In the old state package we had this as a separate manager
state.BackupState, but that doesn't work with our new interfaces because
we handle lineage and serial within the state managers themselves and
don't expose them to callers anymore.
In practice it being built in to the filesystem manager is not a problem
because we only use the backup functionality for local state anyway.
This also slightly adjusts the behavior to be more intuitive. The old
BackupState relied on the implementation detail that Terraform re-persists
the original state early in an apply operation, which meant that by
coincidence it would back up the right snapshot. In this new approach,
we instead take an in-memory copy during State and then write _that_ to
disk in WriteState if the new state seems different, so we're guaranteed
that we'll always write out what we read before any changes were made.
In future we may improve this further, such as keeping multiple
generations of backups, etc. But for now this is intended to preserve the
goals of the original implementation while making its behavior
self-contained and not dependent on coincidences.
This idea of a "state manager" was previously modelled via the
confusingly-named state.State interface, which we've been calling a "state
manager" only in some local variable names in situations where there were
also *terraform.State variables.
As part of reworking our state models to make room for the new type
system, we also need to change what was previously the state.StateReader
interface. Since we've found the previous organization confusing anyway,
here we just copy all of those interfaces over into statemgr where we can
make the relationship to states.State hopefully a little clearer.
This is not yet a complete move of the functionality from "state", since
we're not yet ready to break existing callers. In a future commit we'll
turn the interfaces in the old "state" package into aliases of the
interfaces in this package, and update all the implementers of what will
by then be statemgr.Reader to use *states.State instead of
*terraform.State.
This also includes an adaptation of what was previously state.LocalState
into statemgr.FileSystem, using the new state serialization functionality
from package statefile instead of the old terraform.ReadState and
terraform.WriteState.
Martin Atkins