// Copyright (c) The OpenTofu Authors
// SPDX-License-Identifier: MPL-2.0
// Copyright (c) 2023 HashiCorp, Inc.
// SPDX-License-Identifier: MPL-2.0

package command

import (
	"bytes"
	"context"
	"errors"
	"fmt"
	"log"
	"os"
	"path/filepath"
	"sort"
	"strings"

	"github.com/opentofu/opentofu/internal/backend"
	"github.com/opentofu/opentofu/internal/backend/remote"
	"github.com/opentofu/opentofu/internal/cloud"
	"github.com/opentofu/opentofu/internal/command/arguments"
	"github.com/opentofu/opentofu/internal/command/clistate"
	"github.com/opentofu/opentofu/internal/command/views"
	"github.com/opentofu/opentofu/internal/encryption"
	"github.com/opentofu/opentofu/internal/states"
	"github.com/opentofu/opentofu/internal/states/statemgr"
	"github.com/opentofu/opentofu/internal/tofu"
)

type backendMigrateOpts struct {
	SourceType, DestinationType string
	Source, Destination         backend.Backend
	ViewType                    arguments.ViewType

	// Fields below are set internally when migrate is called

	sourceWorkspace      string
	destinationWorkspace string
	force                bool // if true, won't ask for confirmation
}

// backendMigrateState handles migrating (copying) state from one backend
// to another. This function handles asking the user for confirmation
// as well as the copy itself.
//
// This function can handle all scenarios of state migration regardless
// of the existence of state in either backend.
//
// After migrating the state, the existing state in the first backend
// remains untouched.
//
// This will attempt to lock both states for the migration.
func (m *Meta) backendMigrateState(ctx context.Context, opts *backendMigrateOpts) error {
	log.Printf("[INFO] backendMigrateState: need to migrate from %q to %q backend config", opts.SourceType, opts.DestinationType)
	// We need to check what the named state status is. If we're converting
	// from multi-state to single-state for example, we need to handle that.
	var sourceSingleState, destinationSingleState, sourceTFC, destinationTFC bool

	_, sourceTFC = opts.Source.(*cloud.Cloud)
	_, destinationTFC = opts.Destination.(*cloud.Cloud)

	sourceWorkspaces, sourceSingleState, err := retrieveWorkspaces(ctx, opts.Source, opts.SourceType)
	if err != nil {
		return err
	}
	destinationWorkspaces, destinationSingleState, err := retrieveWorkspaces(ctx, opts.Destination, opts.SourceType)
	if err != nil {
		return err
	}

	// Set up defaults
	opts.sourceWorkspace = backend.DefaultStateName
	opts.destinationWorkspace = backend.DefaultStateName
	opts.force = m.forceInitCopy

	// Disregard remote OpenTofu version for the state source backend. If it's a
	// Terraform Cloud remote backend, we don't care about the remote version,
	// as we are migrating away and will not break a remote workspace.
	m.ignoreRemoteVersionConflict(opts.Source)

	// Disregard remote OpenTofu version if instructed to do so via CLI flag.
	if m.ignoreRemoteVersion {
		m.ignoreRemoteVersionConflict(opts.Destination)
	} else {
		// Check the remote OpenTofu version for the state destination backend. If
		// it's a Terraform Cloud remote backend, we want to ensure that we don't
		// break the workspace by uploading an incompatible state file.
		for _, workspace := range destinationWorkspaces {
			diags := m.remoteVersionCheck(opts.Destination, workspace)
			if diags.HasErrors() {
				return diags.Err()
			}
		}
		// If there are no specified destination workspaces, perform a remote
		// backend version check with the default workspace.
		// Ensure that we are not dealing with Terraform Cloud migrations, as it
		// does not support the default name.
		if len(destinationWorkspaces) == 0 && !destinationTFC {
			diags := m.remoteVersionCheck(opts.Destination, backend.DefaultStateName)
			if diags.HasErrors() {
				return diags.Err()
			}
		}
	}

	// Determine migration behavior based on whether the source/destination
	// supports multi-state.
	switch {
	case sourceTFC || destinationTFC:
		return m.backendMigrateTFC(ctx, opts)

	// Single-state to single-state. This is the easiest case: we just
	// copy the default state directly.
	case sourceSingleState && destinationSingleState:
		return m.backendMigrateState_s_s(ctx, opts)

	// Single-state to multi-state. This is easy since we just copy
	// the default state and ignore the rest in the destination.
	case sourceSingleState && !destinationSingleState:
		return m.backendMigrateState_s_s(ctx, opts)

	// Multi-state to single-state. If the source has more than the default
	// state this is complicated since we have to ask the user what to do.
	case !sourceSingleState && destinationSingleState:
		// If the source only has one state and it is the default,
		// treat it as if it doesn't support multi-state.
		if len(sourceWorkspaces) == 1 && sourceWorkspaces[0] == backend.DefaultStateName {
			return m.backendMigrateState_s_s(ctx, opts)
		}

		return m.backendMigrateState_S_s(ctx, opts)

