Currently, Deckhand fails to perform substitution given
a substitution dependency chain for a group of documents.
That is, if A depends on B and C depends on B for substitution
then substitution will fail. Deckhand, at present, can only
perform substitution if A depends on B and C depends on B
and D depends on B, for example: In this case, the dependency
chain is no bigger than 2.
However, for cases where the substitution dependency chain
is larger than 2, then the dependency is no longer trivial. This
is because the substitution dependencies form a DAG that must
be topologically sorted in order to derive the correct order of
substitution. Once the documents are correctly sorted according
to this scheme, then substitution can be carried out as usual.
This PS makes the aforementioned changes to Deckhand's layering
module to make substitution work for non-cyclical dependencies:
A DAG is used to topologically sort the documents according to
their substitution dependency chain. A unit and functional
test has been added to verify the code.
If a cycle is detected, a critical error is thrown. Unit tests
have been added to validate this case as well.
Change-Id: Iaca3963f44aec6c897ad9fd690ce314a3a4d97a2
e4abca1cd7