PSS: Add `savedStateStore` method to `Meta` (#37558)

* Add test coverage for Meta's `savedBackend` method

* Add new Meta `savedStateStore` method and test coverage

* Streamline test - remove unneeded assertions and update comments

* Remove marks from config before configuring the provider

* Remove marks from config before configuring the state store

* Add test case for savedStateStore to assert marks aren't passed

* Fix call to ConfigureStateStore

* Show that tests pass despite not trying to remove marks

* Allow Config methods to add marks when reading pluggable state store config from the backend state file

* This code is now necessary to let the tests pass

* Stop adding marks to PSS-related config when it's parsed from the backend state file

* Stop removing marks that aren't there

* Remove unnecessary test related to marks

internal/command/meta_backend.go

@@ -27,6 +27,7 @@ import (
 	"github.com/hashicorp/terraform/internal/backend/backendrun"
 	backendInit "github.com/hashicorp/terraform/internal/backend/init"
 	backendLocal "github.com/hashicorp/terraform/internal/backend/local"
+	backendPluggable "github.com/hashicorp/terraform/internal/backend/pluggable"
 	"github.com/hashicorp/terraform/internal/cloud"
 	"github.com/hashicorp/terraform/internal/command/arguments"
 	"github.com/hashicorp/terraform/internal/command/clistate"
@@ -40,6 +41,7 @@ import (
 	"github.com/hashicorp/terraform/internal/states/statemgr"
 	"github.com/hashicorp/terraform/internal/terraform"
 	"github.com/hashicorp/terraform/internal/tfdiags"
+	tfversion "github.com/hashicorp/terraform/version"
 )
 
 // BackendOpts are the options used to initialize a backendrun.OperationsBackend.
@@ -1526,6 +1528,147 @@ func (m *Meta) updateSavedBackendHash(cHash int, sMgr *clistate.LocalState) tfdi
 	return diags
 }
 
