Merge pull request #12347 from hashicorp/b-env-migrate

command: migrating envs when changing backends

backend/backend.go

@@ -13,9 +13,13 @@ import (
 	"github.com/hashicorp/terraform/terraform"
 )
 
+// This is the name of the default, initial state that every backend
+// must have. This state cannot be deleted.
 const DefaultStateName = "default"
 
-// Error value to return when a named state operation isn't supported
+// Error value to return when a named state operation isn't supported.
+// This must be returned rather than a custom error so that the Terraform
+// CLI can detect it and handle it appropriately.
 var ErrNamedStatesNotSupported = errors.New("named states not supported")
 
 // Backend is the minimal interface that must be implemented to enable Terraform.

backend/local/backend.go

@@ -47,9 +47,13 @@ type Local struct {
 	//
 	// StateBackupPath is the local path where a backup file will be written.
 	// Set this to "-" to disable state backup.
+	//
+	// StateEnvPath is the path to the folder containing environments. This
+	// defaults to DefaultEnvDir if not set.
 	StatePath       string
 	StateOutPath    string
 	StateBackupPath string
+	StateEnvDir     string
 
 	// We only want to create a single instance of a local state, so store them
 	// here as they're loaded.
@@ -266,6 +270,13 @@ func (b *Local) init() {
 			"path": &schema.Schema{
 				Type:     schema.TypeString,
 				Optional: true,
+				Default:  "",
+			},
+
+			"environment_dir": &schema.Schema{
+				Type:     schema.TypeString,
+				Optional: true,
+				Default:  "",
 			},
 		},
 
@@ -288,6 +299,13 @@ func (b *Local) schemaConfigure(ctx context.Context) error {
 		b.StateOutPath = path
 	}
 
+	if raw, ok := d.GetOk("environment_dir"); ok {
+		path := raw.(string)
+		if path != "" {
+			b.StateEnvDir = path
+		}
+	}
+
 	return nil
 }
 
@@ -302,12 +320,17 @@ func (b *Local) StatePaths(name string) (string, string, string) {
 		name = backend.DefaultStateName
 	}
 
+	envDir := DefaultEnvDir
+	if b.StateEnvDir != "" {
+		envDir = b.StateEnvDir
+	}
+
 	if name == backend.DefaultStateName {
 		if statePath == "" {
 			statePath = DefaultStateFilename
 		}
 	} else {
-		statePath = filepath.Join(DefaultEnvDir, name, DefaultStateFilename)
+		statePath = filepath.Join(envDir, name, DefaultStateFilename)
 	}
 
 	if stateOutPath == "" {
@@ -330,7 +353,12 @@ func (b *Local) createState(name string) error {
 		return nil
 	}
 
-	stateDir := filepath.Join(DefaultEnvDir, name)
+	envDir := DefaultEnvDir
+	if b.StateEnvDir != "" {
+		envDir = b.StateEnvDir
+	}
+
+	stateDir := filepath.Join(envDir, name)
 	s, err := os.Stat(stateDir)
 	if err == nil && s.IsDir() {
 		// no need to check for os.IsNotExist, since that is covered by os.MkdirAll

backend/local/testing.go

@@ -5,6 +5,8 @@ import (
 	"path/filepath"
 	"testing"
 
+	"github.com/hashicorp/terraform/backend"
+	"github.com/hashicorp/terraform/state"
 	"github.com/hashicorp/terraform/terraform"
 )
 
@@ -56,6 +58,38 @@ func TestLocalProvider(t *testing.T, b *Local, name string) *terraform.MockResou
 	return p
 }
 
+// TestNewLocalSingle is a factory for creating a TestLocalSingleState.
+// This function matches the signature required for backend/init.
+func TestNewLocalSingle() backend.Backend {
+	return &TestLocalSingleState{}
+}
+
+// TestLocalSingleState is a backend implementation that wraps Local
+// and modifies it to only support single states (returns
+// ErrNamedStatesNotSupported for multi-state operations).
+//
+// This isn't an actual use case, this is exported just to provide a
+// easy way to test that behavior.
+type TestLocalSingleState struct {
+	Local
+}
+
+func (b *TestLocalSingleState) State(name string) (state.State, error) {
+	if name != backend.DefaultStateName {
+		return nil, backend.ErrNamedStatesNotSupported
+	}
+
+	return b.Local.State(name)
+}
+
+func (b *TestLocalSingleState) States() ([]string, error) {
+	return nil, backend.ErrNamedStatesNotSupported
+}
+
+func (b *TestLocalSingleState) DeleteState(string) error {
+	return backend.ErrNamedStatesNotSupported
+}
+
 func testTempDir(t *testing.T) string {
 	d, err := ioutil.TempDir("", "tf")
 	if err != nil {

command/command_test.go

@@ -449,6 +449,28 @@ func testInteractiveInput(t *testing.T, answers []string) func() {
 	}
 }
 
