The shadow graph was incredibly useful during the 0.7 cycle but these days
it is idle, since we're not planning any significant graph-related changes
for the forseeable future.
The shadow graph infrastructure is somewhat burdensome since any change
to the ResourceProvider interface must have shims written. Since we _are_
expecting changes to the ResourceProvider interface in the next few
releases, I'm calling "YAGNI" on the shadow graph support to reduce our
maintenence burden.
If we do end up wanting to use shadow graph again in future, we'll always
be able to pull it out of version control and then make whatever changes
we skipped making in the mean time, but we can avoid that cost in the
mean time while we don't have any evidence that we'll need to pay it.
This switches to the Go "context" package for cancellation and threads
the context through all the way to evaluation to allow behavior based on
stopping deep within graph execution.
This also adds the Stop API to provisioners so they can quickly exit
when stop is called.
Implement debugInfo and the DebugGraph
DebugInfo will be a global variable through which graph debug
information can we written to a compressed archive. The DebugInfo
methods are all safe for concurrent use, and noop with a nil receiver.
The API outside of the terraform package will be to call SetDebugInfo
to create the archive, and CloseDebugInfo() to properly close the file.
Each write to the archive will be flushed and sync'ed individually, so
in the event of a crash or a missing call to Close, the archive can
still be recovered.
The DebugGraph is a representation of a terraform Graph to be written to
the debug archive, currently in dot format. The DebugGraph also contains
an internal buffer with Printf and Write methods to add to this buffer.
The buffer will be written to an accompanying file in the debug archive
along with the graph.
This also adds a GraphNodeDebugger interface. Any node implementing
`NodeDebug() string` can output information to annotate the debug graph
node, and add the data to the log. This interface may change or be
removed to provide richer options for debugging graph nodes.
The new graph builders all delegate the build to the BasicGraphBuilder.
Having a Name field lets us differentiate the actual builder
implementation in the debug graphs.
This is necessary so that the shadow version can actually keep track of
what provider is used for what. Before, providers for different alises
were just initialized but the factory had no idea. Arguably this is fine
but when trying to build a shadow graph this presents challenges.
With these changes, we now pass an opaque "uid" through that is used to
keep track of the providers and what real maps to what shadow.
