glib/gio/gmemoryoutputstream.c

/* 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/>.
 *
 * Authors:
 *   Christian Kellner <gicmo@gnome.org>
 *   Krzysztof Kosiński <tweenk.pl@gmail.com>
 */

#include "config.h"
#include "gmemoryoutputstream.h"
#include "goutputstream.h"
#include "gpollableoutputstream.h"
#include "gseekable.h"
#include "gtask.h"
#include "gioerror.h"
#include "string.h"
#include "glibintl.h"


/**
 * SECTION:gmemoryoutputstream
 * @short_description: Streaming output operations on memory chunks
 * @include: gio/gio.h
 * @see_also: #GMemoryInputStream
 *
 * #GMemoryOutputStream is a class for using arbitrary
 * memory chunks as output for GIO streaming output operations.
 *
 * As of GLib 2.34, #GMemoryOutputStream trivially implements
 * #GPollableOutputStream: it always polls as ready.
 */

#define MIN_ARRAY_SIZE  16

enum {
  PROP_0,
  PROP_DATA,
  PROP_SIZE,
  PROP_DATA_SIZE,
  PROP_REALLOC_FUNCTION,
  PROP_DESTROY_FUNCTION
};

struct _GMemoryOutputStreamPrivate
{
  gpointer       data; /* Write buffer */
  gsize          len; /* Current length of the data buffer. Can change with resizing. */
  gsize          valid_len; /* The part of data that has been written to */
  gsize          pos; /* Current position in the stream. Distinct from valid_len,
                         because the stream is seekable. */

  GReallocFunc   realloc_fn;
  GDestroyNotify destroy;
};

static void     g_memory_output_stream_set_property (GObject      *object,
                                                     guint         prop_id,
                                                     const GValue *value,
                                                     GParamSpec   *pspec);
static void     g_memory_output_stream_get_property (GObject      *object,
                                                     guint         prop_id,
                                                     GValue       *value,
                                                     GParamSpec   *pspec);
static void     g_memory_output_stream_finalize     (GObject      *object);

static gssize   g_memory_output_stream_write       (GOutputStream *stream,
                                                    const void    *buffer,
                                                    gsize          count,
                                                    GCancellable  *cancellable,
                                                    GError       **error);

static gboolean g_memory_output_stream_close       (GOutputStream  *stream,
                                                    GCancellable   *cancellable,
                                                    GError        **error);

static void     g_memory_output_stream_close_async  (GOutputStream        *stream,
                                                     int                   io_priority,
                                                     GCancellable         *cancellable,
                                                     GAsyncReadyCallback   callback,
                                                     gpointer              data);
static gboolean g_memory_output_stream_close_finish (GOutputStream        *stream,
                                                     GAsyncResult         *result,
                                                     GError              **error);

static void     g_memory_output_stream_seekable_iface_init (GSeekableIface  *iface);
static goffset  g_memory_output_stream_tell                (GSeekable       *seekable);
static gboolean g_memory_output_stream_can_seek            (GSeekable       *seekable);
static gboolean g_memory_output_stream_seek                (GSeekable       *seekable,
                                                           goffset          offset,
                                                           GSeekType        type,
                                                           GCancellable    *cancellable,
                                                           GError         **error);
static gboolean g_memory_output_stream_can_truncate        (GSeekable       *seekable);
static gboolean g_memory_output_stream_truncate            (GSeekable       *seekable,
                                                           goffset          offset,
                                                           GCancellable    *cancellable,
                                                           GError         **error);

static gboolean g_memory_output_stream_is_writable       (GPollableOutputStream *stream);
static GSource *g_memory_output_stream_create_source     (GPollableOutputStream *stream,
                                                          GCancellable          *cancellable);

static void g_memory_output_stream_pollable_iface_init (GPollableOutputStreamInterface *iface);