+// testInputMap configures tests so that the given answers are returned
+// for calls to Input when the right question is asked. The key is the
+// question "Id" that is used.
+func testInputMap(t *testing.T, answers map[string]string) func() {
+	// Disable test mode so input is called
+	test = false
+
+	// Setup reader/writers
+	defaultInputReader = bytes.NewBufferString("")
+	defaultInputWriter = new(bytes.Buffer)
+
+	// Setup answers
+	testInputResponse = nil
+	testInputResponseMap = answers
+
+	// Return the cleanup
+	return func() {
+		test = true
+		testInputResponseMap = nil
+	}
+}
+
 // testBackendState is used to make a test HTTP server to test a configured
 // backend. This returns the complete state that can be saved. Use
 // `testStateFileRemote` to write the returned state.

command/meta_backend.go

@@ -646,38 +646,19 @@ func (m *Meta) backend_c_r_S(
 			return nil, fmt.Errorf(strings.TrimSpace(errBackendLocalRead), err)
 		}
 
-		env := m.Env()
-
-		localState, err := localB.State(env)
-		if err != nil {
-			return nil, fmt.Errorf(strings.TrimSpace(errBackendLocalRead), err)
-		}
-		if err := localState.RefreshState(); err != nil {
-			return nil, fmt.Errorf(strings.TrimSpace(errBackendLocalRead), err)
-		}
-
 		// Initialize the configured backend
 		b, err := m.backend_C_r_S_unchanged(c, sMgr)
 		if err != nil {
 			return nil, fmt.Errorf(
 				strings.TrimSpace(errBackendSavedUnsetConfig), s.Backend.Type, err)
 		}
-		backendState, err := b.State(env)
-		if err != nil {
-			return nil, fmt.Errorf(
-				strings.TrimSpace(errBackendSavedUnsetConfig), s.Backend.Type, err)
-		}
-		if err := backendState.RefreshState(); err != nil {
-			return nil, fmt.Errorf(
-				strings.TrimSpace(errBackendSavedUnsetConfig), s.Backend.Type, err)
-		}
 
 		// Perform the migration
 		err = m.backendMigrateState(&backendMigrateOpts{
 			OneType: s.Backend.Type,
 			TwoType: "local",
-			One:     backendState,
-			Two:     localState,
+			One:     b,
+			Two:     localB,
 		})
 		if err != nil {
 			return nil, err
@@ -762,16 +743,6 @@ func (m *Meta) backend_c_R_S(
 		return nil, fmt.Errorf(errBackendLocalRead, err)
 	}
 
-	env := m.Env()
-
-	localState, err := localB.State(env)
-	if err != nil {
-		return nil, fmt.Errorf(errBackendLocalRead, err)
-	}
-	if err := localState.RefreshState(); err != nil {
-		return nil, fmt.Errorf(errBackendLocalRead, err)
-	}
-
 	// Grab the state
 	s := sMgr.State()
 
@@ -795,22 +766,13 @@ func (m *Meta) backend_c_R_S(
 		if err != nil {
 			return nil, err
 		}
-		oldState, err := oldB.State(env)
-		if err != nil {
-			return nil, fmt.Errorf(
-				strings.TrimSpace(errBackendSavedUnsetConfig), s.Backend.Type, err)
-		}
-		if err := oldState.RefreshState(); err != nil {
-			return nil, fmt.Errorf(
-				strings.TrimSpace(errBackendSavedUnsetConfig), s.Backend.Type, err)
-		}
 
 		// Perform the migration
 		err = m.backendMigrateState(&backendMigrateOpts{
 			OneType: s.Remote.Type,
 			TwoType: "local",
-			One:     oldState,
-			Two:     localState,
+			One:     oldB,
+			Two:     localB,
 		})
 		if err != nil {
 			return nil, err
@@ -898,33 +860,12 @@ func (m *Meta) backend_C_R_s(
 			return nil, err
 		}
 
