LUAI_MAXSTACK defined privately

LUAI_MAXSTACK is limited to INT_MAX/2, so can use INT_MAX/2 to define
pseudo-indices (LUA_REGISTRYINDEX) in 'lua.h'. A change in the maximum
stack size does not need to change the Lua-C ABI.

ldo.c

@@ -174,6 +174,20 @@ TStatus luaD_rawrunprotected (lua_State *L, Pfunc f, void *ud) {
 #define STACKERRSPACE	200
 
 
+/*
+** LUAI_MAXSTACK limits the size of the Lua stack.
+** It must fit into INT_MAX/2.
+*/
+
+#if !defined(LUAI_MAXSTACK)
+#if 1000000 < (INT_MAX / 2)
+#define LUAI_MAXSTACK           1000000
+#else
+#define LUAI_MAXSTACK           (INT_MAX / 2u)
+#endif
+#endif
+
+
 /* maximum stack size that respects size_t */
 #define MAXSTACK_BYSIZET  ((MAX_SIZET / sizeof(StackValue)) - STACKERRSPACE)
 

ltests.h

@@ -155,7 +155,6 @@ LUA_API void *debug_realloc (void *ud, void *block,
 ** Reduce maximum stack size to make stack-overflow tests run faster.
 ** (But value is still large enough to overflow smaller integers.)
 */
-#undef LUAI_MAXSTACK
 #define LUAI_MAXSTACK   68000
 
 

lua.h

@@ -37,10 +37,10 @@
 
 /*
 ** Pseudo-indices
-** (-LUAI_MAXSTACK is the minimum valid index; we keep some free empty
-** space after that to help overflow detection)
+** (The stack size is limited to INT_MAX/2; we keep some free empty
+** space after that to help overflow detection.)
 */
-#define LUA_REGISTRYINDEX	(-LUAI_MAXSTACK - 1000)
+#define LUA_REGISTRYINDEX	(-(INT_MAX/2 + 1000))
 #define lua_upvalueindex(i)	(LUA_REGISTRYINDEX - (i))
 
 

luaconf.h

@@ -763,20 +763,6 @@
 ** =====================================================================
 */
 
-/*
-@@ LUAI_MAXSTACK limits the size of the Lua stack.
-** CHANGE it if you need a different limit. This limit is arbitrary;
-** its only purpose is to stop Lua from consuming unlimited stack
-** space and to reserve some numbers for pseudo-indices.
-** (It must fit into max(int)/2.)
-*/
-#if 1000000 < (INT_MAX / 2)
-#define LUAI_MAXSTACK		1000000
-#else
-#define LUAI_MAXSTACK		(INT_MAX / 2u)
-#endif
-
-
 /*
 @@ LUA_EXTRASPACE defines the size of a raw memory area associated with
 ** a Lua state with very fast access.