Real zero-copy cow snapshotting

The first iteration of COW snapshotting always made an initial copy when
the snapshot was requested (and reused that copy until the surface was
modified). However, in a few circumstances we can avoid even that copy
so long as the surface is still alive and unmodified between the
snapshotting and its use. In order to do so, we need a new proxy surface
that can automatically perform the copy if the target should disappear
prior to use.

1 files changed, 225 insertions, 0 deletions

diff --git a/src/cairo-surface-snapshot.c b/src/cairo-surface-snapshot.c
new file mode 100644
index 000000000..e59a9e120
--- /dev/null
+++ b/src/cairo-surface-snapshot.c
@@ -0,0 +1,225 @@
+/* cairo - a vector graphics library with display and print output
+ *
+ * Copyright © 2002 University of Southern California
+ * Copyright © 2005 Red Hat, Inc.
+ * Copyright © 2009 Intel Corporation
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it either under the terms of the GNU Lesser General Public
+ * License version 2.1 as published by the Free Software Foundation
+ * (the "LGPL") or, at your option, under the terms of the Mozilla
+ * Public License Version 1.1 (the "MPL"). If you do not alter this
+ * notice, a recipient may use your version of this file under either
+ * the MPL or the LGPL.
+ *
+ * You should have received a copy of the LGPL along with this library
+ * in the file COPYING-LGPL-2.1; if not, write to the Free Software
+ * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
+ * You should have received a copy of the MPL along with this library
+ * in the file COPYING-MPL-1.1
+ *
+ * The contents of this file are subject to the Mozilla Public License
+ * Version 1.1 (the "License"); you may not use this file except in
+ * compliance with the License. You may obtain a copy of the License at
+ * http://www.mozilla.org/MPL/
+ *
+ * This software is distributed on an "AS IS" basis, WITHOUT WARRANTY
+ * OF ANY KIND, either express or implied. See the LGPL or the MPL for
+ * the specific language governing rights and limitations.
+ *
+ * The Original Code is the cairo graphics library.
+ *
+ * The Initial Developer of the Original Code is University of Southern
+ * California.
+ *
+ * Contributor(s):
+ *	Carl D. Worth <cworth@cworth.org>
+ *      Chris Wilson <chris@chris-wilson.co.uk>
+ */
+
+#include "cairoint.h"
+
+#include "cairo-error-private.h"
+#include "cairo-surface-snapshot-private.h"
+
+static cairo_status_t
+_cairo_surface_snapshot_finish (void *abstract_surface)
+{
+    cairo_surface_snapshot_t *surface = abstract_surface;
+
+    cairo_surface_destroy (surface->clone);
+
+    return CAIRO_STATUS_SUCCESS;
+}
+
+static cairo_status_t
+_cairo_surface_snapshot_acquire_source_image (void                    *abstract_surface,
+					      cairo_image_surface_t  **image_out,
+					      void                   **extra_out)
+{
+    cairo_surface_snapshot_t *surface = abstract_surface;
+
+    return _cairo_surface_acquire_source_image (surface->target, image_out, extra_out);
+}
+
+static void
+_cairo_surface_snapshot_release_source_image (void                   *abstract_surface,
+					      cairo_image_surface_t  *image,
+					      void                   *extra)
+{
+    cairo_surface_snapshot_t *surface = abstract_surface;
+
+    _cairo_surface_release_source_image (surface->target, image, extra);
+}
+
+static cairo_bool_t
+_cairo_surface_snapshot_get_extents (void                  *abstract_surface,
+				     cairo_rectangle_int_t *extents)
+{
+    cairo_surface_snapshot_t *surface = abstract_surface;
+
+    return _cairo_surface_get_extents (surface->target, extents);
+}
+
+static const cairo_surface_backend_t _cairo_surface_snapshot_backend = {
+    CAIRO_INTERNAL_SURFACE_TYPE_SNAPSHOT,
+
+    NULL, /* create similar */
+    _cairo_surface_snapshot_finish,
+
+    _cairo_surface_snapshot_acquire_source_image,
+    _cairo_surface_snapshot_release_source_image,
+    NULL, NULL, /* acquire, release dest */
+    NULL, /* clone similar */
+    NULL, /* composite */
+    NULL, /* fill rectangles */
+    NULL, /* composite trapezoids */
+    NULL, /* create span renderer */
+    NULL, /* check span renderer */
