Comment 0 for bug 1792503

"GET /allocation_candidates" now supports "member_of" parameter.
With nested providers present, this should work with the following constraints.

(a) With "member_of" qparam, aggregates on the root should span on the whole tree
 If a root provider is in the aggregate, which has been specified by "member_of" qparam,
the resource providers under that root can be in allocation candidates even the root is absent.

(b) Without "member_of" qparam, sharing resource provider should be shared with the whole tree
If a sharing provider is in the same aggregate with one resource provider (rpA),
and "member_of" hasn't been specified in qparam by user, the sharing provider can be in
allocation candidates with any of the resource providers in the same tree with rpA.

(c) With "member_of" qparam, the range of the share of sharing resource providers should shrink to the resource providers "under the specified aggregates" in a tree.
Here, whether the rp is "under the specified aggregates" is determined with the constraints of (a).
Namely, not only rps that belongs to the aggregates directly are "under the aggregates",
but olso rps whose root is under the aggregates are also "under the aggregates".

So far at Stein PTG time, 2018 Sep. 13th, this constraint is broken in the point that
when placement picks up allocation candidates, the aggregates of nested providers
are assumed as the same as root providers, which means it ignores the aggregates of
the nested provider itself. This results in the lack of allocation candidates when an
aggregate which is on a nested provider but not on the root are specified in member_of
parameter.

This bug is well described in a test case which is submitted shortly.