util.dbuffer: Add efficient shortcuts for discard() in certain cases
If the buffer is already empty, nothing to do. If we're throwing away the
whole buffer, we can just empty it and avoid read_chunk() (which in turn
may collapse()). These shortcuts are much more efficient.
#include <lua.h>
#include <lauxlib.h>
static int lc_xpcall (lua_State *L) {
int ret;
int n_arg = lua_gettop(L);
/* f, msgh, p1, p2... */
luaL_argcheck(L, n_arg >= 2, 2, "value expected");
lua_pushvalue(L, 1); /* f to top */
lua_pushvalue(L, 2); /* msgh to top */
lua_replace(L, 1); /* msgh to 1 */
lua_replace(L, 2); /* f to 2 */
/* msgh, f, p1, p2... */
ret = lua_pcall(L, n_arg - 2, LUA_MULTRET, 1);
lua_pushboolean(L, ret == 0);
lua_replace(L, 1);
return lua_gettop(L);
}
int luaopen_util_compat(lua_State *L) {
lua_createtable(L, 0, 2);
{
lua_pushcfunction(L, lc_xpcall);
lua_setfield(L, -2, "xpcall");
}
return 1;
}