	// Multi-state to multi-state. We merge the states together (migrating
	// each from the source to the destination one by one).
	case !sourceSingleState && !destinationSingleState:
		// If the source only has one state and it is the default,
		// treat it as if it doesn't support multi-state.
		if len(sourceWorkspaces) == 1 && sourceWorkspaces[0] == backend.DefaultStateName {
			return m.backendMigrateState_s_s(ctx, opts)
		}

		return m.backendMigrateState_S_S(ctx, opts)
	}

	return nil
}

//-------------------------------------------------------------------
// State Migration Scenarios
//
// The functions below cover handling all the various scenarios that
// can exist when migrating state. They are named in an immediately not
// obvious format but is simple:
//
// Format: backendMigrateState_s1_s2[_suffix]
//
// When s1 or s2 is lower case, it means that it is a single state backend.
// When either is uppercase, it means that state is a multi-state backend.
// The suffix is used to disambiguate multiple cases with the same type of
// states.
//
//-------------------------------------------------------------------

// Multi-state to multi-state.
func (m *Meta) backendMigrateState_S_S(ctx context.Context, opts *backendMigrateOpts) error {
	log.Print("[INFO] backendMigrateState: migrating all named workspaces")

	migrate := opts.force
	if !migrate {
		var err error
		// Ask the user if they want to migrate their existing remote state
		migrate, err = m.confirm(&tofu.InputOpts{
			Id: "backend-migrate-multistate-to-multistate",
			Query: fmt.Sprintf(
				"Do you want to migrate all workspaces to %q?",
				opts.DestinationType),
			Description: fmt.Sprintf(
				strings.TrimSpace(inputBackendMigrateMultiToMulti),
				opts.SourceType, opts.DestinationType),
		})
		if err != nil {
			return fmt.Errorf(
				"Error asking for state migration action: %w", err)
		}
	}
	if !migrate {
		return fmt.Errorf("Migration aborted by user.")
	}

	// Read all the states
	sourceWorkspaces, err := opts.Source.Workspaces(ctx)
	if err != nil {
		return fmt.Errorf(strings.TrimSpace(
			errMigrateLoadStates), opts.SourceType, err)
	}

	// Sort the states so they're always copied alphabetically
	sort.Strings(sourceWorkspaces)

	// Go through each and migrate
	for _, name := range sourceWorkspaces {
		// Copy the same names
		opts.sourceWorkspace = name
		opts.destinationWorkspace = name

		// Force it, we confirmed above
		opts.force = true

		// Perform the migration
		if err := m.backendMigrateState_s_s(ctx, opts); err != nil {
			return fmt.Errorf(strings.TrimSpace(
				errMigrateMulti), name, opts.SourceType, opts.DestinationType, err)
		}
	}

	return nil
}

// Multi-state to single state.
func (m *Meta) backendMigrateState_S_s(ctx context.Context, opts *backendMigrateOpts) error {
	log.Printf("[INFO] backendMigrateState: destination backend type %q does not support named workspaces", opts.DestinationType)

	currentWorkspace, err := m.Workspace(ctx)
	if err != nil {
		return err
	}

	migrate := opts.force
	if !migrate {
		var err error
		// Ask the user if they want to migrate their existing remote state
		migrate, err = m.confirm(&tofu.InputOpts{
			Id: "backend-migrate-multistate-to-single",
			Query: fmt.Sprintf(
				"Destination state %q doesn't support workspaces.\n"+
					"Do you want to copy only your current workspace?",
				opts.DestinationType),
			Description: fmt.Sprintf(
				strings.TrimSpace(inputBackendMigrateMultiToSingle),
				opts.SourceType, opts.DestinationType, currentWorkspace),
		})
		if err != nil {
			return fmt.Errorf(
				"Error asking for state migration action: %w", err)
		}
	}

	if !migrate {
		return fmt.Errorf("Migration aborted by user.")
	}

	// Copy the default state
	opts.sourceWorkspace = currentWorkspace

	// now switch back to the default env so we can access the new backend
	if err := m.SetWorkspace(backend.DefaultStateName); err != nil {
		return err
	}

	return m.backendMigrateState_s_s(ctx, opts)
}

// Single state to single state, assumed default state name.
func (m *Meta) backendMigrateState_s_s(ctx context.Context, opts *backendMigrateOpts) error {
	log.Printf("[INFO] backendMigrateState: single-to-single migrating %q workspace to %q workspace", opts.sourceWorkspace, opts.destinationWorkspace)

	sourceState, err := opts.Source.StateMgr(ctx, opts.sourceWorkspace)
	if err != nil {
		return fmt.Errorf(strings.TrimSpace(
			errMigrateSingleLoadDefault), opts.SourceType, err)
	}
	if err := sourceState.RefreshState(context.TODO()); err != nil {
		return fmt.Errorf(strings.TrimSpace(
			errMigrateSingleLoadDefault), opts.SourceType, err)
	}

	// Do not migrate workspaces without state.
	if sourceState.State().Empty() {
		log.Print("[TRACE] backendMigrateState: source workspace has empty state, so nothing to migrate")
		return nil
	}

	destinationState, err := opts.Destination.StateMgr(ctx, opts.destinationWorkspace)
	if err == backend.ErrDefaultWorkspaceNotSupported {
		// If the backend doesn't support using the default state, we ask the user
		// for a new name and migrate the default state to the given named state.
		destinationState, err = func() (statemgr.Full, error) {
			log.Print("[TRACE] backendMigrateState: destination doesn't support a default workspace, so we must prompt for a new name")
			name, err := m.promptNewWorkspaceName(opts.DestinationType)
			if err != nil {
				return nil, err
			}

