[test] Compare a failure against the image output

If a backend fails in exactly the same way as the image, then we can
safely assume that the failure is systematic and not an error in the
backend, so change the result to XFAIL.

1 files changed, 46 insertions, 0 deletions

diff --git a/test/cairo-test.c b/test/cairo-test.c
index 55e28dc7..1f495af6 100644
--- a/test/cairo-test.c
+++ b/test/cairo-test.c
@@ -1369,6 +1369,52 @@ REPEAT:
 	    cairo_test_copy_file (test_filename, pass_filename);
 	}
 
+	/* If failed, compare against the current image output,
+	 * and attempt to detect systematic failures.
+	 */
+	if (ret == CAIRO_TEST_FAILURE) {
+	    char *image_out_path;
+
+	    image_out_path =
+		cairo_test_reference_filename (ctx,
+					       base_name,
+					       ctx->test_name,
+					       "image",
+					       "image",
+					       format,
+					       CAIRO_TEST_OUT_SUFFIX,
+					       CAIRO_TEST_PNG_EXTENSION);
+	    if (image_out_path != NULL) {
+		if (cairo_test_files_equal (out_png_path,
+					    image_out_path))
+		{
+		    ret = CAIRO_TEST_XFAILURE;
+		}
+		else
+		{
+		    ref_image =
+			cairo_image_surface_create_from_png (image_out_path);
+		    if (cairo_surface_status (ref_image) == CAIRO_STATUS_SUCCESS)
+		    {
+			diff_status = image_diff (ctx,
+						  test_image, ref_image,
+						  diff_image,
+						  &result);
+			if (diff_status == CAIRO_STATUS_SUCCESS &&
+			    (result.pixels_changed == 0 ||
+			     result.max_diff > target->error_tolerance))
+			{
+			    ret = CAIRO_TEST_XFAILURE;
+			}
+
+			cairo_surface_destroy (ref_image);
+		    }
+		}
+
+		free (image_out_path);
+	    }
+	}
+
 	cairo_surface_destroy (test_image);
 	cairo_surface_destroy (diff_image);
     }