+// Initializing a saved state store from the backend state file (aka 'cache file', aka 'legacy state file')
+func (m *Meta) savedStateStore(sMgr *clistate.LocalState, providerFactory providers.Factory) (backend.Backend, tfdiags.Diagnostics) {
+	// We're preparing a state_store version of backend.Backend.
+	//
+	// The provider and state store will be configured using the backend state file.
+
+	var diags tfdiags.Diagnostics
+	var b backend.Backend
+
+	s := sMgr.State()
+
+	provider, err := providerFactory()
+	if err != nil {
+		diags = diags.Append(fmt.Errorf("error when obtaining provider instance during state store initialization: %w", err))
+		return nil, diags
+	}
+	// We purposefully don't have a deferred call to the provider's Close method here because the calling code needs a
+	// running provider instance inside the returned backend.Backend instance.
+	// Stopping the provider process is the responsibility of the calling code.
+
+	resp := provider.GetProviderSchema()
+
+	if len(resp.StateStores) == 0 {
+		diags = diags.Append(&hcl.Diagnostic{
+			Severity: hcl.DiagError,
+			Summary:  "Provider does not support pluggable state storage",
+			Detail: fmt.Sprintf("There are no state stores implemented by provider %s (%q)",
+				s.StateStore.Provider.Source.Type,
+				s.StateStore.Provider.Source),
+		})
+		return nil, diags
+	}
+
+	stateStoreSchema, exists := resp.StateStores[s.StateStore.Type]
+	if !exists {
+		suggestions := slices.Sorted(maps.Keys(resp.StateStores))
+		suggestion := didyoumean.NameSuggestion(s.StateStore.Type, suggestions)
+		if suggestion != "" {
+			suggestion = fmt.Sprintf(" Did you mean %q?", suggestion)
+		}
+		diags = diags.Append(&hcl.Diagnostic{
+			Severity: hcl.DiagError,
+			Summary:  "State store not implemented by the provider",
+			Detail: fmt.Sprintf("State store %q is not implemented by provider %s (%q)%s",
+				s.StateStore.Type,
+				s.StateStore.Provider.Source.Type,
+				s.StateStore.Provider.Source,
+				suggestion),
+		})
+		return nil, diags
+	}
+
+	// Get the provider config from the backend state file.
+	providerConfigVal, err := s.StateStore.Provider.Config(resp.Provider.Body)
+	if err != nil {
+		diags = diags.Append(
+			&hcl.Diagnostic{
+				Severity: hcl.DiagError,
+				Summary:  "Error reading provider configuration state",
+				Detail: fmt.Sprintf("Terraform experienced an error reading provider configuration for provider %s (%q) while configuring state store %s",
+					s.StateStore.Provider.Source.Type,
+					s.StateStore.Provider.Source,
+					s.StateStore.Type,
+				),
+			},
+		)
+		return nil, diags
+	}
+
+	// Get the state store config from the backend state file.
+	stateStoreConfigVal, err := s.StateStore.Config(stateStoreSchema.Body)
+	if err != nil {
+		diags = diags.Append(
+			&hcl.Diagnostic{
+				Severity: hcl.DiagError,
+				Summary:  "Error reading state store configuration state",
+				Detail: fmt.Sprintf("Terraform experienced an error reading state store configuration for state store %s in provider %s (%q)",
+					s.StateStore.Type,
+					s.StateStore.Provider.Source.Type,
+					s.StateStore.Provider.Source,
+				),
+			},
+		)
+		return nil, diags
+	}
+
+	// Validate and configure the provider
+	//
+	// NOTE: there are no marks we need to remove at this point.
+	// We haven't added marks since the provider config from the backend state was used
+	// because the state-storage provider's config isn't going to be presented to the user via terminal output or diags.
+	validateResp := provider.ValidateProviderConfig(providers.ValidateProviderConfigRequest{
+		Config: providerConfigVal,
+	})
+	diags = diags.Append(validateResp.Diagnostics)
+	if diags.HasErrors() {
+		return nil, diags
+	}
+
+	configureResp := provider.ConfigureProvider(providers.ConfigureProviderRequest{
+		TerraformVersion: tfversion.SemVer.String(),
+		Config:           providerConfigVal,
+	})
+	diags = diags.Append(configureResp.Diagnostics)
+	if diags.HasErrors() {
+		return nil, diags
+	}
+
+	// Validate and configure the state store
+	//
+	// NOTE: there are no marks we need to remove at this point.
+	// We haven't added marks since the state store config from the backend state was used
+	// because the state store's config isn't going to be presented to the user via terminal output or diags.
+	validateStoreResp := provider.ValidateStateStoreConfig(providers.ValidateStateStoreConfigRequest{
+		TypeName: s.StateStore.Type,
+		Config:   stateStoreConfigVal,
+	})
+	diags = diags.Append(validateStoreResp.Diagnostics)
+	if diags.HasErrors() {
+		return nil, diags
+	}
+
+	cfgStoreResp := provider.ConfigureStateStore(providers.ConfigureStateStoreRequest{
+		TypeName: s.StateStore.Type,
+		Config:   stateStoreConfigVal,
+	})
+	diags = diags.Append(cfgStoreResp.Diagnostics)
+	if diags.HasErrors() {
+		return nil, diags
+	}
+
+	// Now we have a fully configured state store, ready to be used.
+	// To make it usable we need to return it in a backend.Backend interface.
+	b, err = backendPluggable.NewPluggable(provider, s.StateStore.Type)
+	if err != nil {
+		diags = diags.Append(err)
+	}
+
+	return b, diags
+}
+
 //-------------------------------------------------------------------
 // Reusable helper functions for backend management
 //-------------------------------------------------------------------

internal/command/meta_backend_test.go

@@ -19,6 +19,7 @@ import (
 	"github.com/hashicorp/terraform/internal/addrs"
 	"github.com/hashicorp/terraform/internal/backend"
 	"github.com/hashicorp/terraform/internal/cloud"
+	"github.com/hashicorp/terraform/internal/command/clistate"
 	"github.com/hashicorp/terraform/internal/command/workdir"
 	"github.com/hashicorp/terraform/internal/configs"
 	"github.com/hashicorp/terraform/internal/configs/configschema"
@@ -35,7 +36,9 @@ import (
 	"github.com/zclconf/go-cty/cty"
 
 	backendInit "github.com/hashicorp/terraform/internal/backend/init"
+	"github.com/hashicorp/terraform/internal/backend/local"
 	backendLocal "github.com/hashicorp/terraform/internal/backend/local"
+	"github.com/hashicorp/terraform/internal/backend/pluggable"
 	backendInmem "github.com/hashicorp/terraform/internal/backend/remote-state/inmem"
 )
 
@@ -2401,6 +2404,97 @@ func TestMetaBackend_configureStateStoreVariableUse(t *testing.T) {
 	}
 }
 
