Function windowsPingOutputIsSuccess

wgengine/netstack/netstack.go:2351–2375

windowsPingOutputIsSuccess reports whether the ping.exe output b contains a success ping response for ip. See https://github.com/tailscale/tailscale/issues/13654 TODO(bradfitz,nickkhyl): delete this and use the proper Windows APIs.

(ip netip.Addr, b []byte)

//
// TODO(bradfitz,nickkhyl): delete this and use the proper Windows APIs.
func windowsPingOutputIsSuccess(ip netip.Addr, b []byte) bool {
	// Look for a line that contains " <ip>: " and then three equal signs.
	// As a special case, the 2nd equal sign may be a '<' character
	// for sub-millisecond pings.
	// This heuristic seems to match the ping.exe output in any language.
	sub := fmt.Appendf(nil, " %s: ", ip)

	eqSigns := func(bb []byte) (n int) {
		for _, b := range bb {
			if b == '=' || (b == '<' && n == 1) {
				n++
			}
		}
		return
	}

	for len(b) > 0 {
		var line []byte
		line, b, _ = bytes.Cut(b, []byte("\n"))
		if _, rest, ok := bytes.Cut(line, sub); ok && eqSigns(rest) == 3 {
			return true
		}
	}
	return false
}