dag.Graph is used as a value, but contains a sync data structure. This
prevents copying the value, and is needed if one wants to upgrade a Graph
to an AcyclicGraph.
The sync.Once only guards the init() method, which can be guarded
internally with nil checks on the fields. The init method could be
removed entirely with a proper constructor later on of we so choose.
The AnnotateVertex and AnnotateEdge Graph methods will allow terraform
to insert arbitray information into the encoded graph stream for later
processing.
The external api provided here is simply
dag.Graph.SetDebugWriter(io.Writer). When a writer is provided to a
Graph, it will immediately encode itself to the stream, and subsequently
encode any additional transformations to the graph. This will allow
easier logging of graph transformations without writing complete graphs
to the logs at every step. Since the marshalGraph can also be dot
encoded, this will allow translation from the JSON logs to dot graphs.
James Bardin