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bytes: avoid intermediate refs in g_bytes_new_from_bytes()

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When referencing a GBytes that is already a slice of another GBytes, we
can avoid referencing the intermediate GBytes and instead reference the
root bytes.

Doing so helps avoid keeping N GBytes instances alive when the
intermediates would have otherwise been finalized.

https://bugzilla.gnome.org/show_bug.cgi?id=792780
This commit is contained in:
Christian Hergert 2018-01-22 15:26:07 -08:00
parent 801accf746
commit 4151bce6db
1 changed files with 26 additions and 1 deletions
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27
glib/gbytes.c
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@ -203,6 +203,12 @@ g_bytes_new_with_free_func (gconstpointer data,
* A reference to @bytes will be held by the newly created #GBytes until
* the byte data is no longer needed.
*
* Since 2.56, if @offset is 0 and @length matches the size of @bytes, then
* @bytes will be returned with the reference count incremented by 1. If @bytes
* is a slice of another #GBytes, then the resulting #GBytes will reference
* the same #GBytes instead of @bytes. This allows consumers to simplify the
* usage of #GBytes when asynchronously writing to streams.
*
* Returns: (transfer full): a new #GBytes
*
* Since: 2.32
@ -212,12 +218,31 @@ g_bytes_new_from_bytes (GBytes *bytes,
gsize offset,
gsize length)
{
gchar *base;
/* Note that length may be 0. */
g_return_val_if_fail (bytes != NULL, NULL);
g_return_val_if_fail (offset <= bytes->size, NULL);
g_return_val_if_fail (offset + length <= bytes->size, NULL);
return g_bytes_new_with_free_func ((gchar *)bytes->data + offset, length,
/* Avoid an extra GBytes if all bytes were requested */
if (offset == 0 && length == bytes->size)
return g_bytes_ref (bytes);
base = (gchar *)bytes->data + offset;
/* Avoid referencing intermediate GBytes. In practice, this should
* only loop once.
*/
while (bytes->free_func == (gpointer)g_bytes_unref)
bytes = bytes->user_data;
g_return_val_if_fail (bytes != NULL, NULL);
g_return_val_if_fail (base >= (gchar *)bytes->data, NULL);
g_return_val_if_fail (base <= (gchar *)bytes->data + bytes->size, NULL);
g_return_val_if_fail (base + length <= (gchar *)bytes->data + bytes->size, NULL);
return g_bytes_new_with_free_func (base, length,
(GDestroyNotify)g_bytes_unref, g_bytes_ref (bytes));
}