Add fixed state atexit stack feature.

Adds following macros:
* MRB_FIXED_STATE_ATEXIT_STACK (not defined by default)
  * When defined enables fixed state atexit stack.
* MRB_FIXED_STATE_ATEXIT_STACK_SIZE (default value: 5)
  * This macro will be ignored when `MRB_FIXED_STATE_ATEXIT_STACK` isn't defined.
  * When `mrb_state_atexit` is called more than this value it will raise runtime error.

include/mrbconf.h

@@ -59,6 +59,12 @@
 /* fixed size GC arena */
 //#define MRB_GC_FIXED_ARENA
 
+/* state atexit stack size */
+//#define MRB_FIXED_STATE_ATEXIT_STACK_SIZE 5
+
+/* fixed size state atexit stack */
+//#define MRB_FIXED_STATE_ATEXIT_STACK
+
 /* -DDISABLE_XXXX to drop following features */
 //#define DISABLE_STDIO		/* use of stdio */
 

include/mruby.h

@@ -52,6 +52,10 @@ typedef void* (*mrb_allocf) (struct mrb_state *mrb, void*, size_t, void *ud);
 #define MRB_GC_ARENA_SIZE 100
 #endif
 
+#ifndef MRB_FIXED_STATE_ATEXIT_STACK_SIZE
+#define MRB_FIXED_STATE_ATEXIT_STACK_SIZE 5
+#endif
+
 typedef struct {
   mrb_sym mid;
   struct RProc *proc;
@@ -173,7 +177,11 @@ typedef struct mrb_state {
 
   void *ud; /* auxiliary data */
 
+#ifdef MRB_FIXED_STATE_ATEXIT_STACK
+  mrb_atexit_func atexit_stack[MRB_FIXED_STATE_ATEXIT_STACK_SIZE];
+#else
   mrb_atexit_func *atexit_stack;
+#endif
   mrb_int atexit_stack_len;
 } mrb_state;
 

src/state.c

@@ -226,7 +226,9 @@ mrb_close(mrb_state *mrb)
     for (i = mrb->atexit_stack_len; i > 0; --i) {
       mrb->atexit_stack[i - 1](mrb);
     }
+#ifndef MRB_FIXED_STATE_ATEXIT_STACK
     mrb_free(mrb, mrb->atexit_stack);
+#endif
   }
 
   /* free */
@@ -268,6 +270,11 @@ mrb_top_self(mrb_state *mrb)
 void
 mrb_state_atexit(mrb_state *mrb, mrb_atexit_func f)
 {
+#ifdef MRB_FIXED_STATE_ATEXIT_STACK
+  if (mrb->atexit_stack_len + 1 > MRB_FIXED_STATE_ATEXIT_STACK_SIZE) {
+    mrb_raise(mrb, E_RUNTIME_ERROR, "exceeded fixed state atexit stack limit");
+  }
+#else
   size_t stack_size;
 
   stack_size = sizeof(mrb_atexit_func) * (mrb->atexit_stack_len + 1);
@@ -276,6 +283,7 @@ mrb_state_atexit(mrb_state *mrb, mrb_atexit_func f)
   } else {
     mrb->atexit_stack = (mrb_atexit_func*)mrb_realloc(mrb, mrb->atexit_stack, stack_size);
   }
+#endif
 
   mrb->atexit_stack[mrb->atexit_stack_len++] = f;
 }