-		env := m.Env()
-
-		oldState, err := oldB.State(env)
-		if err != nil {
-			return nil, fmt.Errorf(
-				strings.TrimSpace(errBackendSavedUnsetConfig), s.Backend.Type, err)
-		}
-		if err := oldState.RefreshState(); err != nil {
-			return nil, fmt.Errorf(
-				strings.TrimSpace(errBackendSavedUnsetConfig), s.Backend.Type, err)
-		}
-
-		// Get the new state
-		newState, err := b.State(env)
-		if err != nil {
-			return nil, fmt.Errorf(strings.TrimSpace(errBackendNewRead), err)
-		}
-		if err := newState.RefreshState(); err != nil {
-			return nil, fmt.Errorf(strings.TrimSpace(errBackendNewRead), err)
-		}
-
 		// Perform the migration
 		err = m.backendMigrateState(&backendMigrateOpts{
 			OneType: s.Remote.Type,
 			TwoType: c.Type,
-			One:     oldState,
-			Two:     newState,
+			One:     oldB,
+			Two:     b,
 		})
 		if err != nil {
 			return nil, err
@@ -979,20 +920,12 @@ func (m *Meta) backend_C_r_s(
 	// If the local state is not empty, we need to potentially do a
 	// state migration to the new backend (with user permission).
 	if localS := localState.State(); !localS.Empty() {
-		backendState, err := b.State(env)
-		if err != nil {
-			return nil, fmt.Errorf(errBackendRemoteRead, err)
-		}
-		if err := backendState.RefreshState(); err != nil {
-			return nil, fmt.Errorf(errBackendRemoteRead, err)
-		}
-
 		// Perform the migration
 		err = m.backendMigrateState(&backendMigrateOpts{
 			OneType: "local",
 			TwoType: c.Type,
-			One:     localState,
-			Two:     backendState,
+			One:     localB,
+			Two:     b,
 		})
 		if err != nil {
 			return nil, err
@@ -1084,33 +1017,12 @@ func (m *Meta) backend_C_r_S_changed(
 				"Error loading previously configured backend: %s", err)
 		}
 
-		env := m.Env()
-
-		oldState, err := oldB.State(env)
-		if err != nil {
-			return nil, fmt.Errorf(
-				strings.TrimSpace(errBackendSavedUnsetConfig), s.Backend.Type, err)
-		}
-		if err := oldState.RefreshState(); err != nil {
-			return nil, fmt.Errorf(
-				strings.TrimSpace(errBackendSavedUnsetConfig), s.Backend.Type, err)
-		}
-
-		// Get the new state
-		newState, err := b.State(env)
-		if err != nil {
-			return nil, fmt.Errorf(strings.TrimSpace(errBackendNewRead), err)
-		}
-		if err := newState.RefreshState(); err != nil {
-			return nil, fmt.Errorf(strings.TrimSpace(errBackendNewRead), err)
-		}
-
 		// Perform the migration
 		err = m.backendMigrateState(&backendMigrateOpts{
 			OneType: s.Backend.Type,
 			TwoType: c.Type,
-			One:     oldState,
-			Two:     newState,
+			One:     oldB,
+			Two:     b,
 		})
 		if err != nil {
 			return nil, err
@@ -1248,33 +1160,12 @@ func (m *Meta) backend_C_R_S_unchanged(
 			return nil, err
 		}
 
-		env := m.Env()
-
-		oldState, err := oldB.State(env)
-		if err != nil {
-			return nil, fmt.Errorf(
-				strings.TrimSpace(errBackendSavedUnsetConfig), s.Remote.Type, err)
-		}
-		if err := oldState.RefreshState(); err != nil {
-			return nil, fmt.Errorf(
-				strings.TrimSpace(errBackendSavedUnsetConfig), s.Remote.Type, err)
-		}
-
-		// Get the new state
-		newState, err := b.State(env)
-		if err != nil {
-			return nil, fmt.Errorf(strings.TrimSpace(errBackendNewRead), err)
-		}
-		if err := newState.RefreshState(); err != nil {
-			return nil, fmt.Errorf(strings.TrimSpace(errBackendNewRead), err)
-		}
-
 		// Perform the migration
 		err = m.backendMigrateState(&backendMigrateOpts{
 			OneType: s.Remote.Type,
 			TwoType: s.Backend.Type,
-			One:     oldState,
-			Two:     newState,
+			One:     oldB,
+			Two:     b,
 		})
 		if err != nil {
 			return nil, err

command/meta_backend_migrate.go

@@ -5,8 +5,10 @@ import (
 	"io/ioutil"
 	"os"
 	"path/filepath"
+	"sort"
 	"strings"
 
