You don’t have to do any significant work within the signal handler itself. In fact, the signal handler can literally be empty. What matters is that as long as SA_RESTART is not set, after the signal handler runs, the interrupted syscall fails with errno set to EINTR. Then the code that did the syscall can check whether a cancellation occurred (and retry the syscall if not).

Disclaimer: I haven’t looked at Zig’s implementation; I’m only going off how the Unix APIs work.