The facade_resource intrinsic function needs to access the parent resource
of a stack, which it does via the stack.parent_resource property. Accessing
this for the first time loads the parent stack and all of its resources if
they were not already. This made sense when all nested stacks in a tree
were handled in-memory at the same time, but now that they are processed by
separate engines it is inefficient.
This change moves responsibility for lazy-loading the parent stack to a
separate ParentResourceProxy class, and makes access more efficient again
by avoiding the loading of resources if the stack was not already in
memory. It also resolves a circular reference between nested stacks and
their parent stacks.
We now have a well-defined API behind which we can potentially make further
efficiency improvements while giving third-party Function plugin developers
confidence that we won't break them without notice.
Change-Id: Ibfd80544889778f3499bcbe421b83f0a5aa6a7f7
Partially-Implements: blueprint stack-definition
Zane Bitter