RFC: Netx - implement -Xclearcache command line option

[Omair Majid]

On 10/04/2010 03:17 PM, Omair Majid wrote:
> On 09/30/2010 05:27 PM, Deepak Bhole wrote:
>> * Omair Majid<omajid at redhat.com> [2010-09-30 16:50]:
>>> On 09/30/2010 04:32 PM, Deepak Bhole wrote:
>>>> * Omair Majid<omajid at redhat.com> [2010-09-30 16:10]:
>>>>> Hi,
>>>>>
>>>>> I posted this patch way back in 2009, but it looks like it got
>>>>> lost/ignored.
>>>>>
>>>
>>> Ok, turns out it was my fault: just noticed
>>> http://mail.openjdk.java.net/pipermail/distro-pkg-dev/2009-August/006666.html.
>>>
>>> Somehow it slipped off my radar.
>>>
>>
>> Doh. Glad it was re-visited. I doubt the issue will be encountered, but
>> we might as well fix it instead of waiting for a bug report.
>>
>>>>> On 07/30/2009 05:18 PM, Omair Majid wrote:
>>>>>> The attached patch adds the option -Xclearcache to Netx.
>>>>>>
>>>>>
>>>>> Which (and I neglected to explain this last time) makes javaws clean
>>>>> out its cache by deleting the contents of ~/.netx/cache/
>>>>>
>>>>
>>>> Hmm, what happens if I have another netx app running and javaws is
>>>> called
>>>> with -Xclearcache? AFAIK the code has no provisioning to detect if a
>>>> cache file has been deleted, and to re-download it... does it?
>>>>
>>>
>>> As far as I can tell, very bad things (not eating babies kind of
>>> bad, but still...). Depending on the exact timing either the VM will
>>> crash or a java Exception might be thrown. If the JVM has opened the
>>> (missing) jar earlier and and wants to open it again to load
>>> additional classes the VM will most probably crash. Otherwise javaws
>>> simply wont be able to find the needed jar and will fail to start.
>>>
>>> I suppose I could make some sort of global application lock to stop
>>> any other javaws instances from starting while -Xclearcache is
>>> running and to make sure -Xclearcache does not start if any other
>>> javaws applications are running.
>>>
>>
>> It would be hard to set up a locking system that can handle odd cases
>> like crashed application, multiple instances of same app., etc.
>>
>
> Yeah, you are right. I thought about it for a bit, and I dont see a
> simple way to make it work using locks.
>
>> Easiest way I think is to execute jps -l and see if there is more than
>> one instance of net.sourceforge.jnlp.runtime.Boot. If there is, refuse
>> to run with -Xclearcache. I am not as worried about the 'don't start
>> while -Xclearcache is running' case, as there is a far smaller window
>> where things can go wrong. Plus if they do, it will be right away (as
>> opposed to an already started app that has modified data, crashing).
>>
>
> That's a great idea! Unfortunately I dont think we can rely on jps. jps
> is part of the JDK as opposed to the JRE - we can not be sure that users
> have it on their machine. I tried looking into the sources for jps to
> find out how it works. The JVM creates /tmp/hsperfdata_$USER/$PID files
> which contain the information that jps can parse and display. The files
> are in a binary format so parsing it is not trivial (and the file format
> is not guaranteed to be final). I am going to dig around a bit to see if
> there is any other simple solution to this
>

I am attaching the updated patch. This one uses an alternate 
implementation based on file locks (exclusive and shared). A normal 
javaws process acquires a shared lock on /tmp/$USER/netx/instance during 
startup (when an instance of Launcher is created), and releases the lock 
when the JVM shutsdown. Multiple javaws processes can share the lock and 
run simultaneously.

The javaws process that is trying to clear the cache checks for the file 
and tries to acquire an exclusive lock. If it succeeds, it goes ahead 
and clears the cache; otherwise it prints out an error to the user and 
exits.

Thoughts?

Cheers,
Omair
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