util.poll: Reject fd == FD_SETSIZE FD_SETSIZE is the maximum total number of FDs supports (well, the highest valid fd number). 0 is a valid fd number, so it supports up to 1023 if FD_SETSIZE is 1024. This is just a fencepost issue, fds > FD_SETSIZE were already correctly rejected.

local xpcall = xpcall;

if select(2, xpcall(function (x) return x end, function () end,  "test")) ~= "test" then
	xpcall = require"prosody.util.compat".xpcall;
end

return {
	xpcall = xpcall;
};