G_DEFINE_TYPE_WITH_CODE (GMemoryOutputStream, g_memory_output_stream, G_TYPE_OUTPUT_STREAM,
                         G_ADD_PRIVATE (GMemoryOutputStream)
                         G_IMPLEMENT_INTERFACE (G_TYPE_SEEKABLE,
                                                g_memory_output_stream_seekable_iface_init);
                         G_IMPLEMENT_INTERFACE (G_TYPE_POLLABLE_OUTPUT_STREAM,
                                                g_memory_output_stream_pollable_iface_init))


static void
g_memory_output_stream_class_init (GMemoryOutputStreamClass *klass)
{
  GOutputStreamClass *ostream_class;
  GObjectClass *gobject_class;

  gobject_class = G_OBJECT_CLASS (klass);
  gobject_class->set_property = g_memory_output_stream_set_property;
  gobject_class->get_property = g_memory_output_stream_get_property;
  gobject_class->finalize     = g_memory_output_stream_finalize;

  ostream_class = G_OUTPUT_STREAM_CLASS (klass);

  ostream_class->write_fn = g_memory_output_stream_write;
  ostream_class->close_fn = g_memory_output_stream_close;
  ostream_class->close_async  = g_memory_output_stream_close_async;
  ostream_class->close_finish = g_memory_output_stream_close_finish;

  /**
   * GMemoryOutputStream:data:
   *
   * Pointer to buffer where data will be written.
   *
   * Since: 2.24
   **/
  g_object_class_install_property (gobject_class,
                                   PROP_DATA,
                                   g_param_spec_pointer ("data",
                                                         P_("Data Buffer"),
                                                         P_("Pointer to buffer where data will be written."),
                                                         G_PARAM_READWRITE | G_PARAM_CONSTRUCT_ONLY |
                                                         G_PARAM_STATIC_STRINGS));

  /**
   * GMemoryOutputStream:size:
   *
   * Current size of the data buffer.
   *
   * Since: 2.24
   **/
  g_object_class_install_property (gobject_class,
                                   PROP_SIZE,
                                   g_param_spec_ulong ("size",
                                                       P_("Data Buffer Size"),
                                                       P_("Current size of the data buffer."),
                                                       0, G_MAXULONG, 0,
                                                       G_PARAM_READWRITE | G_PARAM_CONSTRUCT_ONLY |
                                                       G_PARAM_STATIC_STRINGS));

  /**
   * GMemoryOutputStream:data-size:
   *
   * Size of data written to the buffer.
   *
   * Since: 2.24
   **/
  g_object_class_install_property (gobject_class,
                                   PROP_DATA_SIZE,
                                   g_param_spec_ulong ("data-size",
                                                       P_("Data Size"),
                                                       P_("Size of data written to the buffer."),
                                                       0, G_MAXULONG, 0,
                                                       G_PARAM_READABLE |
                                                       G_PARAM_STATIC_STRINGS));

  /**
   * GMemoryOutputStream:realloc-function: (skip)
   *
   * Function with realloc semantics called to enlarge the buffer.
   *
   * Since: 2.24
   **/
  g_object_class_install_property (gobject_class,
                                   PROP_REALLOC_FUNCTION,
                                   g_param_spec_pointer ("realloc-function",
                                                         P_("Memory Reallocation Function"),
                                                         P_("Function with realloc semantics called to enlarge the buffer."),
                                                         G_PARAM_READWRITE | G_PARAM_CONSTRUCT_ONLY |
                                                         G_PARAM_STATIC_STRINGS));

  /**
   * GMemoryOutputStream:destroy-function: (skip)
   *
   * Function called with the buffer as argument when the stream is destroyed.
   *
   * Since: 2.24
   **/
  g_object_class_install_property (gobject_class,
                                   PROP_DESTROY_FUNCTION,
                                   g_param_spec_pointer ("destroy-function",
                                                         P_("Destroy Notification Function"),
                                                         P_("Function called with the buffer as argument when the stream is destroyed."),
                                                         G_PARAM_READWRITE | G_PARAM_CONSTRUCT_ONLY |
                                                         G_PARAM_STATIC_STRINGS));
}

static void
g_memory_output_stream_pollable_iface_init (GPollableOutputStreamInterface *iface)
{
  iface->is_writable = g_memory_output_stream_is_writable;
  iface->create_source = g_memory_output_stream_create_source;
}

static void
g_memory_output_stream_set_property (GObject      *object,
                                     guint         prop_id,
                                     const GValue *value,
                                     GParamSpec   *pspec)
{
  GMemoryOutputStream        *stream;
  GMemoryOutputStreamPrivate *priv;

  stream = G_MEMORY_OUTPUT_STREAM (object);
  priv = stream->priv;

  switch (prop_id)
    {
    case PROP_DATA:
      priv->data = g_value_get_pointer (value);
      break;
    case PROP_SIZE:
      priv->len = g_value_get_ulong (value);
      break;
    case PROP_REALLOC_FUNCTION:
      priv->realloc_fn = g_value_get_pointer (value);
      break;
    case PROP_DESTROY_FUNCTION:
      priv->destroy = g_value_get_pointer (value);
      break;
    default:
      G_OBJECT_WARN_INVALID_PROPERTY_ID (object, prop_id, pspec);
      break;
    }
}