+	"github.com/hashicorp/terraform/backend"
 	clistate "github.com/hashicorp/terraform/command/state"
 	"github.com/hashicorp/terraform/state"
 	"github.com/hashicorp/terraform/terraform"
@@ -24,30 +26,212 @@ import (
 //
 // This will attempt to lock both states for the migration.
 func (m *Meta) backendMigrateState(opts *backendMigrateOpts) error {
+	// 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 oneSingle, twoSingle bool
+	oneStates, err := opts.One.States()
+	if err == backend.ErrNamedStatesNotSupported {
+		oneSingle = true
+		err = nil
+	}
+	if err != nil {
+		return fmt.Errorf(strings.TrimSpace(
+			errMigrateLoadStates), opts.OneType, err)
+	}
+
+	_, err = opts.Two.States()
+	if err == backend.ErrNamedStatesNotSupported {
+		twoSingle = true
+		err = nil
+	}
+	if err != nil {
+		return fmt.Errorf(strings.TrimSpace(
+			errMigrateLoadStates), opts.TwoType, err)
+	}
+
+	// Setup defaults
+	opts.oneEnv = backend.DefaultStateName
+	opts.twoEnv = backend.DefaultStateName
+	opts.force = false
+
+	// Determine migration behavior based on whether the source/destionation
+	// supports multi-state.
+	switch {
+	// Single-state to single-state. This is the easiest case: we just
+	// copy the default state directly.
+	case oneSingle && twoSingle:
+		return m.backendMigrateState_s_s(opts)
+
+	// Single-state to multi-state. This is easy since we just copy
+	// the default state and ignore the rest in the destination.
+	case oneSingle && !twoSingle:
+		return m.backendMigrateState_s_s(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 !oneSingle && twoSingle:
+		// If the source only has one state and it is the default,
+		// treat it as if it doesn't support multi-state.
+		if len(oneStates) == 1 && oneStates[0] == backend.DefaultStateName {
+			return m.backendMigrateState_s_s(opts)
+		}
+
+		return m.backendMigrateState_S_s(opts)
+
+	// Multi-state to multi-state. We merge the states together (migrating
+	// each from the source to the destination one by one).
+	case !oneSingle && !twoSingle:
+		// If the source only has one state and it is the default,
+		// treat it as if it doesn't support multi-state.
+		if len(oneStates) == 1 && oneStates[0] == backend.DefaultStateName {
+			return m.backendMigrateState_s_s(opts)
+		}
+
+		return m.backendMigrateState_S_S(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(opts *backendMigrateOpts) error {
+	// Ask the user if they want to migrate their existing remote state
+	migrate, err := m.confirm(&terraform.InputOpts{
+		Id: "backend-migrate-multistate-to-multistate",
+		Query: fmt.Sprintf(
+			"Do you want to migrate all environments to %q?",
+			opts.TwoType),
+		Description: fmt.Sprintf(
+			strings.TrimSpace(inputBackendMigrateMultiToMulti),
+			opts.OneType, opts.TwoType),
+	})
+	if err != nil {
+		return fmt.Errorf(
+			"Error asking for state migration action: %s", err)
+	}
+	if !migrate {
+		return fmt.Errorf("Migration aborted by user.")
+	}
+
+	// Read all the states
+	oneStates, err := opts.One.States()
+	if err != nil {
+		return fmt.Errorf(strings.TrimSpace(
+			errMigrateLoadStates), opts.OneType, err)
+	}
+
+	// Sort the states so they're always copied alphabetically
+	sort.Strings(oneStates)
+
+	// Go through each and migrate
+	for _, name := range oneStates {
+		// Copy the same names
+		opts.oneEnv = name
+		opts.twoEnv = name
+
+		// Force it, we confirmed above
+		opts.force = true
+
+		// Perform the migration
+		if err := m.backendMigrateState_s_s(opts); err != nil {
+			return fmt.Errorf(strings.TrimSpace(
+				errMigrateMulti), name, opts.OneType, opts.TwoType, err)
+		}
+	}
+
+	return nil
+}
+
+// Multi-state to single state.
+func (m *Meta) backendMigrateState_S_s(opts *backendMigrateOpts) error {
+	currentEnv := m.Env()
+
+	// Ask the user if they want to migrate their existing remote state
+	migrate, err := m.confirm(&terraform.InputOpts{
+		Id: "backend-migrate-multistate-to-single",
+		Query: fmt.Sprintf(
+			"Destination state %q doesn't support environments (named states).\n"+
+				"Do you want to copy only your current environment?",
+			opts.TwoType),
+		Description: fmt.Sprintf(
+			strings.TrimSpace(inputBackendMigrateMultiToSingle),
+			opts.OneType, opts.TwoType, currentEnv),
+	})
+	if err != nil {
+		return fmt.Errorf(
+			"Error asking for state migration action: %s", err)
+	}
+	if !migrate {
+		return fmt.Errorf("Migration aborted by user.")
+	}
+
+	// Copy the default state
+	opts.oneEnv = currentEnv
+	return m.backendMigrateState_s_s(opts)
+}
+
+// Single state to single state, assumed default state name.
+func (m *Meta) backendMigrateState_s_s(opts *backendMigrateOpts) error {
+	stateOne, err := opts.One.State(opts.oneEnv)
+	if err != nil {
+		return fmt.Errorf(strings.TrimSpace(
+			errMigrateSingleLoadDefault), opts.OneType, err)
+	}
+	if err := stateOne.RefreshState(); err != nil {
+		return fmt.Errorf(strings.TrimSpace(
+			errMigrateSingleLoadDefault), opts.OneType, err)
+	}
+
+	stateTwo, err := opts.Two.State(opts.twoEnv)
+	if err != nil {
+		return fmt.Errorf(strings.TrimSpace(
+			errMigrateSingleLoadDefault), opts.TwoType, err)
+	}
+	if err := stateTwo.RefreshState(); err != nil {
+		return fmt.Errorf(strings.TrimSpace(
+			errMigrateSingleLoadDefault), opts.TwoType, err)
+	}
+
 	lockInfoOne := state.NewLockInfo()
 	lockInfoOne.Operation = "migration"
 	lockInfoOne.Info = "source state"
 