			// Update the name of the destination state.
			opts.destinationWorkspace = name

			destinationState, err := opts.Destination.StateMgr(ctx, opts.destinationWorkspace)
			if err != nil {
				return nil, err
			}

			// Ignore invalid workspace name as it is irrelevant in this context.
			workspace, _ := m.Workspace(ctx)

			// If the currently selected workspace is the default workspace, then set
			// the named workspace as the new selected workspace.
			if workspace == backend.DefaultStateName {
				if err := m.SetWorkspace(opts.destinationWorkspace); err != nil {
					return nil, fmt.Errorf("Failed to set new workspace: %w", err)
				}
			}

			return destinationState, nil
		}()
	}
	if err != nil {
		return fmt.Errorf(strings.TrimSpace(
			errMigrateSingleLoadDefault), opts.DestinationType, err)
	}
	if err := destinationState.RefreshState(context.TODO()); err != nil {
		return fmt.Errorf(strings.TrimSpace(
			errMigrateSingleLoadDefault), opts.DestinationType, err)
	}

	// Check if we need migration at all.
	// This is before taking a lock, because they may also correspond to the same lock.
	source := sourceState.State()
	destination := destinationState.State()

	// no reason to migrate if the state is already there
	if source.Equal(destination) {
		// Equal isn't identical; it doesn't check lineage.
		sm1, _ := sourceState.(statemgr.PersistentMeta)
		sm2, _ := destinationState.(statemgr.PersistentMeta)
		if source != nil && destination != nil {
			if sm1 == nil || sm2 == nil {
				log.Print("[TRACE] backendMigrateState: both source and destination workspaces have no state, so no migration is needed")
				return nil
			}
			if sm1.StateSnapshotMeta().Lineage == sm2.StateSnapshotMeta().Lineage {
				log.Printf("[TRACE] backendMigrateState: both source and destination workspaces have equal state with lineage %q, so no migration is needed", sm1.StateSnapshotMeta().Lineage)
				return nil
			}
		}
	}

	if m.stateLock {
		lockCtx := context.Background()
		vt := arguments.ViewJSON
		// Set default viewtype if none was set as the StateLocker needs to know exactly
		// what viewType we want to have.
		if opts == nil || opts.ViewType != vt {
			vt = arguments.ViewHuman
		}
		view := views.NewStateLocker(vt, m.View)
		locker := clistate.NewLocker(m.stateLockTimeout, view)

		lockerSource := locker.WithContext(lockCtx)
		if diags := lockerSource.Lock(sourceState, "migration source state"); diags.HasErrors() {
			return diags.Err()
		}
		defer lockerSource.Unlock()

		lockerDestination := locker.WithContext(lockCtx)
		if diags := lockerDestination.Lock(destinationState, "migration destination state"); diags.HasErrors() {
			return diags.Err()
		}
		defer lockerDestination.Unlock()

		// We now own a lock, so double check that we have the version
		// corresponding to the lock.
		log.Print("[TRACE] backendMigrateState: refreshing source workspace state")
		if err := sourceState.RefreshState(context.TODO()); err != nil {
			return fmt.Errorf(strings.TrimSpace(
				errMigrateSingleLoadDefault), opts.SourceType, err)
		}
		log.Print("[TRACE] backendMigrateState: refreshing destination workspace state")
		if err := destinationState.RefreshState(context.TODO()); err != nil {
			return fmt.Errorf(strings.TrimSpace(
				errMigrateSingleLoadDefault), opts.SourceType, err)
		}

		source = sourceState.State()
		destination = destinationState.State()
	}

	var confirmFunc func(statemgr.Full, statemgr.Full, *backendMigrateOpts) (bool, error)
	switch {
	// No migration necessary
	case source.Empty() && destination.Empty():
		log.Print("[TRACE] backendMigrateState: both source and destination workspaces have empty state, so no migration is required")
		return nil

	// No migration necessary if we're inheriting state.
	case source.Empty() && !destination.Empty():
		log.Print("[TRACE] backendMigrateState: source workspace has empty state, so no migration is required")
		return nil

	// We have existing state moving into no state. Ask the user if
	// they'd like to do this.
	case !source.Empty() && destination.Empty():
		if opts.SourceType == "cloud" || opts.DestinationType == "cloud" {
			// HACK: backendMigrateTFC has its own earlier prompt for
			// whether to migrate state in the cloud case, so we'll skip
			// this later prompt for Cloud, even though we do still need it
			// for state backends.
			confirmFunc = func(statemgr.Full, statemgr.Full, *backendMigrateOpts) (bool, error) {
				return true, nil // the answer is implied to be "yes" if we reached this point
			}
		} else {
			log.Print("[TRACE] backendMigrateState: destination workspace has empty state, so might copy source workspace state")
			confirmFunc = m.backendMigrateEmptyConfirm
		}

	// Both states are non-empty, meaning we need to determine which
	// state should be used and update accordingly.
	case !source.Empty() && !destination.Empty():
		log.Print("[TRACE] backendMigrateState: both source and destination workspaces have states, so might overwrite destination with source")
		confirmFunc = m.backendMigrateNonEmptyConfirm
	}

	if confirmFunc == nil {
		panic("confirmFunc must not be nil")
	}

	if !opts.force {
		// Abort if we can't ask for input.
		if !m.input {
			log.Print("[TRACE] backendMigrateState: can't prompt for input, so aborting migration")
			return errors.New(strings.TrimSpace(errInteractiveInputDisabled))
		}