static void
g_memory_output_stream_get_property (GObject      *object,
                                     guint         prop_id,
                                     GValue       *value,
                                     GParamSpec   *pspec)
{
  GMemoryOutputStream        *stream;
  GMemoryOutputStreamPrivate *priv;

  stream = G_MEMORY_OUTPUT_STREAM (object);
  priv = stream->priv;

  switch (prop_id)
    {
    case PROP_DATA:
      g_value_set_pointer (value, priv->data);
      break;
    case PROP_SIZE:
      g_value_set_ulong (value, priv->len);
      break;
    case PROP_DATA_SIZE:
      g_value_set_ulong (value, priv->valid_len);
      break;
    case PROP_REALLOC_FUNCTION:
      g_value_set_pointer (value, priv->realloc_fn);
      break;
    case PROP_DESTROY_FUNCTION:
      g_value_set_pointer (value, priv->destroy);
      break;
    default:
      G_OBJECT_WARN_INVALID_PROPERTY_ID (object, prop_id, pspec);
      break;
    }
}

static void
g_memory_output_stream_finalize (GObject *object)
{
  GMemoryOutputStream        *stream;
  GMemoryOutputStreamPrivate *priv;

  stream = G_MEMORY_OUTPUT_STREAM (object);
  priv = stream->priv;
  
  if (priv->destroy)
    priv->destroy (priv->data);

  G_OBJECT_CLASS (g_memory_output_stream_parent_class)->finalize (object);
}

static void
g_memory_output_stream_seekable_iface_init (GSeekableIface *iface)
{
  iface->tell         = g_memory_output_stream_tell;
  iface->can_seek     = g_memory_output_stream_can_seek;
  iface->seek         = g_memory_output_stream_seek;
  iface->can_truncate = g_memory_output_stream_can_truncate;
  iface->truncate_fn  = g_memory_output_stream_truncate;
}


static void
g_memory_output_stream_init (GMemoryOutputStream *stream)
{
  stream->priv = g_memory_output_stream_get_instance_private (stream);
  stream->priv->pos = 0;
  stream->priv->valid_len = 0;
}

/**
 * g_memory_output_stream_new: (skip)
 * @data: (nullable): pointer to a chunk of memory to use, or %NULL
 * @size: the size of @data
 * @realloc_function: (nullable): a function with realloc() semantics (like g_realloc())
 *     to be called when @data needs to be grown, or %NULL
 * @destroy_function: (nullable): a function to be called on @data when the stream is
 *     finalized, or %NULL
 *
 * Creates a new #GMemoryOutputStream.
 *
 * In most cases this is not the function you want.  See
 * g_memory_output_stream_new_resizable() instead.
 *
 * If @data is non-%NULL, the stream will use that for its internal storage.
 *
 * If @realloc_fn is non-%NULL, it will be used for resizing the internal
 * storage when necessary and the stream will be considered resizable.
 * In that case, the stream will start out being (conceptually) empty.
 * @size is used only as a hint for how big @data is.  Specifically,
 * seeking to the end of a newly-created stream will seek to zero, not
 * @size.  Seeking past the end of the stream and then writing will
 * introduce a zero-filled gap.
 *
 * If @realloc_fn is %NULL then the stream is fixed-sized.  Seeking to
 * the end will seek to @size exactly.  Writing past the end will give
 * an 'out of space' error.  Attempting to seek past the end will fail.
 * Unlike the resizable case, seeking to an offset within the stream and
 * writing will preserve the bytes passed in as @data before that point
 * and will return them as part of g_memory_output_stream_steal_data().
 * If you intend to seek you should probably therefore ensure that @data
 * is properly initialised.
 *
 * It is probably only meaningful to provide @data and @size in the case
 * that you want a fixed-sized stream.  Put another way: if @realloc_fn
 * is non-%NULL then it makes most sense to give @data as %NULL and
 * @size as 0 (allowing #GMemoryOutputStream to do the initial
 * allocation for itself).
 *
 * |[<!-- language="C" -->
 * // a stream that can grow
 * stream = g_memory_output_stream_new (NULL, 0, realloc, free);
 *
 * // another stream that can grow
 * stream2 = g_memory_output_stream_new (NULL, 0, g_realloc, g_free);
 *
 * // a fixed-size stream
 * data = malloc (200);
 * stream3 = g_memory_output_stream_new (data, 200, NULL, free);
 * ]|
 *
 * Returns: A newly created #GMemoryOutputStream object.
 **/
GOutputStream *
g_memory_output_stream_new (gpointer       data,
                            gsize          size,
                            GReallocFunc   realloc_function,
                            GDestroyNotify destroy_function)
{
  GOutputStream *stream;

