Add ParentId() to Machine

Add ParentId() to Machine

This branch provides the ability to ask a container for its parent machine's id.

https://code.launchpad.net/~wallyworld/juju-core/add-machine-parent-getter/+merge/168827

(do not edit description out of merge proposal)

[wallyworld, 2013-06-11 22:42:20]

Please take a look.

[thumper, 2013-06-11 22:47:06]

https://codereview.appspot.com/10203043/diff/1/state/machine.go
File state/machine.go (right):

https://codereview.appspot.com/10203043/diff/1/state/machine.go#newcode243
state/machine.go:243: return m.st.Machine(parentId)
Unfortunately this is making an assumption that doesn't always hold.

If I am the provisioner, and I have a state.Machine instance, I won't
necessarily have access to state.

Can we have something that returns ParentId() ?

While this method is OK now, I'm wondering how the API changes are going to
impact it.

[thumper, 2013-06-11 23:07:23]

On 2013/06/11 22:47:06, thumper wrote:
> https://codereview.appspot.com/10203043/diff/1/state/machine.go
> File state/machine.go (right):
> 
> https://codereview.appspot.com/10203043/diff/1/state/machine.go#newcode243
> state/machine.go:243: return m.st.Machine(parentId)
> Unfortunately this is making an assumption that doesn't always hold.
> 
> If I am the provisioner, and I have a state.Machine instance, I won't
> necessarily have access to state.
> 
> Can we have something that returns ParentId() ?
> 
> While this method is OK now, I'm wondering how the API changes are going to
> impact it.

LGTM - any change to the api will need a bucket load of changes.

[fwereade, 2013-06-11 23:43:38]

LGTM with ParentId as suggested.

https://codereview.appspot.com/10203043/diff/1/state/machine.go
File state/machine.go (right):

https://codereview.appspot.com/10203043/diff/1/state/machine.go#newcode241
state/machine.go:241: return nil, nil
There's a general expectation that a func returning (*Foo, error) will give you
an error iff it can't give you a non-nil *Foo, so I don't really like this.

https://codereview.appspot.com/10203043/diff/1/state/machine.go#newcode243
state/machine.go:243: return m.st.Machine(parentId)
On 2013/06/11 22:47:06, thumper wrote:
> Unfortunately this is making an assumption that doesn't always hold.
> 
> If I am the provisioner, and I have a state.Machine instance, I won't
> necessarily have access to state.
> 
> Can we have something that returns ParentId() ?
> 
> While this method is OK now, I'm wondering how the API changes are going to
> impact it.

Ideally, I think, `ParentId() (string, bool)`, in the name of convention wrt the
state package.

[wallyworld, 2013-06-12 00:03:45]

Please take a look.

[wallyworld, 2013-06-12 00:11:10]

*** Submitted:

Add ParentId() to Machine

This branch provides the ability to ask a container for its parent machine's id.

R=thumper, fwereade
CC=
https://codereview.appspot.com/10203043