		// Confirm with the user whether we want to copy state over
		confirm, err := confirmFunc(sourceState, destinationState, opts)
		if err != nil {
			log.Print("[TRACE] backendMigrateState: error reading input, so aborting migration")
			return err
		}
		if !confirm {
			log.Print("[TRACE] backendMigrateState: user cancelled at confirmation prompt, so aborting migration")
			return nil
		}
	}

	// Confirmed! We'll have the statemgr package handle the migration, which
	// includes preserving any lineage/serial information where possible, if
	// both managers support such metadata.
	log.Print("[TRACE] backendMigrateState: migration confirmed, so migrating")
	if err := statemgr.Migrate(destinationState, sourceState); err != nil {
		return fmt.Errorf(strings.TrimSpace(errBackendStateCopy),
			opts.SourceType, opts.DestinationType, err)
	}
	// The backend is currently handled before providers are installed during init,
	// so requiring schemas here could lead to a catch-22 where it requires some manual
	// intervention to proceed far enough for provider installation. To avoid this,
	// when migrating to TFC backend, the initial JSON variant of state won't be generated and stored.
	if err := destinationState.PersistState(context.TODO(), nil); err != nil {
		return fmt.Errorf(strings.TrimSpace(errBackendStateCopy),
			opts.SourceType, opts.DestinationType, err)
	}

	// And we're done.
	return nil
}

func (m *Meta) backendMigrateEmptyConfirm(source, destination statemgr.Full, opts *backendMigrateOpts) (bool, error) {
	var inputOpts *tofu.InputOpts
	if opts.DestinationType == "cloud" {
		inputOpts = &tofu.InputOpts{
			Id:          "backend-migrate-copy-to-empty-cloud",
			Query:       "Do you want to copy existing state to cloud backend?",
			Description: fmt.Sprintf(strings.TrimSpace(inputBackendMigrateEmptyCloud), opts.SourceType),
		}
	} else {
		inputOpts = &tofu.InputOpts{
			Id:    "backend-migrate-copy-to-empty",
			Query: "Do you want to copy existing state to the new backend?",
			Description: fmt.Sprintf(
				strings.TrimSpace(inputBackendMigrateEmpty),
				opts.SourceType, opts.DestinationType),
		}
	}

	return m.confirm(inputOpts)
}

func (m *Meta) backendMigrateNonEmptyConfirm(
	sourceState, destinationState statemgr.Full, opts *backendMigrateOpts) (bool, error) {
	// We need to grab both states so we can write them to a file
	source := sourceState.State()
	destination := destinationState.State()

	// Save both to a temporary
	td, err := os.MkdirTemp("", "terraform")
	if err != nil {
		return false, fmt.Errorf("Error creating temporary directory: %w", err)
	}
	defer os.RemoveAll(td)

	// Helper to write the state
	saveHelper := func(n, path string, s *states.State) error {
		return statemgr.WriteAndPersist(context.TODO(), statemgr.NewFilesystem(path, encryption.StateEncryptionDisabled()), s, nil)
	}

	// Write the states
	sourcePath := filepath.Join(td, fmt.Sprintf("1-%s.tfstate", opts.SourceType))
	destinationPath := filepath.Join(td, fmt.Sprintf("2-%s.tfstate", opts.DestinationType))
	if err := saveHelper(opts.SourceType, sourcePath, source); err != nil {
		return false, fmt.Errorf("Error saving temporary state: %w", err)
	}
	if err := saveHelper(opts.DestinationType, destinationPath, destination); err != nil {
		return false, fmt.Errorf("Error saving temporary state: %w", err)
	}

	// Ask for confirmation
	var inputOpts *tofu.InputOpts
	if opts.DestinationType == "cloud" {
		inputOpts = &tofu.InputOpts{
			Id:    "backend-migrate-to-tfc",
			Query: "Do you want to copy existing state to cloud backend?",
			Description: fmt.Sprintf(
				strings.TrimSpace(inputBackendMigrateNonEmptyCloud),
				opts.SourceType, sourcePath, destinationPath),
		}
	} else {
		inputOpts = &tofu.InputOpts{
			Id:    "backend-migrate-to-backend",
			Query: "Do you want to copy existing state to the new backend?",
			Description: fmt.Sprintf(
				strings.TrimSpace(inputBackendMigrateNonEmpty),
				opts.SourceType, opts.DestinationType, sourcePath, destinationPath),
		}
	}

	// Confirm with the user that the copy should occur
	return m.confirm(inputOpts)
}

func retrieveWorkspaces(ctx context.Context, back backend.Backend, sourceType string) ([]string, bool, error) {
	var singleState bool
	var err error
	workspaces, err := back.Workspaces(ctx)
	if err == backend.ErrWorkspacesNotSupported {
		singleState = true
		err = nil
	}
	if err != nil {
		return nil, singleState, fmt.Errorf(strings.TrimSpace(
			errMigrateLoadStates), sourceType, err)
	}

	return workspaces, singleState, err
}

func (m *Meta) backendMigrateTFC(ctx context.Context, opts *backendMigrateOpts) error {
	_, sourceTFC := opts.Source.(*cloud.Cloud)
	cloudBackendDestination, destinationTFC := opts.Destination.(*cloud.Cloud)

	sourceWorkspaces, sourceSingleState, err := retrieveWorkspaces(ctx, opts.Source, opts.SourceType)
	if err != nil {
		return err
	}
	// to be used below, not yet implemented
	// destinationWorkspaces, destinationSingleState
	_, _, err = retrieveWorkspaces(ctx, opts.Destination, opts.SourceType)
	if err != nil {
		return err
	}

