Add janet_abstract_begin and janet_abstract_end

This will allow some one constructing an abstract to
only make it visible to the garbage collector after it
is in a valid state. If code in the constructing cfunction
panics before janet_abstract_end is called, the GC will not try
to mark the incomplete abstract type. This is often not needed through
careful programming, but should work well.

src/core/abstract.c

@@ -26,10 +26,19 @@
 #endif
 
 /* Create new userdata */
-void *janet_abstract(const JanetAbstractType *atype, size_t size) {
+void *janet_abstract_begin(const JanetAbstractType *atype, size_t size) {
-    JanetAbstractHead *header = janet_gcalloc(JANET_MEMORY_ABSTRACT,
+    JanetAbstractHead *header = janet_gcalloc(JANET_MEMORY_NONE,
                                 sizeof(JanetAbstractHead) + size);
     header->size = size;
     header->type = atype;
     return (void *) & (header->data);
 }
+
+void *janet_abstract_end(void *x) {
+    janet_gc_settype((void *)(janet_gc_header(x)), JANET_MEMORY_ABSTRACT);
+    return x;
+}
+
+void *janet_abstract(const JanetAbstractType *atype, size_t size) {
+    return janet_abstract_end(janet_abstract_begin(atype, size));
+}

src/core/peg.c

@@ -990,6 +990,11 @@ static void peg_unmarshal(void *p, JanetMarshalContext *ctx) {
     peg->bytecode = NULL;
     peg->constants = NULL;
 
+    /* Ensure not too large */
+    if (constants_start + sizeof(Janet) * peg->num_constants > janet_abstract_size(p)) {
+        janet_panic("size mismatch");
+    }
+
     for (size_t i = 0; i < peg->bytecode_len; i++)
         bytecode[i] = (uint32_t) janet_unmarshal_int(ctx);
     for (uint32_t j = 0; j < peg->num_constants; j++)

src/include/janet.h

@@ -1219,7 +1219,9 @@ JANET_API const JanetKV *janet_dictionary_next(const JanetKV *kvs, int32_t cap,
 #define janet_abstract_head(u) ((JanetAbstractHead *)((char *)u - offsetof(JanetAbstractHead, data)))
 #define janet_abstract_type(u) (janet_abstract_head(u)->type)
 #define janet_abstract_size(u) (janet_abstract_head(u)->size)
-JANET_API void *janet_abstract(const JanetAbstractType *type, size_t size);
+JANET_API void *janet_abstract_begin(const JanetAbstractType *type, size_t size);
+JANET_API void *janet_abstract_end(void *);
+JANET_API void *janet_abstract(const JanetAbstractType *type, size_t size); /* begin and end in one call */
 
 /* Native */
 typedef void (*JanetModule)(JanetTable *);