macro: add ONCE macro that evaluates to 1 one time

Every location that this macro is used, it will be true the first
time it's checked, then false each time after that.

This can be useful for things such as one-time logging.

src/fundamental/macro-fundamental.h

@@ -50,6 +50,17 @@
 #define UNIQ_T(x, uniq) CONCATENATE(__unique_prefix_, CONCATENATE(x, uniq))
 #define UNIQ __COUNTER__
 
+/* Note that this works differently from pthread_once(): this macro does
+ * not synchronize code execution, i.e. code that is run conditionalized
+ * on this macro will run concurrently to all other code conditionalized
+ * the same way, there's no ordering or completion enforced. */
+#define ONCE __ONCE(UNIQ_T(_once_, UNIQ))
+#define __ONCE(o)                                                       \
+        ({                                                              \
+                static bool (o) = false;                                \
+                __sync_bool_compare_and_swap(&(o), false, true);        \
+        })
+
 #undef MAX
 #define MAX(a, b) __MAX(UNIQ, (a), UNIQ, (b))
 #define __MAX(aq, a, bq, b)                             \