[rfc][icedtea-web] Check online detection even if checked before

[Jacob Wisor]

On 01/05/2015 at 05:19 PM Jie Kang wrote:
> Hello,
> 
> This patch slightly modifies JNLPRuntime's online detection function to
> always perform the check, even if it has been called before.
> 
> Previously, if the boolean onlineDetected wasn't null, the check would be
> skipped. This caused a bug when downloading multiple resources.
> 
> For each resource being downloaded, we perform the detection check. For two
> consecutive resources: If for the first resource, we detected offline, the
> boolean onlineDetected would be set to false. Then for the second resource,
> the detection check would be skipped, even if that resource was able to be
> connected to. This would result in the second resource being unable to be
> downloaded, even though it definitely could and should be. The patch fixes
> this.
> 
> Thoughts?

Although I did not test it, seems to be working as advertised.

While looking at the code, I was wondering why JNLPRuntime.onlineDetected was
a Boolean object instead of a primitive. And, apart from that, the whole
offline detection mechanism seems overly complex and unreliable to me.
I am also not really convinced that

    public static void detectOnline(URL location) {
        if (onlineDetected != null) {
            return;
        }
        try {
            if (location.getProtocol().equals("file")) {
                return;
            }
            //Checks the offline/online status of the system.
            InetAddress.getByName(location.getHost());
        } catch (UnknownHostException ue) { […]

is the best or most reliable way in J2SE to test for the online/offline status
of a system. Do systems having no DNS server configured or which is unreachable
properly resolve their name? And, even if they do, does this really actually
prove them being online?

Now, I was considering a few different options to reliably detecting a system's
online/offline status. Initially, I was thinking about pinging the default
gateway. However, hosts with public IPs may have their public IP as the default
gateway configured. So, the most reliable way I came up with is to send an echo
broadcast (on the local subnet) request. Of course, this does not prove the
system being connected to the /public/ internet. But this is not neccessary
either because JNLP applications may be (which they often are) served on an
intranet only. So, would you mind investigating this further?

Regards,

Jacob