* Lift snapshot management out of the engine
This PR is a prerequisite for parallelism by addressing a major problem
that the engine has to deal with when performing parallel resource
construction: parallel mutation of the global snapshot. This PR adds
a `SnapshotManager` type that is responsible for maintaining and
persisting the current resource snapshot. It serializes all reads and
writes to the global snapshot and persists the snapshot to persistent
storage upon every write.
As a side-effect of this, the core engine no longer needs to know about
snapshot management at all; all snapshot operations can be handled as
callbacks on deployment events. This will greatly simplify the
parallelization of the core engine.
Worth noting is that the core engine will still need to be able to read
the current snapshot, since it is interested in the dependency graphs
contained within. The full implications of that are out of scope of this
PR.
Remove dead code, Steps no longer need a reference to the plan iterator that created them
Fixing various issues that arise when bringing up pulumi-aws
Line length broke the build
Code review: remove dead field, fix yaml name error
Rebase against master, provide implementation of StackPersister for cloud backend
Code review feedback: comments on MutationStatus, style in snapshot.go
Code review feedback: move SnapshotManager to pkg/backend, change engine to use an interface SnapshotManager
Code review feedback: use a channel for synchronization
Add a comment and a new test
* Maintain two checkpoints, an immutable base and a mutable delta, and
periodically merge the two to produce snapshots
* Add a lot of tests - covers all of the non-error paths of BeginMutation and End
* Fix a test resource provider
* Add a few tests, fix a few issues
* Rebase against master, fixed merge
2c479c172d