gc: use single allocation for prot_array.

* gc.c (prot_array): Add self pointer; arr member
becomes flexible array.
(prot_array_mark): We now check the handle itself for
null, because the whole thing is freed.
(prot_array_free): Function removed.
(prot_array_ops): Wire cobj_destroy_free_op in place
of prot_array_free. This fixes a memory leak because
prot_array_free was not freeing the handle, only
the array.
(gc_prot_array_alloc): Fix to allocate everything
in one swoop and store the self-pointer in the
named member rather than arr[-1]. The self argument
is not required; we drop it. The size argument cannot
be anywhere near INT_PTR_MAX, because such an array
wouldn't fit into virtual memory, so it is always
safe to add a small value to the size.
(prot_array_free): Obtain the self-pointer, and
free the handle, replacing it with a null pointer.

* gc.h (gc_prot_array_alloc): Declaration updated.

* lib.c (ssort_vec): Don't pass self to gc_prot_array_alloc.

* lib.h (container): New macro.

1 files changed, 4 insertions, 0 deletions

diff --git a/lib.h b/lib.h
index 9e71b5f9..dc90aa23 100644
--- a/lib.h
+++ b/lib.h
@@ -46,6 +46,10 @@ typedef double_uintptr_t dbl_ucnum;
 #define coerce(TYPE, EXPR) ((TYPE) (EXPR))
 #endif
 
+#define container(PTR, TYPE, MEMB)                          \
+  convert(TYPE *,                                           \
+          convert(mem_t *, (PTR)) - offsetof(TYPE, MEMB))
+
 #if __STDC_VERSION__ >= 199901L
 #define FLEX_ARRAY
 #else