-	lockIDOne, err := clistate.Lock(opts.One, lockInfoOne, m.Ui, m.Colorize())
+	lockIDOne, err := clistate.Lock(stateOne, lockInfoOne, m.Ui, m.Colorize())
 	if err != nil {
 		return fmt.Errorf("Error locking source state: %s", err)
 	}
-	defer clistate.Unlock(opts.One, lockIDOne, m.Ui, m.Colorize())
+	defer clistate.Unlock(stateOne, lockIDOne, m.Ui, m.Colorize())
 
 	lockInfoTwo := state.NewLockInfo()
 	lockInfoTwo.Operation = "migration"
 	lockInfoTwo.Info = "destination state"
 
-	lockIDTwo, err := clistate.Lock(opts.Two, lockInfoTwo, m.Ui, m.Colorize())
+	lockIDTwo, err := clistate.Lock(stateTwo, lockInfoTwo, m.Ui, m.Colorize())
 	if err != nil {
 		return fmt.Errorf("Error locking destination state: %s", err)
 	}
-	defer clistate.Unlock(opts.Two, lockIDTwo, m.Ui, m.Colorize())
+	defer clistate.Unlock(stateTwo, lockIDTwo, m.Ui, m.Colorize())
 
-	one := opts.One.State()
-	two := opts.Two.State()
+	one := stateOne.State()
+	two := stateTwo.State()
 
-	var confirmFunc func(opts *backendMigrateOpts) (bool, error)
+	var confirmFunc func(state.State, state.State, *backendMigrateOpts) (bool, error)
 	switch {
 	// No migration necessary
 	case one.Empty() && two.Empty():
@@ -72,21 +256,23 @@ func (m *Meta) backendMigrateState(opts *backendMigrateOpts) error {
 		panic("confirmFunc must not be nil")
 	}
 
+	if !opts.force {
 		// Confirm with the user whether we want to copy state over
-	confirm, err := confirmFunc(opts)
+		confirm, err := confirmFunc(stateOne, stateTwo, opts)
 		if err != nil {
 			return err
 		}
 		if !confirm {
 			return nil
 		}
+	}
 
 	// Confirmed! Write.
-	if err := opts.Two.WriteState(one); err != nil {
+	if err := stateTwo.WriteState(one); err != nil {
 		return fmt.Errorf(strings.TrimSpace(errBackendStateCopy),
 			opts.OneType, opts.TwoType, err)
 	}
-	if err := opts.Two.PersistState(); err != nil {
+	if err := stateTwo.PersistState(); err != nil {
 		return fmt.Errorf(strings.TrimSpace(errBackendStateCopy),
 			opts.OneType, opts.TwoType, err)
 	}
@@ -95,9 +281,9 @@ func (m *Meta) backendMigrateState(opts *backendMigrateOpts) error {
 	return nil
 }
 
-func (m *Meta) backendMigrateEmptyConfirm(opts *backendMigrateOpts) (bool, error) {
+func (m *Meta) backendMigrateEmptyConfirm(one, two state.State, opts *backendMigrateOpts) (bool, error) {
 	inputOpts := &terraform.InputOpts{
-		Id: "backend-migrate-to-backend",
+		Id: "backend-migrate-copy-to-empty",
 		Query: fmt.Sprintf(
 			"Do you want to copy state from %q to %q?",
 			opts.OneType, opts.TwoType),
@@ -124,10 +310,11 @@ func (m *Meta) backendMigrateEmptyConfirm(opts *backendMigrateOpts) (bool, error
 	}
 }
 
