The filesystem backend has the option of using a different file for its
initial read.
Previously we were incorrectly writing the contents of that file out into
the backup file, rather than the prior contents of the output file. Now
we will always read the output file in RefreshState in order to decide
what we will back up but then we will optionally additionally read the
input file and prefer its content as the "current" state snapshot.
This is verified by command.TestMetaBackend_planLocalStatePath and
TestMetaBackend_configureNew, which are both now passing.
This was failing because we now handle the settings for the local backend
a little differently as a result of decoding it with the HCL2 machinery.
Specifically, the backend.State* fields are now assumed to be what is
given in configuration, and any CLI overrides are maintained separately
in OverrideState* fields so that they can be imposed "just in time" in
StatePaths.
This is particularly important because OverrideStatePath (when set) is
used regardless of workspace name, while StatePath is a suitable value
only for the "default" workspace, with others needing to be constructed
from StateWorkspaceDir instead.
In our recent refactoring of the state manager interfaces we made serial
and lineage management the responsibility of the state managers
themselves, not exposing them at all to most callers, and allowing for
simple state managers that don't implement them at all.
However, we do have some specific cases where we need to preserve these
properly when available, such as migration between backends, and the
"terraform state push" and "terraform state pull" commands.
These new functions and their associated optional interface allow the
logic here to be captured in one place and access via some simple
calls. Separating this from the main interface leaves things simple for
the normal uses of state managers.
Since these functions are mostly just thin wrappers around other
functionality, they are not yet well-tested directly, but will be
indirectly tested through the tests of their callers. A subsequent commit
will add more unit tests here.
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