Implement double buffering

Until this point we was using one single buffer. This cause tearing when
they are scanned while we are drawing.

This refactorize our buffer and damage tracking to use two buffers. One
buffer, the back_buffer, is our dirty area. The other one, the
display_buffer, is left untouched.

Now we only flip buffers on adequate moments, indicated by the
compositor sending the frame callback event. We can draw multiple
time between those events, and we store all damaged areas. We only
schedule frame callbacks when needed, after storing a new damaged area.

Once flipped, we backport the damaged area from the new display_buffer
to the new back_buffer. Which generally means, for wvkbd, one rectangle
for each one of the surf and popup_surf.

Signed-off-by: Maarten van Gompel <proycon@anaproy.nl>

drw.c

@@ -6,34 +6,102 @@
 #include "shm_open.h"
 #include "math.h"
 
+void drwsurf_handle_frame_cb(void* data, struct wl_callback* callback,
+    uint32_t time)
+{
+    struct drwsurf *ds = data;
+    wl_callback_destroy(ds->frame_cb);
+    ds->frame_cb = NULL;
+
+    cairo_rectangle_int_t r = {0};
+    for (int i = 0; i < cairo_region_num_rectangles(ds->back_buffer->damage); i++) {
+        cairo_region_get_rectangle(ds->back_buffer->damage, i, &r);
+        wl_surface_damage(ds->surf, r.x, r.y, r.width, r.height);
+    };
+
+    drwsurf_flip(ds);
+}
+
+const struct wl_callback_listener frame_listener = {
+    .done = drwsurf_handle_frame_cb
+};
+
+void drwsurf_register_frame_cb(struct drwsurf *ds)
+{
+    if (ds->frame_cb)
+        return;
+    if (!ds->attached)
+        return;
+    ds->frame_cb = wl_surface_frame(ds->surf);
+    wl_callback_add_listener(ds->frame_cb, &frame_listener, ds);
+    wl_surface_commit(ds->surf);
+}
+
+void drwsurf_damage(struct drwsurf *ds, uint32_t x, uint32_t y, uint32_t w, uint32_t h)
+{
+    cairo_rectangle_int_t rect = { x, y, w, h };
+    cairo_region_union_rectangle(ds->back_buffer->damage, &rect);
+    drwsurf_register_frame_cb(ds);
+}
+
 void
 drwsurf_resize(struct drwsurf *ds, uint32_t w, uint32_t h, double s)
 {
-    if (ds->buf) {
-        munmap(ds->pool_data, ds->size);
-        wl_buffer_destroy(ds->buf);
-        ds->buf = NULL;
-    }
-
     ds->scale = s;
     ds->width = ceil(w * s);
     ds->height = ceil(h * s);
 
-    setup_buffer(ds);
+    setup_buffer(ds, ds->back_buffer);
+    setup_buffer(ds, ds->display_buffer);
+}
+
+void
+drwsurf_backport(struct drwsurf *ds)
+{
+    cairo_save(ds->back_buffer->cairo);
+
+    cairo_scale(ds->back_buffer->cairo, 1/ds->scale, 1/ds->scale);
+    cairo_set_operator(ds->back_buffer->cairo, CAIRO_OPERATOR_SOURCE);
+
+    cairo_rectangle_int_t r = {0};
+    for (int i = 0; i < cairo_region_num_rectangles(ds->display_buffer->damage); i++) {
+        cairo_region_get_rectangle(ds->display_buffer->damage, i, &r);
+
+        cairo_set_source_surface(ds->back_buffer->cairo, ds->display_buffer->cairo_surf, 0, 0);
+        cairo_rectangle(
+            ds->back_buffer->cairo,
+            r.x * ds->scale,
+            r.y * ds->scale,
+            r.width * ds->scale,
+            r.height * ds->scale
+        );
+        cairo_fill(ds->back_buffer->cairo);
+    };
+
+    cairo_restore(ds->back_buffer->cairo);
+    cairo_region_subtract(ds->display_buffer->damage, ds->display_buffer->damage);
 }
 
 void
 drwsurf_flip(struct drwsurf *ds)
 {
-    wl_surface_attach(ds->surf, ds->buf, 0, 0);
+    wl_surface_attach(ds->surf, ds->back_buffer->buf, 0, 0);
     wl_surface_commit(ds->surf);
+    ds->attached = true;
+
+    struct drwbuf *tmp = ds->back_buffer;
+    ds->back_buffer = ds->display_buffer;
+    ds->display_buffer = tmp;
+
+    drwsurf_backport(ds);
 }
 
 void
-drw_draw_text(struct drwsurf *d, Color color, uint32_t x, uint32_t y,
+drw_draw_text(struct drwsurf *ds, Color color, uint32_t x, uint32_t y,
               uint32_t w, uint32_t h, uint32_t b, const char *label,
               PangoFontDescription *font_description)
 {
+    struct drwbuf *d = ds->back_buffer;
 
     cairo_save(d->cairo);
 
