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Re: [Libvir] generator.py -- why?



On Mon, Aug 28, 2006 at 11:53:09PM -0400, pvetere redhat com wrote:
> Hi all.  So, I found a bug in the python bindings that I'd really like to fix,
> but when I sat down to do so I quickly found myself mired in a swampy mess of
> code generation: generator.py.  :-)  Now, I feel compelled to ask -- why don't
> we just have a static libvirt.py file that is WYSIWYG?  The generator.py
> program alone is longer than the file it generates, and having a static file
> would not only make the code easier to read, but would also make bug fixing
> much simpler.  But, I'm sure there's got to be a good reason for it.  :-)

  Historic, the same generator is used by libxml2 and libxslt at least.

> Here's a program that produces the bug I tried to address:
> 
> import libvirt
> def get_domain(dom_name):
>     conn = libvirt.openReadOnly(None)
>     domain = conn.lookupByName(dom_name)
>     return domain
> d = get_domain("mydomain")
> print d.info()
> 
> This is a fairly typical "factory" pattern I could imagine people using.  The
> problem is that the connection object falls out of scope after the get_domain
> routine ends, and is therefore garbage collected.  This leaves the "d" domain
> object with no valid connection, resulting in a "libvir: Xen error : failed Xen
> syscall  ioctl  3166208" error.  The simple solution would be for the connection
> object to pass a reference to itself into each domain object it creates.  The
> domain objects would then maintain the reference until they are destroyed.

  Hum, right, but really even at the C level you want to keep the connection
around as long as you manipulate the domain.

> I valiantly tried to follow the table-based patterns used in generator.py to fix
> this problem, but I found myself inventing somewhat arbitrary rules just to
> force the code I wanted it to output.
> 
> And hence, $SUBJECT.  :-)

  Okay, this may look surprizing, but well ...
Now we can probably add a reference back from the domain to the connection,
but still dropping any handle to the connection is kind of weird.

Daniel

-- 
Red Hat Virtualization group http://redhat.com/virtualization/
Daniel Veillard      | virtualization library  http://libvirt.org/
veillard redhat com  | libxml GNOME XML XSLT toolkit  http://xmlsoft.org/
http://veillard.com/ | Rpmfind RPM search engine  http://rpmfind.net/


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