-func (m *Meta) backendMigrateNonEmptyConfirm(opts *backendMigrateOpts) (bool, error) {
+func (m *Meta) backendMigrateNonEmptyConfirm(
+	stateOne, stateTwo state.State, opts *backendMigrateOpts) (bool, error) {
 	// We need to grab both states so we can write them to a file
-	one := opts.One.State()
-	two := opts.Two.State()
+	one := stateOne.State()
+	two := stateTwo.State()
 
 	// Save both to a temporary
 	td, err := ioutil.TempDir("", "terraform")
@@ -188,9 +375,48 @@ func (m *Meta) backendMigrateNonEmptyConfirm(opts *backendMigrateOpts) (bool, er
 
 type backendMigrateOpts struct {
 	OneType, TwoType string
-	One, Two         state.State
+	One, Two         backend.Backend
+
+	// Fields below are set internally when migrate is called
+
+	oneEnv string // source env
+	twoEnv string // dest env
+	force  bool   // if true, won't ask for confirmation
 }
 
+const errMigrateLoadStates = `
+Error inspecting state in %q: %s
+
+Prior to changing backends, Terraform inspects the source and destionation
+states to determine what kind of migration steps need to be taken, if any.
+Terraform 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 state from %q: %s
+
+Terraform failed to load the default state from %[1]q.
+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 environment %q from %q to %q:
+
+%s
+
+Terraform copies environments in alphabetical order. Any environments
+alphabetically earlier than this one have been copied. Any environments
+later than this haven't been modified in the destination. No environments
+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 environments again.
+`
+
 const errBackendStateCopy = `
 Error copying state from %q to %q: %s
 
@@ -215,3 +441,26 @@ Two (%[2]q): %[4]s
 Do you want to copy the state from %[1]q to %[2]q? Enter "yes" to copy
 and "no" to start with the existing state in %[2]q.
 `
+
+const inputBackendMigrateMultiToSingle = `
+The existing backend %[1]q supports environments and you currently are
+using more than one. The target backend %[2]q doesn't support environments.
+If you continue, Terraform will offer to copy your current environment
+%[3]q to the default environment in the target. Your existing environments
+in the source backend won't be modified. If you want to switch environments,
+back them up, or cancel altogether, answer "no" and Terraform will abort.
+`
+
+const inputBackendMigrateMultiToMulti = `
+Both the existing backend %[1]q and the target backend %[2]q support
+environments. When migrating between backends, Terraform will copy all
+environments (with the same names). THIS WILL OVERWRITE any conflicting
+states in the destination.
+
+Terraform initialization doesn't currently migrate only select environments.
+If you want to migrate a select number of environments, you must manually
+pull and push those states.
+
+If you answer "yes", Terraform will migrate all states. If you answer
+"no", Terraform will abort.
+`

command/meta_backend_test.go

@@ -4,10 +4,14 @@ import (
 	"io/ioutil"
 	"os"
 	"path/filepath"
+	"reflect"
+	"sort"
 	"strings"
 	"testing"
 
 	"github.com/hashicorp/terraform/backend"
+	backendinit "github.com/hashicorp/terraform/backend/init"
+	backendlocal "github.com/hashicorp/terraform/backend/local"
 	"github.com/hashicorp/terraform/helper/copy"
 	"github.com/hashicorp/terraform/state"
 	"github.com/hashicorp/terraform/terraform"
@@ -951,6 +955,340 @@ func TestMetaBackend_configuredChangeCopy(t *testing.T) {
 	}
 }
 