@@ -58,8 +126,10 @@ drw_draw_text(struct drwsurf *d, Color color, uint32_t x, uint32_t y,
 }
 
 void
-drw_do_clear(struct drwsurf *d, uint32_t x, uint32_t y, uint32_t w, uint32_t h)
+drw_do_clear(struct drwsurf *ds, uint32_t x, uint32_t y, uint32_t w, uint32_t h)
 {
+    struct drwbuf *d = ds->back_buffer;
+
     cairo_save(d->cairo);
 
     cairo_set_operator(d->cairo, CAIRO_OPERATOR_CLEAR);
@@ -70,9 +140,11 @@ drw_do_clear(struct drwsurf *d, uint32_t x, uint32_t y, uint32_t w, uint32_t h)
 }
 
 void
-drw_do_rectangle(struct drwsurf *d, Color color, uint32_t x, uint32_t y,
+drw_do_rectangle(struct drwsurf *ds, Color color, uint32_t x, uint32_t y,
                  uint32_t w, uint32_t h, bool over, int rounding)
 {
+    struct drwbuf *d = ds->back_buffer;
+
     cairo_save(d->cairo);
 
     if (over) {
@@ -128,41 +200,55 @@ drw_over_rectangle(struct drwsurf *d, Color color, uint32_t x, uint32_t y,
 }
 
 uint32_t
-setup_buffer(struct drwsurf *drwsurf)
+setup_buffer(struct drwsurf *drwsurf, struct drwbuf *drwbuf)
 {
+    int prev_size = drwbuf->size;
     int stride = drwsurf->width * 4;
-    drwsurf->size = stride * drwsurf->height;
+    drwbuf->size = stride * drwsurf->height;
 
-    int fd = allocate_shm_file(drwsurf->size);
+    int fd = allocate_shm_file(drwbuf->size);
     if (fd == -1) {
         return 1;
     }
 
-    drwsurf->pool_data =
-        mmap(NULL, drwsurf->size, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
-    if (drwsurf->pool_data == MAP_FAILED) {
+    if (drwbuf->pool_data)
+        munmap(drwbuf->pool_data, prev_size);
+    drwbuf->pool_data =
+        mmap(NULL, drwbuf->size, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
+    if (drwbuf->pool_data == MAP_FAILED) {
         close(fd);
         return 1;
     }
 
+    if (drwbuf->buf)
+        wl_buffer_destroy(drwbuf->buf);
     struct wl_shm_pool *pool =
-        wl_shm_create_pool(drwsurf->ctx->shm, fd, drwsurf->size);
-    drwsurf->buf =
+        wl_shm_create_pool(drwsurf->ctx->shm, fd, drwbuf->size);
+    drwbuf->buf =
         wl_shm_pool_create_buffer(pool, 0, drwsurf->width, drwsurf->height,
                                   stride, WL_SHM_FORMAT_ARGB8888);
     wl_shm_pool_destroy(pool);
     close(fd);
 
-    cairo_surface_t *s = cairo_image_surface_create_for_data(
-        drwsurf->pool_data, CAIRO_FORMAT_ARGB32, drwsurf->width,
+
+    if (drwbuf->cairo_surf)
+        cairo_surface_destroy(drwbuf->cairo_surf);
+    drwbuf->cairo_surf = cairo_image_surface_create_for_data(
+        drwbuf->pool_data, CAIRO_FORMAT_ARGB32, drwsurf->width,
         drwsurf->height, stride);
 
-    drwsurf->cairo = cairo_create(s);
-    cairo_scale(drwsurf->cairo, drwsurf->scale, drwsurf->scale);
-    cairo_set_antialias(drwsurf->cairo, CAIRO_ANTIALIAS_NONE);
-    drwsurf->layout = pango_cairo_create_layout(drwsurf->cairo);
-    pango_layout_set_auto_dir(drwsurf->layout, false);
-    cairo_save(drwsurf->cairo);
+    if (drwbuf->damage)
+        cairo_region_destroy(drwbuf->damage);
+    drwbuf->damage = cairo_region_create();
+
+    if (drwbuf->cairo)
+        cairo_destroy(drwbuf->cairo);
+    drwbuf->cairo = cairo_create(drwbuf->cairo_surf);
+    cairo_scale(drwbuf->cairo, drwsurf->scale, drwsurf->scale);
+    cairo_set_antialias(drwbuf->cairo, CAIRO_ANTIALIAS_NONE);
+    drwbuf->layout = pango_cairo_create_layout(drwbuf->cairo);
+    pango_layout_set_auto_dir(drwbuf->layout, false);
+    cairo_save(drwbuf->cairo);
 
     return 0;
 }