Clean up Xapi_vm_lifecycle.check_operation_error

[gaborigloi]

Get rid of the unnecessary get_info function.

Signed-off-by: Gabor Igloi gabor.igloi@citrix.com

[coveralls]

Coverage decreased (-0.02%) to 14.103% when pulling ffb7a84 on gaborigloi:xapi_vm_lifecycle_check_operation_error_cleanup into ab5bca8 on xapi-project:master.

[gaborigloi]

Realised that maybe we did things this way for performance reasons: get the info from the db only once, and use that info when we loop through multiple VM operations here, to avoid doing the same DB accesses multiple times. With this change we do a Db.VM.get_record_internal, Db.VM_guest_metrics.get_record_internal, and a Db.VBD.get_VDI & Db.VDI.get_sm_config for each VBD & VDI, during each iteration (however we could mostly eliminate the last two by only computing them when necessary and making them lazy). Not sure whether this is a performance issue, and how expensive/slow these DB accesses are.

[gaborigloi]

We could actually restore the old performance of this function, without using get_info and introducing duplication:

let check_operation_error ~__context ~ref =
  let vmr = Db.VM.get_record_internal ~__context ~self:ref in
  let vmgmr = lazy (maybe_get_guest_metrics ~__context ~ref:(vmr.Db_actions.vM_guest_metrics)) in
  (fun ~op ~strict ->
      let x = lazy ... in
      ...
  )

Then, when we call it once only, the code remains the same. But when we call it in a loop with multiple operations, like in update_allowed_operations, we can do:

let check_operation_error ~__context ~ref = check_operation_error ~context ~ref in
loop (
  ... check_operation_error ~op ~strict:true
)