Hooks: Protect against reentrancy.

Previously, we made the user deal with this themselves, but that's not good
enough; if hooks may allocate, we should test the allocation pathways down
hooks.  If we're doing that, we might as well actually implement the protection
for the user.

include/jemalloc/internal/hook.h

@@ -25,9 +25,9 @@
  * and only calls the alloc hook).
  *
  * Reentrancy:
- *   Is not protected against.  If your hooks allocate, then the hooks will be
- *   called again.  Note that you can guard against this with a thread-local
- *   "in_hook" bool.
+ *   Reentrancy is guarded against from within the hook implementation.  If you
+ *   call allocator functions from within a hook, the hooks will not be invoked
+ *   again.
  * Threading:
  *   The installation of a hook synchronizes with all its uses.  If you can
  *   prove the installation of a hook happens-before a jemalloc entry point,

include/jemalloc/internal/tsd.h

@@ -66,6 +66,7 @@ typedef ql_elm(tsd_t) tsd_link_t;
 #define MALLOC_TSD							\
     O(tcache_enabled,		bool,			bool)		\
     O(arenas_tdata_bypass,	bool,			bool)		\
+    O(in_hook,			bool,			bool)		\
     O(reentrancy_level,		int8_t,			int8_t)		\
     O(narenas_tdata,		uint32_t,		uint32_t)	\
     O(offset_state,		uint64_t,		uint64_t)	\
@@ -85,6 +86,7 @@ typedef ql_elm(tsd_t) tsd_link_t;
     ATOMIC_INIT(tsd_state_uninitialized),				\
     TCACHE_ENABLED_ZERO_INITIALIZER,					\
     false,								\
+    false,								\
     0,									\
     0,									\
     0,									\