Re: [Xen-devel] [BUG] xs.watch and xs.unwatch are unreliable

[David Vrabel <david.vrabel@xxxxxxxxxx>]

On 03/03/16 17:02, Wei Liu wrote:
> On Thu, Mar 03, 2016 at 04:47:12PM +0000, David Vrabel wrote:
>> On 01/03/16 20:17, Sergei Lebedev wrote:
>>> Hi list,
>>>
>>> I’ve initially wanted to report another inconsistency in
>>> ``xen.lowlevel.xs`` documentation, but this time the issue is more
>>> subtle.
>>
>> [...]
>>
>>> Here’s another example with a string token
>>>
>>>     >>> token1 = str(100000000000000000000000000000)
>>>     >>> token2 = str(100000000000000000000000000000)
>>>     >>> token1 == token2
>>>     True
>>>     >>> h.watch("@introduceDomain", token1)
>>>     >>> h.unwatch("@introduceDomain", token2)
>>>     Traceback (most recent call last):
>>>       File "<stdin>", line 1, in <module>
>>>     xen.lowlevel.xs.Error: (2, 'No such file or directory’)
>>>
>>> I’m not sure what would be the best way to handle this as there might
>>> be existing code relying on this undocumented behaviour. What do you
>>> think?
>>
>> I think you're stuck with this behaviour.  If you fix it there's a risk
>> of breaking existing applications by unwatch removing the wrong watch.
>>
> 
> I'm not sure I follow this. Do you have an example why it would remove
> the wrong watch?

  token1 = str(100000000000000000000000000000)
  token2 = str(100000000000000000000000000000)

  h.watch("path", token1)
  h.watch("path", token2)

Created two unique tokens.

  h.unwatch("path", token1)

Which watch should be removed if token1 and token2 no longer have a
unique token?  Although, I'm not sure of the behaviour of adding two
watches with the same token.

It also occurs to me that if this area is going to be improved, it
should be the kernel that provides the token since it has to be unique
across all users.

David

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