+// Changing a configured backend that supports only single states to another
+// backend that only supports single states.
+func TestMetaBackend_configuredChangeCopy_singleState(t *testing.T) {
+	// Create a temporary working directory that is empty
+	td := tempDir(t)
+	copy.CopyDir(testFixturePath("backend-change-single-to-single"), td)
+	defer os.RemoveAll(td)
+	defer testChdir(t, td)()
+
+	// Register the single-state backend
+	backendinit.Set("local-single", backendlocal.TestNewLocalSingle)
+	defer backendinit.Set("local-single", nil)
+
+	// Ask input
+	defer testInputMap(t, map[string]string{
+		"backend-migrate-to-new":        "yes",
+		"backend-migrate-copy-to-empty": "yes",
+	})()
+
+	// Setup the meta
+	m := testMetaBackend(t, nil)
+
+	// Get the backend
+	b, err := m.Backend(&BackendOpts{Init: true})
+	if err != nil {
+		t.Fatalf("bad: %s", err)
+	}
+
+	// Check the state
+	s, err := b.State(backend.DefaultStateName)
+	if err != nil {
+		t.Fatalf("bad: %s", err)
+	}
+	if err := s.RefreshState(); err != nil {
+		t.Fatalf("bad: %s", err)
+	}
+	state := s.State()
+	if state == nil {
+		t.Fatal("state should not be nil")
+	}
+	if state.Lineage != "backend-change" {
+		t.Fatalf("bad: %#v", state)
+	}
+
+	// Verify no local state
+	if _, err := os.Stat(DefaultStateFilename); err == nil {
+		t.Fatal("file should not exist")
+	}
+
+	// Verify no local backup
+	if _, err := os.Stat(DefaultStateFilename + DefaultBackupExtension); err == nil {
+		t.Fatal("file should not exist")
+	}
+}
+
+// Changing a configured backend that supports multi-state to a
+// backend that only supports single states. The multi-state only has
+// a default state.
+func TestMetaBackend_configuredChangeCopy_multiToSingleDefault(t *testing.T) {
+	// Create a temporary working directory that is empty
+	td := tempDir(t)
+	copy.CopyDir(testFixturePath("backend-change-multi-default-to-single"), td)
+	defer os.RemoveAll(td)
+	defer testChdir(t, td)()
+
+	// Register the single-state backend
+	backendinit.Set("local-single", backendlocal.TestNewLocalSingle)
+	defer backendinit.Set("local-single", nil)
+
+	// Ask input
+	defer testInputMap(t, map[string]string{
+		"backend-migrate-to-new":        "yes",
+		"backend-migrate-copy-to-empty": "yes",
+	})()
+
+	// Setup the meta
+	m := testMetaBackend(t, nil)
+
+	// Get the backend
+	b, err := m.Backend(&BackendOpts{Init: true})
+	if err != nil {
+		t.Fatalf("bad: %s", err)
+	}
+
+	// Check the state
+	s, err := b.State(backend.DefaultStateName)
+	if err != nil {
+		t.Fatalf("bad: %s", err)
+	}
+	if err := s.RefreshState(); err != nil {
+		t.Fatalf("bad: %s", err)
+	}
+	state := s.State()
+	if state == nil {
+		t.Fatal("state should not be nil")
+	}
+	if state.Lineage != "backend-change" {
+		t.Fatalf("bad: %#v", state)
+	}
+
+	// Verify no local state
+	if _, err := os.Stat(DefaultStateFilename); err == nil {
+		t.Fatal("file should not exist")
+	}
+
+	// Verify no local backup
+	if _, err := os.Stat(DefaultStateFilename + DefaultBackupExtension); err == nil {
+		t.Fatal("file should not exist")
+	}
+}
+
+// Changing a configured backend that supports multi-state to a
+// backend that only supports single states.
+func TestMetaBackend_configuredChangeCopy_multiToSingle(t *testing.T) {
+	// Create a temporary working directory that is empty
+	td := tempDir(t)
+	copy.CopyDir(testFixturePath("backend-change-multi-to-single"), td)
+	defer os.RemoveAll(td)
+	defer testChdir(t, td)()
+
+	// Register the single-state backend
+	backendinit.Set("local-single", backendlocal.TestNewLocalSingle)
+	defer backendinit.Set("local-single", nil)
+
+	// Ask input
+	defer testInputMap(t, map[string]string{
+		"backend-migrate-to-new":               "yes",
+		"backend-migrate-multistate-to-single": "yes",
+		"backend-migrate-copy-to-empty":        "yes",
+	})()
+
+	// Setup the meta
+	m := testMetaBackend(t, nil)
+
+	// Get the backend
+	b, err := m.Backend(&BackendOpts{Init: true})
+	if err != nil {
+		t.Fatalf("bad: %s", err)
+	}
+
+	// Check the state
+	s, err := b.State(backend.DefaultStateName)
+	if err != nil {
+		t.Fatalf("bad: %s", err)
+	}
+	if err := s.RefreshState(); err != nil {
+		t.Fatalf("bad: %s", err)
+	}
+	state := s.State()
+	if state == nil {