	// from TFC to non-TFC backend
	if sourceTFC && !destinationTFC {
		// From Terraform Cloud to another backend. This is not yet implemented, and
		// we recommend people to use the TFC API.
		return fmt.Errorf("%s", strings.TrimSpace(errTFCMigrateNotYetImplemented))
	}

	// Everything below, by the above two conditionals, now assumes that the
	// destination is always Terraform Cloud (TFC).

	sourceSingle := sourceSingleState || (len(sourceWorkspaces) == 1)
	if sourceSingle {
		if cloudBackendDestination.WorkspaceMapping.Strategy() == cloud.WorkspaceNameStrategy {
			// If we know the name via WorkspaceNameStrategy, then set the
			// destinationWorkspace to the new Name and skip the user prompt. Here the
			// destinationWorkspace is not set to `default` thereby we will create it
			// in TFC if it does not exist.
			opts.destinationWorkspace = cloudBackendDestination.WorkspaceMapping.Name
		}

		currentWorkspace, err := m.Workspace(ctx)
		if err != nil {
			return err
		}
		opts.sourceWorkspace = currentWorkspace

		log.Printf("[INFO] backendMigrateTFC: single-to-single migration from source %s to destination %q", opts.sourceWorkspace, opts.destinationWorkspace)

		// If the current workspace is has no state we do not need to ask
		// if they want to migrate the state.
		sourceState, err := opts.Source.StateMgr(ctx, currentWorkspace)
		if err != nil {
			return err
		}
		if err := sourceState.RefreshState(context.TODO()); err != nil {
			return err
		}
		if sourceState.State().Empty() {
			log.Printf("[INFO] backendMigrateTFC: skipping migration because source %s is empty", opts.sourceWorkspace)
			return nil
		}

		// Run normal single-to-single state migration.
		// This will handle both situations where the new cloud backend
		// configuration is using a workspace.name strategy or workspace.tags
		// strategy.
		//
		// We do prompt first though, because state migration is mandatory
		// for moving to Cloud and the user should get an opportunity to
		// confirm that first.
		if migrate, err := m.promptSingleToCloudSingleStateMigration(opts); err != nil {
			return err
		} else if !migrate {
			return nil //skip migrating but return successfully
		}

		return m.backendMigrateState_s_s(ctx, opts)
	}

	destinationTagsStrategy := cloudBackendDestination.WorkspaceMapping.Strategy() == cloud.WorkspaceTagsStrategy
	destinationNameStrategy := cloudBackendDestination.WorkspaceMapping.Strategy() == cloud.WorkspaceNameStrategy

	multiSource := !sourceSingleState && len(sourceWorkspaces) > 1
	if multiSource && destinationNameStrategy {
		currentWorkspace, err := m.Workspace(ctx)
		if err != nil {
			return err
		}

		opts.sourceWorkspace = currentWorkspace
		opts.destinationWorkspace = cloudBackendDestination.WorkspaceMapping.Name
		if err := m.promptMultiToSingleCloudMigration(opts); err != nil {
			return err
		}

		log.Printf("[INFO] backendMigrateTFC: multi-to-single migration from source %s to destination %q", opts.sourceWorkspace, opts.destinationWorkspace)

		return m.backendMigrateState_s_s(ctx, opts)
	}

	// Multiple sources, and using tags strategy. So migrate every source
	// workspace over to new one, prompt for workspace name pattern (*),
	// and start migrating, and create tags for each workspace.
	if multiSource && destinationTagsStrategy {
		log.Printf("[INFO] backendMigrateTFC: multi-to-multi migration from source workspaces %q", sourceWorkspaces)
		return m.backendMigrateState_S_TFC(ctx, opts, sourceWorkspaces)
	}

	// TODO(omar): after the check for sourceSingle is done, everything following
	// it has to be multi. So rework the code to not need to check for multi, and
	// return m.backendMigrateState_S_TFC here.
	return nil
}

// migrates a multi-state backend to Terraform Cloud
func (m *Meta) backendMigrateState_S_TFC(ctx context.Context, opts *backendMigrateOpts, sourceWorkspaces []string) error {
	log.Print("[TRACE] backendMigrateState: migrating all named workspaces")

	currentWorkspace, err := m.Workspace(ctx)
	if err != nil {
		return err
	}
	newCurrentWorkspace := ""

	// This map is used later when doing the migration per source/destination.
	// If a source has 'default' and has state, then we ask what the new name should be.
	// And further down when we actually run state migration for each
	// source/destination workspace, we use this new name (where source is 'default')
	// and set as destinationWorkspace. If the default workspace does not have
	// state we will not prompt the user for a new name because empty workspaces
	// do not get migrated.
	defaultNewName := map[string]string{}
	for i := 0; i < len(sourceWorkspaces); i++ {
		if sourceWorkspaces[i] == backend.DefaultStateName {
			// For the default workspace we want to look to see if there is any state
			// before we ask for a workspace name to migrate the default workspace into.
			sourceState, err := opts.Source.StateMgr(ctx, backend.DefaultStateName)
			if err != nil {
				return fmt.Errorf(strings.TrimSpace(
					errMigrateSingleLoadDefault), opts.SourceType, err)
			}
			// RefreshState is what actually pulls the state to be evaluated.
			if err := sourceState.RefreshState(context.TODO()); err != nil {
				return fmt.Errorf(strings.TrimSpace(
					errMigrateSingleLoadDefault), opts.SourceType, err)
			}
			if !sourceState.State().Empty() {
				newName, err := m.promptNewWorkspaceName(opts.DestinationType)
				if err != nil {
					return err
				}
				defaultNewName[sourceWorkspaces[i]] = newName
			}
		}
	}