+    NULL, /* copy_page */
+    NULL, /* show_page */
+    _cairo_surface_snapshot_get_extents,
+};
+
+static void
+_cairo_surface_snapshot_copy_on_write (cairo_surface_t *surface)
+{
+    cairo_surface_snapshot_t *snapshot = (cairo_surface_snapshot_t *) surface;
+    cairo_image_surface_t *image;
+    cairo_image_surface_t *clone;
+    void *extra;
+    cairo_status_t status;
+
+    /* We need to make an image copy of the original surface since the
+     * snapshot may exceed the lifetime of the original device, i.e.
+     * when we later need to use the snapshot the data may have already
+     * been lost.
+     */
+
+    status = _cairo_surface_acquire_source_image (snapshot->target, &image, &extra);
+    if (unlikely (status)) {
+	snapshot->target = _cairo_surface_create_in_error (status);
+	return;
+    }
+
+    clone = (cairo_image_surface_t *)
+	_cairo_image_surface_create_with_pixman_format (NULL,
+							image->pixman_format,
+							image->width,
+							image->height,
+							0);
+    if (likely (clone->base.status == CAIRO_STATUS_SUCCESS)) {
+	pixman_image_composite (PIXMAN_OP_SRC,
+				image->pixman_image, NULL, clone->pixman_image,
+				0, 0,
+				0, 0,
+				0, 0,
+				image->width, image->height);
+	clone->base.is_clear = FALSE;
+
+	snapshot->clone = &clone->base;
+    } else {
+	snapshot->clone = &clone->base;
+    }
+
+    _cairo_surface_release_source_image (snapshot->target, image, extra);
+    snapshot->target = snapshot->clone;
+}
+
+/**
+ * _cairo_surface_snapshot
+ * @surface: a #cairo_surface_t
+ *
+ * Make an immutable reference to @surface. It is an error to call a
+ * surface-modifying function on the result of this function. The
+ * resulting 'snapshot' is a lazily copied-on-write surface i.e. it
+ * remains a reference to the original surface until that surface is
+ * written to again, at which time a copy is made of the original surface
+ * and the snapshot then points to that instead. Multiple snapshots of the
+ * same unmodified surface point to the same copy.
+ *
+ * The caller owns the return value and should call
+ * cairo_surface_destroy() when finished with it. This function will not
+ * return %NULL, but will return a nil surface instead.
+ *
+ * Return value: The snapshot surface. Note that the return surface
+ * may not necessarily be of the same type as @surface.
+ **/
+cairo_surface_t *
+_cairo_surface_snapshot (cairo_surface_t *surface)
+{
+    cairo_surface_snapshot_t *snapshot;
+    cairo_status_t status;
+
+    if (unlikely (surface->status))
+	return _cairo_surface_create_in_error (surface->status);
+
+    if (surface->finished)
+	return _cairo_surface_create_in_error (_cairo_error (CAIRO_STATUS_SURFACE_FINISHED));
+
+    if (surface->snapshot_of != NULL)
+	return cairo_surface_reference (surface);
+
+    if (surface->backend->snapshot != NULL) {
+	cairo_surface_t *snap;
+
+	snap = surface->backend->snapshot (surface);
+	if (snap != NULL)
+	    return snap;
+    }
+
+    snapshot = (cairo_surface_snapshot_t *)
+	_cairo_surface_has_snapshot (surface, &_cairo_surface_snapshot_backend);
+    if (snapshot != NULL)
+	return cairo_surface_reference (&snapshot->base);
+
+    snapshot = malloc (sizeof (cairo_surface_snapshot_t));
+    if (unlikely (snapshot == NULL))
+	return _cairo_surface_create_in_error (_cairo_error (CAIRO_STATUS_SURFACE_FINISHED));
+
+    _cairo_surface_init (&snapshot->base,
+			 &_cairo_surface_snapshot_backend,
+			 surface->content);
+
+    snapshot->target = surface;
+    snapshot->clone = NULL;
+
+    status = _cairo_surface_copy_mime_data (&snapshot->base, surface);
+    if (unlikely (status)) {
+	cairo_surface_destroy (&snapshot->base);
+	return _cairo_surface_create_in_error (status);
+    }
+
+    snapshot->base.device_transform = surface->device_transform;
+    snapshot->base.device_transform_inverse = surface->device_transform_inverse;
+
+    status = _cairo_surface_attach_snapshot (surface,
+					     &snapshot->base,
+					     _cairo_surface_snapshot_copy_on_write);
+    if (unlikely (status)) {
+	cairo_surface_destroy (&snapshot->base);
+	return _cairo_surface_create_in_error (status);
+    }
+
+    return &snapshot->base;
+}