Codereview request: 8007710 runtime/7158988/FieldMonitor.java fails with com.sun.jdi.VMDisconnectedException: Connection closed

[shanliang]

Staffan Larsen wrote:
> I think what you need to do is wait for the VMStartEvent before you add requests to the VM. Note this paragraph from the VirtualMachine doc:
>
>  Note that a target VM launched by a launching connector is not
>  guaranteed to be stable until after the VMStartEvent has been
>  received.
>   
I may miss something here, I believe VMStartEvent must be the first 
event, when the test got ClassPrepareEvent, it must already received 
VMStartEvent.
> I think adding code that looks something like this will make the test stable:
>
>     VirtualMachine vm = launchTarget(CLASS_NAME);
>     EventQueue eventQueue = vm.eventQueue();
>
>     boolean started = false;
>     while(!started) {
>       EventSet eventSet = eventQueue.remove();
>       for (Event event : eventSet) {
>         if (event instanceof VMStartEvent) {
>           started = true;
>         }
>         if (event instanceof VMDeathEvent
>             || event instanceof VMDisconnectEvent) {
>           throw new Error("VM died before it started...:"+event);
>         }
>       }
>     }
>
>     System.out.println("Vm launched");
>   
The code you proposed could improve the test, it made sure that 
TestPostFieldModification was started, but I am afraid that it did not 
address the issue causing the failure, the issue I believe was that 
TestPostFieldModification exited before or during FieldMonitor called 
addFieldWatch(), that was why addFieldWatch() received 
VMDisconnectedException. When the test was treating ClassPrepareEvent, 
even if VMDeathEvent or VMDisconnectEvent arrived, it must be still 
waiting in the eventQueue because it arrived after ClassPrepareEvent.

My fix was to not allow TestPostFieldModification to exit before 
addFieldWatch() was done. 
>   
> There is also no reason to call addFieldWatch() before the ClassPrepareEvent has been received. The call to vm..classesByName() will just return an empty list anyway.
>   
I do not know why the test called addFieldWatch before ClassPrepareEvent 
had been received, but yes the returned list was empty, so agree to 
remove it.
> While you are in there you can also remove the unused StringBuffer near the top of main().
>   
Yes it was already removed in version 01

Here is the new webrev:
http://cr.openjdk.java.net/~sjiang/JDK-8007710/02/

Thanks,
Shanliang
>  
> Thanks,
> /Staffan
>
> On 11 feb 2014, at 18:30, shanliang <shanliang.jiang at oracle.com> wrote:
>
>   
>> Here is the new fix in which FieldMonitor will write to TestPostFieldModification, to inform the latter to quit, as suggested bu Jaroslav
>>   http://cr.openjdk.java.net/~sjiang/JDK-8007710/01/
>>
>> Thanks,
>> Shanliang
>>
>> shanliang wrote:
>>     
>>> shanliang wrote:
>>>       
>>>> Jaroslav Bachorik wrote:
>>>>         
>>>>> On 11.2.2014 16:31, shanliang wrote:
>>>>>           
>>>>>> Staffan Larsen wrote:
>>>>>>             
>>>>>>> Hi Shanliang,
>>>>>>>
>>>>>>> I can’t quite see how the test can fail in this way. When the
>>>>>>> ClassPrepareEvent happens, the debuggee will be suspended. So when
>>>>>>> addFieldWatch() is called, the debuggee should not have moved.
>>>>>>>               
>>>>>> I am not expert of jdi so I may miss something here. I checked the
>>>>>> failure trace and saw the report exception happen when FieldMonitor
>>>>>> received ClassPrepareEvent and was doing addFieldWatch. FieldMonitor did
>>>>>> call "vm.resume()" before treating events.
>>>>>>             
>>>>> AFAICS, calling vm.resume() results in an almost immediate debuggee death. The gc() invoking thread "d" is flagged as a deamon and as such doesn't prevent the process from exiting. The other thread is not a daemon but will finish in only few cycles.
>>>>>           
>>>> I looked at the class com.sun.jdi.VirtualMachine, here is the Javadoc of the method "resume":
>>>>   /**
>>>>    * Continues the execution of the application running in this
>>>>    * virtual machine. All threads are resumed as documented in
>>>>    * {@link ThreadReference#resume}.
>>>>    *
>>>>    * @throws VMCannotBeModifiedException if the VirtualMachine is read-only - see {@link VirtualMachine#canBeModified()}.
>>>>    *
>>>>    * @see #suspend
>>>>    */
>>>>   void resume();
>>>> My understanding is that the debuggee resumes to work after this call, instead to die?
>>>>         
>>> In fact the problem is here, the vm (TestPostFieldModification) should not die before FieldMonitor finishes addFieldWatch.
>>>
>>> Shanliang
>>>       
>>>>>> I reproduced the bug by add sleep(1000) after vm.resume() but before
>>>>>> calling eventQueue.remove();
>>>>>>             
>>>>> It looks like some kind of synchronization between the debugger and the debuggee is necessary. But I wonder if you should better use the process.getOuptuptStream() to write and flush a message for the debugee indicating that it can exit. And in the debugee you would just do System.in.read() as the last statement in the main() method. Seems more robust than involving files.
>>>>>           
>>>> It could work, but creating a file in the testing directory should have no issue, but yes maybe less performance.
>>>>
>>>> Thanks,
>>>> Shanliang
>>>>         
>>>>> Cheers,
>>>>>
>>>>> -JB-
>>>>>
>>>>>           
>>>>>> Thanks,
>>>>>> Shanliang
>>>>>>             
>>>>>>> One problem I do see with the test is that it does not wait for a
>>>>>>> VMStartEvent before setting up requests. I’m not sure if that could
>>>>>>> cause the failure in the bug report, though.
>>>>>>>
>>>>>>> /Staffan
>>>>>>>
>>>>>>> On 11 feb 2014, at 15:13, shanliang <shanliang.jiang at oracle.com> wrote:
>>>>>>>
>>>>>>>               
>>>>>>>> Hi ,
>>>>>>>>
>>>>>>>> The problem could be that FieldMonitor did not have enough time to
>>>>>>>> "addFieldWatch" but the vm to monitor (TestPostFieldModification) was
>>>>>>>> already ended.
>>>>>>>>
>>>>>>>> So we should make sure that TestPostFieldModification exits after
>>>>>>>> FieldMonitor has done necessary. The solution proposed here is that
>>>>>>>> FieldMonitor creates a file after adding field watching, and
>>>>>>>> TestPostFieldModification quits only after finding the file.
>>>>>>>>
>>>>>>>> web:
>>>>>>>> http://icncweb.fr.oracle.com/~shjiang/webrev/8007710/00/
>>>>>>>>
>>>>>>>> bug:
>>>>>>>> https://bugs.openjdk.java.net/browse/JDK-8007710
>>>>>>>>
>>>>>>>> Thanks,
>>>>>>>> Shanliang
>>>>>>>>                 
>>>>>>             
>
>   

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