Re: [libvirt] [PATCH alternative 2/2] Change contract of virDomainGetID to make it safer
On Wed, Sep 03, 2008 at 05:22:45PM +0100, Richard W.M. Jones wrote:
This changes the contract of the existing virDomainGetID call so
that
it is guaranteed to return the ID provided that the @domain parameter
is not NULL or corrupted.
Actually this isn't entirely an accurate description. Inactive domains
do not have an ID of -1. This is merely a sentinal we use internally.
Inactive domains simply do not have an ID at all. Thus we return the
-1 error code if it is asked for. This is why tools like virt-manager
virsh do not display '-1' for ID - they simply leave the space blank.
This should be compatible with all preceeding versions of libvirt,
since all they have ever done is to check the @domain parameter and
return the dom->id field.
However it might not be forwards compatible with future versions: At
the moment there is an odd distinction between the local and remote
case. In the local case, the dom->id field is set to -1 when the
domain goes (mostly anyhow, not always). In the remote case it is not
set, because this is not known.
In practice, this never really matters. All significant libvirt
callers grab a new virDomain object from the remote end each time,
thus getting an updated ID. virDomain objects seem to be individually
very short-lived.
It is however a bug that the remote driver does not update its
internally ID field after performing Create/Destroy operations
on VMs, because it knows the ID will have changed at this point.
Rich.
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Index: src/libvirt.c
===================================================================
RCS file: /data/cvs/libvirt/src/libvirt.c,v
retrieving revision 1.155
diff -u -r1.155 libvirt.c
--- src/libvirt.c 2 Sep 2008 15:00:09 -0000 1.155
+++ src/libvirt.c 3 Sep 2008 16:16:47 -0000
@@ -1878,7 +1878,9 @@
*
* Get the hypervisor ID number for the domain
*
- * Returns the domain ID number or (unsigned int) -1 in case of error
+ * Returns the domain ID number or (unsigned int) -1 if the domain is
+ * not running. If @domain is NULL or its memory is corrupted
+ * then this can also return (unsigned int) -1.
I think it needs to make it clear that an inactive domain does
not have an ID of -1 - this is simply an error code indicating
the domain has not valid ID at this time.
Daniel
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