  stream = g_object_new (G_TYPE_MEMORY_OUTPUT_STREAM,
                         "data", data,
                         "size", size,
                         "realloc-function", realloc_function,
                         "destroy-function", destroy_function,
                         NULL);

  return stream;
}

/**
 * g_memory_output_stream_new_resizable:
 *
 * Creates a new #GMemoryOutputStream, using g_realloc() and g_free()
 * for memory allocation.
 *
 * Since: 2.36
 */
GOutputStream *
g_memory_output_stream_new_resizable (void)
{
  return g_memory_output_stream_new (NULL, 0, g_realloc, g_free);
}

/**
 * g_memory_output_stream_get_data:
 * @ostream: a #GMemoryOutputStream
 *
 * Gets any loaded data from the @ostream.
 *
 * Note that the returned pointer may become invalid on the next
 * write or truncate operation on the stream.
 *
 * Returns: (transfer none): pointer to the stream's data, or %NULL if the data
 *    has been stolen
 **/
gpointer
g_memory_output_stream_get_data (GMemoryOutputStream *ostream)
{
  g_return_val_if_fail (G_IS_MEMORY_OUTPUT_STREAM (ostream), NULL);

  return ostream->priv->data;
}

/**
 * g_memory_output_stream_get_size:
 * @ostream: a #GMemoryOutputStream
 *
 * Gets the size of the currently allocated data area (available from
 * g_memory_output_stream_get_data()).
 *
 * You probably don't want to use this function on resizable streams.
 * See g_memory_output_stream_get_data_size() instead.  For resizable
 * streams the size returned by this function is an implementation
 * detail and may be change at any time in response to operations on the
 * stream.
 *
 * If the stream is fixed-sized (ie: no realloc was passed to
 * g_memory_output_stream_new()) then this is the maximum size of the
 * stream and further writes will return %G_IO_ERROR_NO_SPACE.
 *
 * In any case, if you want the number of bytes currently written to the
 * stream, use g_memory_output_stream_get_data_size().
 *
 * Returns: the number of bytes allocated for the data buffer
 */
gsize
g_memory_output_stream_get_size (GMemoryOutputStream *ostream)
{
  g_return_val_if_fail (G_IS_MEMORY_OUTPUT_STREAM (ostream), 0);

  return ostream->priv->len;
}

/**
 * g_memory_output_stream_get_data_size:
 * @ostream: a #GMemoryOutputStream
 *
 * Returns the number of bytes from the start up to including the last
 * byte written in the stream that has not been truncated away.
 *
 * Returns: the number of bytes written to the stream
 *
 * Since: 2.18
 */
gsize
g_memory_output_stream_get_data_size (GMemoryOutputStream *ostream)
{
  g_return_val_if_fail (G_IS_MEMORY_OUTPUT_STREAM (ostream), 0);

  return ostream->priv->valid_len;
}

/**
 * g_memory_output_stream_steal_data:
 * @ostream: a #GMemoryOutputStream
 *
 * Gets any loaded data from the @ostream. Ownership of the data
 * is transferred to the caller; when no longer needed it must be
 * freed using the free function set in @ostream's
 * #GMemoryOutputStream:destroy-function property.
 *
 * @ostream must be closed before calling this function.
 *
 * Returns: (transfer full): the stream's data, or %NULL if it has previously
 *    been stolen
 *
 * Since: 2.26
 **/
gpointer
g_memory_output_stream_steal_data (GMemoryOutputStream *ostream)
{
  gpointer data;

  g_return_val_if_fail (G_IS_MEMORY_OUTPUT_STREAM (ostream), NULL);
  g_return_val_if_fail (g_output_stream_is_closed (G_OUTPUT_STREAM (ostream)), NULL);

  data = ostream->priv->data;
  ostream->priv->data = NULL;

