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

[Jacob Wisor]

On 01/06/2015 at 01:25 PM Jacob Wisor wrote:
> 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?

Oh, I forgot: I am sure we do not want to make a DNS request for every resource
we need to download, even when subsequent DNS resolution requests are pulled
from the local DNS cache. So, although this patch fixes the bug, it is very
inefficient and impractical because it induces an avoidable performance penalty.
In other words, the online detection *logic* itself needs to be fixed.

@Jie Kang:
Thank you for spotting this bug.

Regards,

Jacob