[osx] Improve os_getcwd() on Lua>=52.

osx.c

@@ -478,34 +478,65 @@ static int os_mkdir(lua_State *L)
 
 static int os_getcwd(lua_State *L)
 {
-    char *p = NULL;
+#if LUA_VERSION_NUM >= 502
+    luaL_Buffer buf;
+    char *p;
+    size_t size = 128;
+
+    luaL_buffinit(L, &buf);
+    while (true) {
+        p = luaL_prepbuffsize(&buf, size);
+        if (getcwd(p, size))
+            break;  // got working directory
+
+        if (errno != ERANGE)
+            return os_pusherror(L, NULL);
+
+        size <<= 1;
+        if (size == 0) {
+            // PARANOID
+            errno = ERANGE;
+            return os_pusherror(L, NULL);
+        }
+    }
+
+    luaL_pushresultsize(&buf, strlen(p));
+    return 1;
+#else
+    // Unfortunately this leaks memory if Lua ever decides to longjmp().
+    // After 12 years since Lua 5.2 introduction,
+    // that's what we get with LuaJIT.
+    char *buf = NULL;
     size_t size = 128;
 
     while (true) {
-        char *np = realloc(p, size);
+        char *p = realloc(buf, size);
-        if (!np)
+        if (!p)
             goto fail;
 
-        p = np;
+        buf = p;
-        if (getcwd(p, size))
+        if (getcwd(buf, size))
             break;  // got working directory
 
         if (errno != ERANGE)
             goto fail;
 
         size <<= 1;
-        if (size == 0)  // PARANOID
+        if (size == 0) {
+            // PARANOID
+            errno = ERANGE;
             goto fail;
         }
+    }
 
-    lua_pushstring(L, p);
+    lua_pushstring(L, buf);
-    free(p);
+    free(buf);
     return 1;
 
 fail:
-    free(p);
+    free(buf);
-
     return os_pusherror(L, NULL);
+#endif
 }
 
 static int os_chdir(lua_State *L)