[vbox-dev] Race condition?

[Klaus Espenlaub]

Hi Max,

On 23.02.2013 03:17, Maxime Dor wrote:
> Hello,
>
> Given the following Java code snipet :
[...]
> I am running this using the 4.2 API (from the 4.2.4 SDK to be precise)
> against Web Services running on a Debian Testing box.
>
> When running my Unit tests, I keep having an intermitent error on the
> following part :
>> if (session.getState().equals(SessionState.Locked)) {
>> session.unlockMachine();
>> }
> /session.unlockMachine()/ throws an exception with : "/The session is
> not locked (session state: Unlocked) (0x8000FFFF)/". This exception is
> thrown about 1 out of 5 times I use this method or a copy of this one
> with only/.saveState()/ instead of/.powerDown()/ on the machine's
> console object.
>
> Am I missing something? From what I see here, it looks like the session
> state changes between the IF statement and the actual unlockMachine()
> call, looking like a race condition - or a bug?

You're not missing anything, and it's not a bug either. It's an inherent 
API race, since there is nothing preventing session state changes 
between the if statement and the unlockMachine() call (and there can't 
be anything preventing it).

The cause is that the operations you're doing (saveState/powerDown) are 
terminating the VM, and at the very end of those operations the VM 
session will be taken down (which implies that the associated shared 
session locks are gone too). Your API client code is racing with exactly 
this session cleanup which can extend slightly after the operation 
already signaled completion through the corresponding IProgress object.

So it's up to well-written code to be prepared for such spurious session 
state changes (which might happen for other reasons actually, e.g. if 
the VM process is terminated concurrently by some other API client, 
which can happen even if your code doesn't). Your code has to handle it.


Klaus

>
> Thank you for your insight.
>
> Best regards,
> Max
>
>
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