  return data;
}

/**
 * g_memory_output_stream_steal_as_bytes:
 * @ostream: a #GMemoryOutputStream
 *
 * Returns data from the @ostream as a #GBytes. @ostream must be
 * closed before calling this function.
 *
 * Returns: (transfer full): the stream's data
 *
 * Since: 2.34
 **/
GBytes *
g_memory_output_stream_steal_as_bytes (GMemoryOutputStream *ostream)
{
  GBytes *result;

  g_return_val_if_fail (G_IS_MEMORY_OUTPUT_STREAM (ostream), NULL);
  g_return_val_if_fail (g_output_stream_is_closed (G_OUTPUT_STREAM (ostream)), NULL);

  result = g_bytes_new_with_free_func (ostream->priv->data,
                                       ostream->priv->valid_len,
                                       ostream->priv->destroy,
                                       ostream->priv->data);
  ostream->priv->data = NULL;

  return result;
}

static gboolean
array_resize (GMemoryOutputStream  *ostream,
              gsize                 size,
              gboolean              allow_partial,
              GError              **error)
{
  GMemoryOutputStreamPrivate *priv;
  gpointer data;
  gsize len;

  priv = ostream->priv;

  if (priv->len == size)
    return TRUE;

  if (!priv->realloc_fn)
    {
      if (allow_partial &&
          priv->pos < priv->len)
        return TRUE; /* Short write */

      g_set_error_literal (error,
                           G_IO_ERROR,
                           G_IO_ERROR_NO_SPACE,
                           _("Memory output stream not resizable"));
      return FALSE;
    }

  len = priv->len;
  data = priv->realloc_fn (priv->data, size);

  if (size > 0 && !data)
    {
      if (allow_partial &&
          priv->pos < priv->len)
        return TRUE; /* Short write */

      g_set_error_literal (error,
                           G_IO_ERROR,
                           G_IO_ERROR_NO_SPACE,
                           _("Failed to resize memory output stream"));
      return FALSE;
    }

  if (size > len)
    memset ((guint8 *)data + len, 0, size - len);

  priv->data = data;
  priv->len = size;

  if (priv->len < priv->valid_len)
    priv->valid_len = priv->len;

  return TRUE;
}

static gsize
g_nearest_pow (gsize num)
{
  gsize n = 1;

  while (n < num && n > 0)
    n <<= 1;

  return n;
}

static gssize
g_memory_output_stream_write (GOutputStream  *stream,
                              const void     *buffer,
                              gsize           count,
                              GCancellable   *cancellable,
                              GError        **error)
{
  GMemoryOutputStream        *ostream;
  GMemoryOutputStreamPrivate *priv;
  guint8   *dest;
  gsize new_size;

  ostream = G_MEMORY_OUTPUT_STREAM (stream);
  priv = ostream->priv;

  if (count == 0)
    return 0;

  /* Check for address space overflow, but only if the buffer is resizable.
     Otherwise we just do a short write and don't worry. */
  if (priv->realloc_fn && priv->pos + count < priv->pos)
    goto overflow;

  if (priv->pos + count > priv->len)
    {
      /* At least enough to fit the write, rounded up for greater than
       * linear growth.
       *
       * Assuming that we're using something like realloc(), the kernel
       * will overcommit memory to us, so doubling the size each time
       * will keep the number of realloc calls low without wasting too
       * much memory.
       */
      new_size = g_nearest_pow (priv->pos + count);
      /* Check for overflow again. We have checked if
         pos + count > G_MAXSIZE, but now check if g_nearest_pow () has
         overflowed */
      if (new_size == 0)
        goto overflow;

      new_size = MAX (new_size, MIN_ARRAY_SIZE);
      if (!array_resize (ostream, new_size, TRUE, error))
        return -1;
    }

  /* Make sure we handle short writes if the array_resize
     only added part of the required memory */
  count = MIN (count, priv->len - priv->pos);

  dest = (guint8 *)priv->data + priv->pos;
  memcpy (dest, buffer, count);
  priv->pos += count;

  if (priv->pos > priv->valid_len)
    priv->valid_len = priv->pos;

  return count;

 overflow:
  /* Overflow: buffer size would need to be bigger than G_MAXSIZE. */
  g_set_error_literal (error,
                       G_IO_ERROR,
                       G_IO_ERROR_NO_SPACE,
                       _("Amount of memory required to process the write is "
                         "larger than available address space"));
  return -1;
}

static gboolean
g_memory_output_stream_close (GOutputStream  *stream,
                              GCancellable   *cancellable,
                              GError        **error)
{
  return TRUE;
}

static void
g_memory_output_stream_close_async (GOutputStream       *stream,
                                    int                  io_priority,
                                    GCancellable        *cancellable,
                                    GAsyncReadyCallback  callback,
                                    gpointer             data)
{
  GTask *task;

  task = g_task_new (stream, cancellable, callback, data);
  g_task_set_source_tag (task, g_memory_output_stream_close_async);

