Allow FQDN lookup functions to take context

[ycombinator]

This PR changes the FQDN lookup functions to take a context.Context, thus allowing callers of these functions to pass a context with a timeout to prevent indefinitely waiting for DNS resolution.

[andrewkroh]

This is a breaking change to the API. I suggest adding an FQDNWithContext(context.Context) unless we are prepared to rollout a github.com/elastic/go-sysinfo/v2.

[ycombinator]

Done in 8ef547d.

[cmacknz]

The type of ips here changed from []net.IP to []net.IPAddr which doesn't have the same implementation of String(), since net.IPAddr embeds IP and extends it with a zone.

If you want the exact same conversion as before you want to call ip.IP.String() I think.

func (a *IPAddr) String() string {
	if a == nil {
		return "<nil>"
	}
	ip := ipEmptyString(a.IP)
	if a.Zone != "" {
		return ip + "%" + a.Zone
	}
	return ip
}

The original call to net.LookupIP stripped off the IPv6 zone: https://cs.opensource.google/go/go/+/refs/tags/go1.21.6:src/net/lookup.go;l=195

// LookupIP looks up host using the local resolver.
// It returns a slice of that host's IPv4 and IPv6 addresses.
func LookupIP(host string) ([]IP, error) {
	addrs, err := DefaultResolver.LookupIPAddr(context.Background(), host)
	if err != nil {
		return nil, err
	}
	ips := make([]IP, len(addrs))
	for i, ia := range addrs {
		ips[i] = ia.IP
	}
	return ips, nil
}

[ycombinator]

The type of ips here changed from []net.IP to []net.IPAddr...

Are you sure? Before, ips was initialized as:

ips, err := net.LookupIP(hostname)

Now, it's initialized as:

ips, err := net.DefaultResolver.LookupIP(ctx, "ip", hostname)

In both cases, the type of ips is the same: []IP.

• net.LookupIP: https://cs.opensource.google/go/go/+/refs/tags/go1.21.6:src/net/lookup.go;l=193-205
• net.DefaultResolver.LookupIP: https://cs.opensource.google/go/go/+/refs/tags/go1.21.6:src/net/lookup.go;l=213-234

[cmacknz]

Ah you are right for some reason I ended up focusing on LookupIPAddr as if it were called early, I think this was because net.LookupIP calls LookupIPAddr but func (r *Resolver) LookupIP does something else.

I don't have enough brainpower left for the day or network knowledge to know if that is significant, I was trying to quickly check if this worked the same as it did before.