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

[Sergei Lebedev <sergei.a.lebedev@xxxxxxxxx>]

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?

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