  /* will always return TRUE */
  g_memory_output_stream_close (stream, cancellable, NULL);

  g_task_return_boolean (task, TRUE);
  g_object_unref (task);
}

static gboolean
g_memory_output_stream_close_finish (GOutputStream  *stream,
                                     GAsyncResult   *result,
                                     GError        **error)
{
  g_return_val_if_fail (g_task_is_valid (result, stream), FALSE);

  return g_task_propagate_boolean (G_TASK (result), error);
}

static goffset
g_memory_output_stream_tell (GSeekable *seekable)
{
  GMemoryOutputStream *stream;
  GMemoryOutputStreamPrivate *priv;

  stream = G_MEMORY_OUTPUT_STREAM (seekable);
  priv = stream->priv;

  return priv->pos;
}

static gboolean
g_memory_output_stream_can_seek (GSeekable *seekable)
{
  return TRUE;
}

static gboolean
g_memory_output_stream_seek (GSeekable    *seekable,
                             goffset        offset,
                             GSeekType      type,
                             GCancellable  *cancellable,
                             GError       **error)
{
  GMemoryOutputStream        *stream;
  GMemoryOutputStreamPrivate *priv;
  goffset absolute;

  stream = G_MEMORY_OUTPUT_STREAM (seekable);
  priv = stream->priv;

  switch (type)
    {
    case G_SEEK_CUR:
      absolute = priv->pos + offset;
      break;

    case G_SEEK_SET:
      absolute = offset;
      break;

    case G_SEEK_END:
      /* For resizable streams, we consider the end to be the data
       * length.  For fixed-sized streams, we consider the end to be the
       * size of the buffer.
       */
      if (priv->realloc_fn)
        absolute = priv->valid_len + offset;
      else
        absolute = priv->len + offset;
      break;

    default:
      g_set_error_literal (error,
                           G_IO_ERROR,
                           G_IO_ERROR_INVALID_ARGUMENT,
                           _("Invalid GSeekType supplied"));

      return FALSE;
    }

  if (absolute < 0)
    {
      g_set_error_literal (error,
                           G_IO_ERROR,
                           G_IO_ERROR_INVALID_ARGUMENT,
                           _("Requested seek before the beginning of the stream"));
      return FALSE;
    }

  /* Can't seek past the end of a fixed-size stream.
   *
   * Note: seeking to the non-existent byte at the end of a fixed-sized
   * stream is valid (eg: a 1-byte fixed sized stream can have position
   * 0 or 1).  Therefore '>' is what we want.
   * */
  if (priv->realloc_fn == NULL && absolute > priv->len)
    {
      g_set_error_literal (error,
                           G_IO_ERROR,
                           G_IO_ERROR_INVALID_ARGUMENT,
                           _("Requested seek beyond the end of the stream"));
      return FALSE;
    }

  priv->pos = absolute;

  return TRUE;
}

static gboolean
g_memory_output_stream_can_truncate (GSeekable *seekable)
{
  GMemoryOutputStream *ostream;
  GMemoryOutputStreamPrivate *priv;

  ostream = G_MEMORY_OUTPUT_STREAM (seekable);
  priv = ostream->priv;

  /* We do not allow truncation of fixed-sized streams */
  return priv->realloc_fn != NULL;
}

static gboolean
g_memory_output_stream_truncate (GSeekable     *seekable,
                                 goffset        offset,
                                 GCancellable  *cancellable,
                                 GError       **error)
{
  GMemoryOutputStream *ostream = G_MEMORY_OUTPUT_STREAM (seekable);

  if (!array_resize (ostream, offset, FALSE, error))
    return FALSE;

  ostream->priv->valid_len = offset;

  return TRUE;
}

static gboolean
g_memory_output_stream_is_writable (GPollableOutputStream *stream)
{
  return TRUE;
}

static GSource *
g_memory_output_stream_create_source (GPollableOutputStream *stream,
                                      GCancellable          *cancellable)
{
  GSource *base_source, *pollable_source;

  base_source = g_timeout_source_new (0);
  pollable_source = g_pollable_source_new_full (stream, base_source, cancellable);
  g_source_unref (base_source);

  return pollable_source;
}