[PATCH 4/5] compositor: Version the backend configuration structures

[Bryce Harrington]

With this struct versioning, it is possible to add new options without
breaking the ABI, as long as all additions are made to the end of a
struct and nothing existing is modified or removed.  When things are
added, the structure's size will increase, and we'll use this size as
our minor version number.  If existing things need to be changed, then
the major version, struct_version, is incremented to indicate the ABI
break.

>From our call site in main we record these major and minor version as
struct_version and struct_size.  The backend then verifies these against
its own assumptions.  So long as the backend's struct is equal or larger
than what was passed in and the major versions are equal, we're good;
but if it is larger, then this is a fatal error.

Signed-off-by: Bryce Harrington <bryce at osg.samsung.com>
---
 src/compositor-drm.c | 10 ++++++++--
 src/compositor.h     | 16 ++++++++++++++++
 src/main.c           |  2 ++
 3 files changed, 26 insertions(+), 2 deletions(-)

diff --git a/src/compositor-drm.c b/src/compositor-drm.c
index 5ddedb9..9bce285 100644
--- a/src/compositor-drm.c
+++ b/src/compositor-drm.c
@@ -3203,8 +3203,14 @@ backend_init(struct weston_compositor *compositor, int *argc, char *argv[],
              struct weston_backend_config *config_base)
 {
 	struct drm_backend *b;
-	struct weston_drm_backend_config *config =
-				(struct weston_drm_backend_config *)config_base;
+	struct weston_drm_backend_config *config;
+
+	if (config_base == NULL ||
+	    config_base->struct_version != 1 ||
+	    config_base->struct_size > sizeof(struct weston_drm_backend_config))
+		return -1;
+
+	config = (struct weston_drm_backend_config *)config_base;
 
 	b = drm_backend_create(compositor, config);
 	if (b == NULL)
diff --git a/src/compositor.h b/src/compositor.h
index 30462cf..3e52703 100644
--- a/src/compositor.h
+++ b/src/compositor.h
@@ -684,6 +684,22 @@ struct weston_backend_output_config {
  * data.
  */
 struct weston_backend_config {
+	/** Major version for the backend-specific config struct
+	 *
+	 * This version must match exactly what the backend expects, otherwise
+	 * the struct is incompatible.
+	 */
+	uint32_t struct_version;
+
+	/** Minor version of the backend-specific config struct
+	 *
+	 * This must be set to sizeof(struct backend-specific config).
+	 * If the value here is smaller than what the backend expects, the
+	 * extra config members will assume their default values.
+	 *
+	 * A value greater than what the backend expects is incompatible.
+	 */
+	size_t struct_size;
 };
 
 struct weston_backend {
diff --git a/src/main.c b/src/main.c
index 66c054e..7370292 100644
--- a/src/main.c
+++ b/src/main.c
@@ -748,6 +748,8 @@ load_drm_backend(struct weston_compositor *c, const char *backend,
 					 "gbm-format", &config->base.format,
 					 NULL);
 
+	config->base.base.struct_version = 1;
+	config->base.base.struct_size = sizeof(struct weston_drm_backend_config);
 	config->base.configure_output = drm_configure_output;
 
 	if (load_backend_new(c, backend, &config->base.base) < 0) {
-- 
1.9.1