	// Fetch the pattern that will be used to rename the workspaces for Terraform Cloud.
	//
	// * For the general case, this will be a pattern provided by the user.
	//
	// * Specifically for a migration from the "remote" backend using 'prefix', we will
	//   instead 'migrate' the workspaces using a pattern based on the old prefix+name,
	//   not allowing a user to accidentally input the wrong pattern to line up with
	//   what the remote backend was already using before (which presumably already
	//   meets the naming considerations for Terraform Cloud).
	//   In other words, this is a fast-track migration path from the remote backend, retaining
	//   how things already are in Terraform Cloud with no user intervention needed.
	pattern := ""
	if remoteBackend, ok := opts.Source.(*remote.Remote); ok {
		if err := m.promptRemotePrefixToCloudTagsMigration(opts); err != nil {
			return err
		}
		pattern = remoteBackend.WorkspaceNamePattern()
		log.Printf("[TRACE] backendMigrateTFC: Remote backend reports workspace name pattern as: %q", pattern)
	}

	if pattern == "" {
		pattern, err = m.promptMultiStateMigrationPattern(opts.SourceType)
		if err != nil {
			return err
		}
	}

	// Go through each and migrate
	for _, name := range sourceWorkspaces {

		// Copy the same names
		opts.sourceWorkspace = name
		if newName, ok := defaultNewName[name]; ok {
			// this has to be done before setting destinationWorkspace
			name = newName
		}
		opts.destinationWorkspace = strings.ReplaceAll(pattern, "*", name)

		// Force it, we confirmed above
		opts.force = true

		// Perform the migration
		log.Printf("[INFO] backendMigrateTFC: multi-to-multi migration, source workspace %q to destination workspace %q", opts.sourceWorkspace, opts.destinationWorkspace)
		if err := m.backendMigrateState_s_s(ctx, opts); err != nil {
			return fmt.Errorf(strings.TrimSpace(
				errMigrateMulti), name, opts.SourceType, opts.DestinationType, err)
		}

		if currentWorkspace == opts.sourceWorkspace {
			newCurrentWorkspace = opts.destinationWorkspace
		}
	}

	// After migrating multiple workspaces, we need to reselect the current workspace as it may
	// have been renamed. Query the backend first to be sure it now exists.
	workspaces, err := opts.Destination.Workspaces(ctx)
	if err != nil {
		return err
	}

	var workspacePresent bool
	for _, name := range workspaces {
		if name == newCurrentWorkspace {
			workspacePresent = true
		}
	}

	// If we couldn't select the workspace automatically from the backend (maybe it was empty
	// and wasn't migrated, for instance), ask the user to select one instead and be done.
	if !workspacePresent {
		if err = m.selectWorkspace(ctx, opts.Destination); err != nil {
			return err
		}
		return nil
	}

	// The newly renamed current workspace is present, so we'll automatically select it for the
	// user, as well as display the equivalent of 'workspace list' to show how the workspaces
	// were changed (as well as the newly selected current workspace).
	if err = m.SetWorkspace(newCurrentWorkspace); err != nil {
		return err
	}

	m.Ui.Output(m.Colorize().Color("[reset][bold]Migration complete! Your workspaces are as follows:[reset]"))
	var out bytes.Buffer
	for _, name := range workspaces {
		if name == newCurrentWorkspace {
			out.WriteString("* ")
		} else {
			out.WriteString("  ")
		}
		out.WriteString(name + "\n")
	}

	m.Ui.Output(out.String())

	return nil
}

func (m *Meta) promptSingleToCloudSingleStateMigration(opts *backendMigrateOpts) (bool, error) {
	if !m.input {
		log.Print("[TRACE] backendMigrateState: can't prompt for input, so aborting migration")
		return false, errors.New(strings.TrimSpace(errInteractiveInputDisabled))
	}
	migrate := opts.force
	if !migrate {
		var err error
		migrate, err = m.confirm(&tofu.InputOpts{
			Id:          "backend-migrate-state-single-to-cloud-single",
			Query:       "Do you wish to proceed?",
			Description: strings.TrimSpace(tfcInputBackendMigrateStateSingleToCloudSingle),
		})
		if err != nil {
			return false, fmt.Errorf("Error asking for state migration action: %w", err)
		}
	}

	return migrate, nil
}

func (m *Meta) promptRemotePrefixToCloudTagsMigration(opts *backendMigrateOpts) error {
	if !m.input {
		log.Print("[TRACE] backendMigrateState: can't prompt for input, so aborting migration")
		return errors.New(strings.TrimSpace(errInteractiveInputDisabled))
	}
	migrate := opts.force
	if !migrate {
		var err error
		migrate, err = m.confirm(&tofu.InputOpts{
			Id:          "backend-migrate-remote-multistate-to-cloud",
			Query:       "Do you wish to proceed?",
			Description: strings.TrimSpace(tfcInputBackendMigrateRemoteMultiToCloud),
		})
		if err != nil {
			return fmt.Errorf("Error asking for state migration action: %w", err)
		}
	}

