Until now we've not really cared much about the state snapshot produced
by the previous Terraform operation, except to use it as a jumping-off
point for our refresh step.
However, we'd like to be able to report to an end-user whenever Terraform
detects a change that occurred outside of Terraform, because that's often
helpful context for understanding why a plan contains changes that don't
seem to have corresponding changes in the configuration.
As part of reporting that we'll need to keep track of the state as it
was before we did any refreshing work, so we can then compare that against
the state after refreshing. To retain enough data to achieve that, the
existing Plan field State is now two fields: PrevRunState and PriorState.
This also includes a very shallow change in the core package to make it
populate something somewhat-reasonable into this field so that integration
tests can function reasonably. However, this shallow implementation isn't
really sufficient for real-world use of PrevRunState because we'll
actually need to update PrevRunState as part of planning in order to
incorporate the results of any provider-specific state upgrades to make
the PrevRunState objects compatible with the current provider schema, or
else our diffs won't be valid. This deeper awareness of PrevRunState in
Terraform Core will follow in a subsequent commit, prior to anything else
making use of Plan.PrevRunState.
The new format is radically different in than the old in physical
structure, but still has the same logical parts: the plan itself, a
snapshot of the input configuration, and a snapshot of the state as it
existed when the plan was created.
Rather than creating plan-specific serializations of state and config, the
new format instead leans on the existing file formats implemented
elsewhere, wrapping the result up in a zip archive with some internal file
naming conventions.
The plan portion of the file is serialized with protobuf, consistent with
our general strategy of replacing all use of encoding/gob with protobuf
moving forward.
Martin Atkins