+func TestSavedBackend(t *testing.T) {
+	// Create a temporary working directory
+	td := t.TempDir()
+	testCopyDir(t, testFixturePath("backend-unset"), td) // Backend state file describes local backend, config lacks backend config
+	t.Chdir(td)
+
+	// Make a state manager for the backend state file,
+	// read state from file
+	m := testMetaBackend(t, nil)
+	statePath := filepath.Join(m.DataDir(), DefaultStateFilename)
+	sMgr := &clistate.LocalState{Path: statePath}
+	err := sMgr.RefreshState()
+	if err != nil {
+		t.Fatalf("unexpected error: %s", err)
+	}
+
+	// Code under test
+	b, diags := m.savedBackend(sMgr)
+	if diags.HasErrors() {
+		t.Fatalf("unexpected errors: %s", diags.Err())
+	}
+
+	// The test fixtures used in this test include a backend state file describing
+	// a local backend with the non-default path value below (local-state.tfstate)
+	localB, ok := b.(*local.Local)
+	if !ok {
+		t.Fatalf("expected the returned backend to be a local backend, matching the test fixtures.")
+	}
+	if localB.StatePath != "local-state.tfstate" {
+		t.Fatalf("expected the local backend to be configured using the backend state file, but got unexpected configuration values.")
+	}
+}
+
+func TestSavedStateStore(t *testing.T) {
+	t.Run("the returned state store is configured with the backend state and not the current config", func(t *testing.T) {
+		// Create a temporary working directory
+		td := t.TempDir()
+		testCopyDir(t, testFixturePath("state-store-changed"), td) // Fixtures with config that differs from backend state file
+		t.Chdir(td)
+
+		// Make a state manager for accessing the backend state file,
+		// and read the backend state from file
+		m := testMetaBackend(t, nil)
+		statePath := filepath.Join(m.DataDir(), DefaultStateFilename)
+		sMgr := &clistate.LocalState{Path: statePath}
+		err := sMgr.RefreshState()
+		if err != nil {
+			t.Fatalf("unexpected error: %s", err)
+		}
+
+		// Prepare provider factories for use
+		mock := testStateStoreMock(t)
+		factory := func() (providers.Interface, error) {
+			return mock, nil
+		}
+		mock.ConfigureProviderFn = func(req providers.ConfigureProviderRequest) providers.ConfigureProviderResponse {
+			// Assert that the state store is configured using backend state file values from the fixtures
+			config := req.Config.AsValueMap()
+			if config["region"].AsString() != "old-value" {
+				t.Fatalf("expected the provider to be configured with values from the backend state file (the string \"old-value\"), not the config. Got: %#v", config)
+			}
+			return providers.ConfigureProviderResponse{}
+		}
+		mock.ConfigureStateStoreFn = func(req providers.ConfigureStateStoreRequest) providers.ConfigureStateStoreResponse {
+			// Assert that the state store is configured using backend state file values from the fixtures
+			config := req.Config.AsValueMap()
+			if config["value"].AsString() != "old-value" {
+				t.Fatalf("expected the state store to be configured with values from the backend state file (the string \"old-value\"), not the config. Got: %#v", config)
+			}
+			return providers.ConfigureStateStoreResponse{}
+		}
+
+		// Code under test
+		b, diags := m.savedStateStore(sMgr, factory)
+		if diags.HasErrors() {
+			t.Fatalf("unexpected errors: %s", diags.Err())
+		}
+
+		if _, ok := b.(*pluggable.Pluggable); !ok {
+			t.Fatalf(
+				"expected savedStateStore to return a backend.Backend interface with concrete type %s, but got something else: %#v",
+				"*pluggable.Pluggable",
+				b,
+			)
+		}
+	})
+
+	// NOTE: the mock's functions include assertions about the values passed to
+	// the ConfigureProvider and ConfigureStateStore methods
+}
+
 func TestMetaBackend_GetStateStoreProviderFactory(t *testing.T) {
 	// See internal/command/e2etest/meta_backend_test.go for test case
 	// where a provider factory is found using a local provider cache

internal/command/testdata/state-store-changed/.terraform/terraform.tfstate

@@ -10,7 +10,9 @@
         "provider": {
             "version": "1.2.3",
             "source": "registry.terraform.io/my-org/foo",
-            "config": {},
+            "config": {
+                "region": "old-value"
+            },
             "hash": 12345
         },
         "hash": 12345

internal/command/testdata/state-store-changed/main.tf

@@ -5,7 +5,9 @@ terraform {
     }
   }
   state_store "test_store" {
-    provider "test" {}
+    provider "test" {
+      region = "changed-value" # changed versus backend state file
+    }
 
     value = "changed-value" # changed versus backend state file
   }