	if !migrate {
		return fmt.Errorf("Migration aborted by user.")
	}

	return nil
}

// Multi-state to single state.
func (m *Meta) promptMultiToSingleCloudMigration(opts *backendMigrateOpts) error {
	if !m.input {
		log.Print("[TRACE] backendMigrateState: can't prompt for input, so aborting migration")
		return errors.New(strings.TrimSpace(errInteractiveInputDisabled))
	}
	migrate := opts.force
	if !migrate {
		var err error
		// Ask the user if they want to migrate their existing remote state
		migrate, err = m.confirm(&tofu.InputOpts{
			Id:    "backend-migrate-multistate-to-single",
			Query: "Do you want to copy only your current workspace?",
			Description: fmt.Sprintf(
				strings.TrimSpace(tfcInputBackendMigrateMultiToSingle),
				opts.SourceType, opts.destinationWorkspace),
		})
		if err != nil {
			return fmt.Errorf("Error asking for state migration action: %w", err)
		}
	}

	if !migrate {
		return fmt.Errorf("Migration aborted by user.")
	}

	return nil
}

func (m *Meta) promptNewWorkspaceName(destinationType string) (string, error) {
	message := fmt.Sprintf("[reset][bold][yellow]The %q backend configuration only allows "+
		"named workspaces![reset]", destinationType)
	if destinationType == "cloud" {
		if !m.input {
			log.Print("[TRACE] backendMigrateState: can't prompt for input, so aborting migration")
			return "", errors.New(strings.TrimSpace(errInteractiveInputDisabled))
		}
		message = `[reset][bold][yellow]Cloud backend requires all workspaces to be given an explicit name.[reset]`
	}
	name, err := m.UIInput().Input(context.Background(), &tofu.InputOpts{
		Id:          "new-state-name",
		Query:       message,
		Description: strings.TrimSpace(inputBackendNewWorkspaceName),
	})
	if err != nil {
		return "", fmt.Errorf("Error asking for new state name: %w", err)
	}

	return name, nil
}

func (m *Meta) promptMultiStateMigrationPattern(sourceType string) (string, error) {
	// This is not the first prompt a user would be presented with in the migration to TFC, so no
	// guard on m.input is needed here.
	renameWorkspaces, err := m.UIInput().Input(context.Background(), &tofu.InputOpts{
		Id:          "backend-migrate-multistate-to-tfc",
		Query:       fmt.Sprintf("[reset][bold][yellow]%s[reset]", "Would you like to rename your workspaces?"),
		Description: fmt.Sprintf(strings.TrimSpace(tfcInputBackendMigrateMultiToMulti), sourceType),
	})
	if err != nil {
		return "", fmt.Errorf("Error asking for state migration action: %w", err)
	}
	if renameWorkspaces != "2" && renameWorkspaces != "1" {
		return "", fmt.Errorf("Please select 1 or 2 as part of this option.")
	}
	if renameWorkspaces == "2" {
		// this means they did not want to rename their workspaces, and we are
		// returning a generic '*' that means use the same workspace name during
		// migration.
		return "*", nil
	}

	pattern, err := m.UIInput().Input(context.Background(), &tofu.InputOpts{
		Id:          "backend-migrate-multistate-to-tfc-pattern",
		Query:       fmt.Sprintf("[reset][bold][yellow]%s[reset]", "How would you like to rename your workspaces?"),
		Description: strings.TrimSpace(tfcInputBackendMigrateMultiToMultiPattern),
	})
	if err != nil {
		return "", fmt.Errorf("Error asking for state migration action: %w", err)
	}
	if !strings.Contains(pattern, "*") {
		return "", fmt.Errorf("The pattern must have an '*'")
	}

	if count := strings.Count(pattern, "*"); count > 1 {
		return "", fmt.Errorf("The pattern '*' cannot be used more than once.")
	}

	return pattern, nil
}

const errMigrateLoadStates = `
Error inspecting states in the %q backend:
    %w

Prior to changing backends, OpenTofu inspects the source and destination
states to determine what kind of migration steps need to be taken, if any.
OpenTofu failed to load the states. The data in both the source and the
destination remain unmodified. Please resolve the above error and try again.
`

const errMigrateSingleLoadDefault = `
Error loading default state from the %q backend:
    %w

State migration cannot occur unless the state can be loaded. Backend
modification and state migration has been aborted. The state in both the
source and the destination remain unmodified. Please resolve the
above error and try again.
`

const errMigrateMulti = `
Error migrating the workspace %q from the previous %q backend
to the newly configured %q backend:
    %w

OpenTofu copies workspaces in alphabetical order. Any workspaces
alphabetically earlier than this one have been copied. Any workspaces
later than this haven't been modified in the destination. No workspaces
in the source state have been modified.

Please resolve the error above and run the initialization command again.
This will attempt to copy (with permission) all workspaces again.
`

const errBackendStateCopy = `
Error copying state from the previous %q backend to the newly configured
%q backend:
    %w

The state in the previous backend remains intact and unmodified. Please resolve
the error above and try again.
`

const errTFCMigrateNotYetImplemented = `
Migrating state from cloud backend to another backend is not yet implemented.
`

const errInteractiveInputDisabled = `
Can't ask approval for state migration when interactive input is disabled.

