Proposal: New virDomainLookup function to indicate Not found vs Error

["Richard W.M. Jones" <rjones@xxxxxxxxxx>]

virt-install has some code which waits for a domain to appear just after 
it has been created.  It looks like the loop attached to the end of this 
email, and is functional but has two problems.

Problem (1) is that self.conn.lookupByName doesn't distinguish between a 
"Not found" domain and an actual error.  For example there is no way to 
tell the difference between being unable to contact xend (an actual 
error), and being able to contact xend, but xend not being able to find 
the domain (not found).

As shown here:

  >>> import libvirt
  >>> conn = libvirt.open ("xen+tls:///")
  >>> d = conn.lookupByName ("Domain-0")
  >>> d = conn.lookupByName ("doesnotexist")
  [...]
  libvirt.libvirtError: virDomainLookupByName() failed

then I deliberately kill the remote daemon:

  >>> d = conn.lookupByName ("doesnotexist")
  libvir: Remote error : Error in the push function.
  [...]

The first exception is a Not found condition (not an error) whereas the 
second is an error.

Problem (2) is that virterror is over anxious to print error messages to 
stderr, even if the caller can handle them and even if (as in the Not 
found case) they don't indicate errors.  In practical terms this means 
that the virt-install loop attached below may print out 1 or 2 error 
messages even when it is functioning normally.  You'll see an error like 
this appearing [sic]:

  libvir: Xen Daemon error : GET operation failed:

Since it's difficult to change the LookupBy* functions without changing 
the ABI, I suspect that the best thing to do is going to be to add a new 
call with better semantics.  Therefore I suggest:

  virDomainPtr *
    virDomainLookup (virConnectPtr conn, int flags,
                     int id, char *str, int *error);

  where flags is one of:
    VIR_LOOKUP_BY_ID, VIR_LOOKUP_BY_NAME, VIR_LOOKUP_BY_UUID
    or VIR_LOOKUP_BY_UUID_STRING

The return values are:
  ret = domain, *error = 0 => found it
  ret = NULL, *error = 0 => not found
  ret = NULL, *error = 1 => error (check virterror)

Addition 1: There would be a similar function virNetworkLookup, but 
without needing the 'id' parameter because networks don't have IDs.

Addition 2: Change the driver internals so that they don't call 
virterror in the not found case.  (This requires quite a bit of 
rejigging in xend_internal, but is not too hard).

Addition 3: Language bindings could be modified to detect this function 
and if present change their existing LookupBy* functions to use the new 
interface.

Thoughts?

Rich.

----------------------------------------------------

This is the troublesome loop:

  logging.debug("Created guest, looking to see if it is running")
  # sleep in .25 second increments until either a) we find
  # our domain or b) it's been 5 seconds.  this is so that
  # we can try to gracefully handle domain creation failures
  num = 0
  d = None
  while num < (5 / .25): # 5 seconds, .25 second sleeps
     try:
       d = self.conn.lookupByName(self.name)
       break
     except libvirt.libvirtError, e:
       logging.debug("No guest running yet " + str(e))
       pass
     num += 1
     time.sleep(0.25)


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