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

[Wei Liu <wei.liu2@xxxxxxxxxx>]

I've CC'ed some people who might have an idea whether they are replying
on this behaviour. I doubt that but let's better be sure...

On Tue, Mar 01, 2016 at 11:17:54PM +0300, 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.
> 
> Both ``xs.watch`` and ``xs.unwatch`` accept two arguments: a path to watch 
> and a token. According to the documentation, the second argument must be a 
> string, which makes sense, since the token should be sent directly to 
> XenStore. Instead of doing the simple thing (transmitting the token as-is), 
> the implementation makes a new token from *the pointer* to the token object 
> and sends it instead:
> 
>     PyObject *token;
>     char token_str[MAX_STRLEN(unsigned long) + 1];
> 
>     snprintf(token_str, sizeof(token_str), "%li", (unsigned long)token);
> 
> This does work for simple cases, e.g. if a token is a string literal 
> 
>     >>> from xen.lowlevel.xs import xs
>     >>> h = xs()
>     >>> h.watch(“@introduceDomain”, “token”)
>     >>> h.unwatch(“@introduceDomain”, “token”)
> 
> or a small number
> 
>     >>> h.watch(“@introduceDomain”, 42)
>     >>> h.unwatch(“@introduceDomain”, 42)
> 
> But in the general case this is broken
> 
>     >>> h.watch("@introduceDomain", 100000000000000000000000000000)
>     >>> h.unwatch("@introduceDomain", 100000000000000000000000000000)
>     Traceback (most recent call last):
>       File "<stdin>", line 1, in <module>
>     xen.lowlevel.xs.Error: (2, 'No such file or directory’)
> 
> 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?
> 

In any case this looks like a real bug.

The fix (as you said) is to transmit the token. I don't think your
proposed fix would break existing users though.

Wei.


> Regards,
> Sergei
> _______________________________________________
> Xen-devel mailing list
> Xen-devel@xxxxxxxxxxxxx
> http://lists.xen.org/xen-devel

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