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Re: [libvirt] virConnectClose question



Dne 19.10.2010 17:32, Matthias Bolte napsal(a):
2010/10/19 Radek Hladik<radek eadresa cz>:
Hello,
        I did find out that in some cases virConnectClose does not close the
connection at all. It only returns -1 which by the documentation means
error:
Returns: 0 in case of success or -1 in case of error.

Check virGetLastError() for the error.

There were no errors reported neither by virGetLastError nor Error callback function.


But there is no other indication of what error it is. I am almost sure that
it is because some open domains or other libvirt objects. If this is the
case it would be good to add notice like: "You need to free all
domains,....,etc before calling virConnectClose". And is there any
possibility how to force closing the connection? I know that it is a good
programming manner to close all objects before closing the master object but
I did run into this in php-libvrit development and it is quite painful.
PHP is keeping a list of all resources (objects from libvirt) and is calling
my destructors to close it. Unfortunately there are some problems with this:
* when garbage collector clears my resource my destructor is not called. It
is probably my mistake that I do not set it correctly somewhere but I am not
able to find out where.
* I can not influence the order of calls. It seems that there were some
discussions and that for quite some time PHP is calling destructors in
reverse order of creation. That would be good but I am not sure whether it
is granted.
The problem with this all is that if I for some reason fail to free some
domain object the connection is not closed. But as PHP does reuse threads
the connection will be open forever and connection limit for libvirtd get
drained very fast.
So basically my question is: should I implement some list of all objects I
did create and free them before calling virConnectClose or is there any
other way how to "terminate" libvirt connection for good?

Radek


Okay, all libvirt objects like connection, domain, storage pool etc
are reference counted. When you lookup a domain over a connection then
the domain adds an additional reference to the connection. This
ensures that the connection is open as long as there are objects that
have been looked up over it.

You don't need to free all objects before you can close the
connection. You just need to match all virConnectOpen calls by a
virConnectClose call and all lookup calls for other objects by the
matching free calls.

virConnectPtr conn = virConnectOpen("qemu:///system", 0); // conn.refs = 1
virDomainPtr domain = virDomainLookupByName(conn, "vm1"); // conn.refs
= 2, domain.refs = 1
...
virConnectClose(conn); // conn.refs = 1, but conn is still open
because domain has a reference to it
virDomainFree(domain); // conn.refs = 0, domain.refs = 0, domain gets
freed and releases its reference to conn, no conn gets closed

So virConnectClose returning -1 has nothing to do with the order of
close/free calls.

I think you can only make virConnectClose return -1 when you call it
on an invalid or already closed connection.

Matthias
My problem is that I may not close some i.e. domain or storage pool and I still need to close the connection. Basically I need to destroy the libvirt connection because PHP script is already ended.* Normally when PHP script "generates" objects, i.e. lookup, I just pass it to PHP as resource and expect PHP calling my destructor for that resource and that it will call it in correct order (domains before connection). This seems to fail in some cases (as I did say before). So I am thinking about having list of all abjects and checking whether they are freed. I just wanted to know whether something like this is not already in libvirt. You saying that libvirt keeps only reference count means that this must be done in my layer. I do understand that the order of freeing is not important but it is important to i.e. close all domains before closing connection. And as PHP does not know about the content of the resource it does the most clever thing to do - call destructors in reverse order. But this does not seem to be guaranteed.

thanks for your answer

Radek

*) it may sound as bad programming habit to "forget" to free something and it definetely is but I need some sort of backup. Because one thread in PHP will run hundreds of PHP scripts and keeping a connection open is much worse than even small memory leak that may happen.


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