  source = g_source_new (&cancellable_source_funcs, sizeof (GCancellableSource));

  g_source_set_name (source, "GCancellable");

  g_source_set_dispose_function (source, cancellable_source_dispose);

  cancellable_source = (GCancellableSource *)source;

  if (cancellable)

    {

      cancellable_source->cancellable = g_object_ref (cancellable);

      /* We intentionally don't use g_cancellable_connect() here,

       * because we don't want the "at most once" behavior.

       */

      cancellable_source->cancelled_handler =

        g_signal_connect (cancellable, "cancelled",

                          G_CALLBACK (cancellable_source_cancelled),

                          source);

      if (g_cancellable_is_cancelled (cancellable))

        g_source_set_ready_time (source, 0);

    }

  return source;

}