If non-interactive operation is desired, consider adding the "-force-copy"
option. Otherwise, please remove the "-input=false" option and try again.
`

const tfcInputBackendMigrateMultiToMultiPattern = `
Enter a pattern with an asterisk (*) to rename all workspaces based on their
previous names. The asterisk represents the current workspace name.

For example, if a workspace is currently named 'prod', the pattern 'app-*' would yield
'app-prod' for a new workspace name; 'app-*-region1' would  yield 'app-prod-region1'.
`

const tfcInputBackendMigrateMultiToMulti = `
Unlike typical OpenTofu workspaces representing an environment associated with a particular
configuration (e.g. production, staging, development), cloud backend workspaces are named uniquely
across all configurations used within an organization. A typical strategy to start with is
<COMPONENT>-<ENVIRONMENT>-<REGION> (e.g. networking-prod-us-east, networking-staging-us-east).

When migrating existing workspaces from the backend %[1]q to cloud backend, would you like to
rename your workspaces? Enter 1 or 2.

1. Yes, I'd like to rename all workspaces according to a pattern I will provide.
2. No, I would not like to rename my workspaces. Migrate them as currently named.
`

const tfcInputBackendMigrateMultiToSingle = `
The previous backend %[1]q has multiple workspaces, but cloud backend has
been configured to use a single workspace (%[2]q). By continuing, you will
only migrate your current workspace. If you wish to migrate all workspaces
from the previous backend, you may cancel this operation and use the 'tags'
strategy in your workspace configuration block instead.

Enter "yes" to proceed or "no" to cancel.
`

const tfcInputBackendMigrateStateSingleToCloudSingle = `
As part of migrating to cloud backend, OpenTofu can optionally copy your
current workspace state to the configured cloud backend workspace.

Answer "yes" to copy the latest state snapshot to the configured
cloud backend workspace.

Answer "no" to ignore the existing state and just activate the configured
cloud backend workspace with its existing state, if any.

Should OpenTofu migrate your existing state?
`

const tfcInputBackendMigrateRemoteMultiToCloud = `
When migrating from the 'remote' backend to OpenTofu's native integration
with cloud backend, OpenTofu will automatically create or use existing
workspaces based on the previous backend configuration's 'prefix' value.

When the migration is complete, workspace names in OpenTofu will match the
fully qualified cloud backend workspace name. If necessary, the workspace
tags configured in the 'cloud' option block will be added to the associated
cloud backend workspaces.

Enter "yes" to proceed or "no" to cancel.
`

const inputBackendMigrateEmpty = `
Pre-existing state was found while migrating the previous %q backend to the
newly configured %q backend. No existing state was found in the newly
configured %[2]q backend. Do you want to copy this state to the new %[2]q
backend? Enter "yes" to copy and "no" to start with an empty state.
`

const inputBackendMigrateEmptyCloud = `
Pre-existing state was found while migrating the previous %q backend to cloud backend.
No existing state was found in cloud backend. Do you want to copy this state to cloud backend?
Enter "yes" to copy and "no" to start with an empty state.
`

const inputBackendMigrateNonEmpty = `
Pre-existing state was found while migrating the previous %q backend to the
newly configured %q backend. An existing non-empty state already exists in
the new backend. The two states have been saved to temporary files that will be
removed after responding to this query.

Previous (type %[1]q): %[3]s
New      (type %[2]q): %[4]s

Do you want to overwrite the state in the new backend with the previous state?
Enter "yes" to copy and "no" to start with the existing state in the newly
configured %[2]q backend.
`

const inputBackendMigrateNonEmptyCloud = `
Pre-existing state was found while migrating the previous %q backend to
cloud backend. An existing non-empty state already exists in cloud backend.
The two states have been saved to temporary files that will be removed after
responding to this query.

Previous (type %[1]q): %[2]s
New      (cloud backend): %[3]s

Do you want to overwrite the state in cloud backend with the previous state?
Enter "yes" to copy and "no" to start with the existing state in cloud backend.
`

const inputBackendMigrateMultiToSingle = `
The existing %[1]q backend supports workspaces and you currently are
using more than one. The newly configured %[2]q backend doesn't support
workspaces. If you continue, OpenTofu will copy your current workspace %[3]q
to the default workspace in the new backend. Your existing workspaces in the
source backend won't be modified. If you want to switch workspaces, back them
up, or cancel altogether, answer "no" and OpenTofu will abort.
`

const inputBackendMigrateMultiToMulti = `
Both the existing %[1]q backend and the newly configured %[2]q backend
support workspaces. When migrating between backends, OpenTofu will copy
all workspaces (with the same names). THIS WILL OVERWRITE any conflicting
states in the destination.

OpenTofu initialization doesn't currently migrate only select workspaces.
If you want to migrate a select number of workspaces, you must manually
pull and push those states.

If you answer "yes", OpenTofu will migrate all states. If you answer
"no", OpenTofu will abort.
`

const inputBackendNewWorkspaceName = `
Please provide a new workspace name (e.g. dev, test) that will be used
to migrate the existing default workspace.
`

const inputBackendSelectWorkspace = `
This is expected behavior when the selected workspace did not have an
existing non-empty state. Please enter a number to select a workspace:

%s
`
