shm: track busy buffers’ age, and add compile-time option to force double buffering

By default, age all matching buffers that are busy (i.e. in use by the
compositor).

This allows us to detect whether we can apply the current frame’s
damage directly, or if we need to prepare the buffer first (e.g. copy
old buffer, or re-apply last frame’s damage etc).

render.c

@@ -2048,6 +2048,11 @@ grid_render(struct terminal *term)
         term->is_searching != term->render.was_searching ||
         term->render.margins)
     {
+        if (buf->age > 0) {
+            LOG_DBG("compositor double buffers (age=%d): last=%p, cur=%p",
+                    buf->age, (void*)term->render.last_buf, (void*)buf);
+        }
+
         if (term->render.last_buf != NULL &&
             term->render.last_buf->width == buf->width &&
             term->render.last_buf->height == buf->height &&
@@ -2080,6 +2085,7 @@ grid_render(struct terminal *term)
         term->render.was_searching = term->is_searching;
     }
 
+    buf->age = 0;
     tll_foreach(term->grid->scroll_damage, it) {
         switch (it->item.type) {
         case DAMAGE_SCROLL:

shm.c

@@ -30,6 +30,8 @@
 
 #define TIME_SCROLL 0
 
+#define FORCED_DOUBLE_BUFFERING 0
+
 /*
  * Maximum memfd size allowed.
  *
@@ -222,6 +224,8 @@ shm_get_buffer(struct wl_shm *shm, int width, int height, unsigned long cookie,
         tll_remove(buffers, it);
     }
 
+    struct buffer *cached = NULL;
+
     tll_foreach(buffers, it) {
         if (it->item.width != width)
             continue;
@@ -230,16 +234,27 @@ shm_get_buffer(struct wl_shm *shm, int width, int height, unsigned long cookie,
         if (it->item.cookie != cookie)
             continue;
 
-        if (!it->item.busy) {
-            LOG_DBG("cookie=%lx: re-using buffer from cache (buf=%p)",
-                    cookie, (void *)&it->item);
-            it->item.busy = true;
-            it->item.purge = false;
-            xassert(it->item.pix_instances == pix_instances);
-            return &it->item;
-        }
+        if (it->item.busy)
+            it->item.age++;
+        else
+#if FORCED_DOUBLE_BUFFERING
+            if (it->item.age == 0)
+                it->item.age++;
+            else
+#endif
+            {
+                LOG_DBG("cookie=%lx: re-using buffer from cache (buf=%p)",
+                        cookie, (void *)&it->item);
+                it->item.busy = true;
+                it->item.purge = false;
+                xassert(it->item.pix_instances == pix_instances);
+                cached = &it->item;
+            }
     }
 
+    if (cached != NULL)
+        return cached;
+
     /* Purge old buffers associated with this cookie */
     tll_foreach(buffers, it) {
         if (it->item.cookie != cookie)
@@ -376,9 +391,9 @@ shm_get_buffer(struct wl_shm *shm, int width, int height, unsigned long cookie,
             .scrollable = scrollable,
             .real_mmapped = real_mmapped,
             .mmap_size = memfd_size,
-            .offset = 0}
-            )
-        );
+            .offset = 0,
+            .age = 0,
+        }));
 
     struct buffer *ret = &tll_back(buffers);
     if (!instantiate_offset(shm, ret, initial_offset))

shm.h

@@ -32,6 +32,8 @@ struct buffer {
 
     bool scrollable;
     bool purge;            /* True if this buffer should be destroyed */
+
+    int age;
 };
 
 struct buffer *shm_get_buffer(