New function 'setCstacklimit'

Added new functions to dynamically set the C-stack limit
('lua_setCstacklimit' in the C-API, 'debug.setCstacklimit' in Lua).

lstate.c

@@ -96,6 +96,29 @@ void luaE_setdebt (global_State *g, l_mem debt) {
 }
 
 
+LUA_API int lua_setCstacklimit (lua_State *L, unsigned int limit) {
+  global_State *g = G(L);
+  int ccalls;
+  luaE_freeCI(L);  /* release unused CIs */
+  ccalls = getCcalls(L);
+  if (limit >= 40000)
+    return 0;  /* out of bounds */
+  limit += CSTACKERR;
+  if (L != g-> mainthread)
+    return 0;  /* only main thread can change the C stack */
+  else if (ccalls <= CSTACKERR)
+    return 0;  /* handling overflow */
+  else {
+    int diff = limit - g->Cstacklimit;
+    if (ccalls + diff <= CSTACKERR)
+      return 0;  /* new limit would cause an overflow */
+    g->Cstacklimit = limit;  /* set new limit */
+    L->nCcalls += diff;  /* correct 'nCcalls' */
+    return limit - diff - CSTACKERR;  /* success; return previous limit */
+  }
+}
+
+
 /*
 ** Decrement count of "C calls" and check for overflows. In case of
 ** a stack overflow, check appropriate error ("regular" overflow or
@@ -121,7 +144,7 @@ void luaE_enterCcall (lua_State *L) {
       else if (ncalls >= CSTACKMARK) {
         /* not in error-handling zone; raise the error now */
         L->nCcalls = (CSTACKMARK - 1);  /* enter error-handling zone */
-        luaG_runerror(L, "C stack overflow1");
+        luaG_runerror(L, "C stack overflow");
       }
       /* else stack is in the error-handling zone;
          allow message handler to work */
@@ -263,7 +286,7 @@ static void preinit_thread (lua_State *L, global_State *g) {
   L->stacksize = 0;
   L->twups = L;  /* thread has no upvalues */
   L->errorJmp = NULL;
-  L->nCcalls = LUAI_MAXCSTACK + CSTACKERR;
+  L->nCcalls = CSTACKTHREAD;
   L->hook = NULL;
   L->hookmask = 0;
   L->basehookcount = 0;
@@ -365,6 +388,7 @@ LUA_API lua_State *lua_newstate (lua_Alloc f, void *ud) {
   preinit_thread(L, g);
   g->allgc = obj2gco(L);  /* by now, only object is the main thread */
   L->next = NULL;
+  g->Cstacklimit = L->nCcalls = LUAI_MAXCSTACK;
   g->frealloc = f;
   g->ud = ud;
   g->warnf = NULL;