+		t.Fatal("state should not be nil")
+	}
+	if state.Lineage != "backend-change" {
+		t.Fatalf("bad: %#v", state)
+	}
+
+	// Verify no local state
+	if _, err := os.Stat(DefaultStateFilename); err == nil {
+		t.Fatal("file should not exist")
+	}
+
+	// Verify no local backup
+	if _, err := os.Stat(DefaultStateFilename + DefaultBackupExtension); err == nil {
+		t.Fatal("file should not exist")
+	}
+
+	// Verify existing environments exist
+	envPath := filepath.Join(backendlocal.DefaultEnvDir, "env2", backendlocal.DefaultStateFilename)
+	if _, err := os.Stat(envPath); err != nil {
+		t.Fatal("env should exist")
+	}
+}
+
+// Changing a configured backend that supports multi-state to a
+// backend that only supports single states.
+func TestMetaBackend_configuredChangeCopy_multiToSingleCurrentEnv(t *testing.T) {
+	// Create a temporary working directory that is empty
+	td := tempDir(t)
+	copy.CopyDir(testFixturePath("backend-change-multi-to-single"), td)
+	defer os.RemoveAll(td)
+	defer testChdir(t, td)()
+
+	// Register the single-state backend
+	backendinit.Set("local-single", backendlocal.TestNewLocalSingle)
+	defer backendinit.Set("local-single", nil)
+
+	// Ask input
+	defer testInputMap(t, map[string]string{
+		"backend-migrate-to-new":               "yes",
+		"backend-migrate-multistate-to-single": "yes",
+		"backend-migrate-copy-to-empty":        "yes",
+	})()
+
+	// Setup the meta
+	m := testMetaBackend(t, nil)
+
+	// Change env
+	if err := m.SetEnv("env2"); err != nil {
+		t.Fatalf("bad: %s", err)
+	}
+
+	// Get the backend
+	b, err := m.Backend(&BackendOpts{Init: true})
+	if err != nil {
+		t.Fatalf("bad: %s", err)
+	}
+
+	// Check the state
+	s, err := b.State(backend.DefaultStateName)
+	if err != nil {
+		t.Fatalf("bad: %s", err)
+	}
+	if err := s.RefreshState(); err != nil {
+		t.Fatalf("bad: %s", err)
+	}
+	state := s.State()
+	if state == nil {
+		t.Fatal("state should not be nil")
+	}
+	if state.Lineage != "backend-change-env2" {
+		t.Fatalf("bad: %#v", state)
+	}
+
+	// Verify no local state
+	if _, err := os.Stat(DefaultStateFilename); err == nil {
+		t.Fatal("file should not exist")
+	}
+
+	// Verify no local backup
+	if _, err := os.Stat(DefaultStateFilename + DefaultBackupExtension); err == nil {
+		t.Fatal("file should not exist")
+	}
+
+	// Verify existing environments exist
+	envPath := filepath.Join(backendlocal.DefaultEnvDir, "env2", backendlocal.DefaultStateFilename)
+	if _, err := os.Stat(envPath); err != nil {
+		t.Fatal("env should exist")
+	}
+}
+
+// Changing a configured backend that supports multi-state to a
+// backend that also supports multi-state.
+func TestMetaBackend_configuredChangeCopy_multiToMulti(t *testing.T) {
+	// Create a temporary working directory that is empty
+	td := tempDir(t)
+	copy.CopyDir(testFixturePath("backend-change-multi-to-multi"), td)
+	defer os.RemoveAll(td)
+	defer testChdir(t, td)()
+
+	// Ask input
+	defer testInputMap(t, map[string]string{
+		"backend-migrate-to-new":                   "yes",
+		"backend-migrate-multistate-to-multistate": "yes",
+	})()
+
+	// Setup the meta
+	m := testMetaBackend(t, nil)
+
+	// Get the backend
+	b, err := m.Backend(&BackendOpts{Init: true})
+	if err != nil {
+		t.Fatalf("bad: %s", err)
+	}
+
+	// Check resulting states
+	states, err := b.States()
+	if err != nil {
+		t.Fatalf("bad: %s", err)
+	}
+
+	sort.Strings(states)
+	expected := []string{"default", "env2"}
+	if !reflect.DeepEqual(states, expected) {
+		t.Fatalf("bad: %#v", states)
+	}
+
+	{
+		// Check the default state
+		s, err := b.State(backend.DefaultStateName)
+		if err != nil {
+			t.Fatalf("bad: %s", err)
+		}
+		if err := s.RefreshState(); err != nil {
+			t.Fatalf("bad: %s", err)
+		}
+		state := s.State()
+		if state == nil {
+			t.Fatal("state should not be nil")
+		}
+		if state.Lineage != "backend-change" {
+			t.Fatalf("bad: %#v", state)
+		}
+	}
+
+	{
+		// Check the other state
+		s, err := b.State("env2")
+		if err != nil {
+			t.Fatalf("bad: %s", err)
+		}
+		if err := s.RefreshState(); err != nil {
+			t.Fatalf("bad: %s", err)
+		}
+		state := s.State()
+		if state == nil {
+			t.Fatal("state should not be nil")
+		}
+		if state.Lineage != "backend-change-env2" {
+			t.Fatalf("bad: %#v", state)
+		}
+	}
+
+	// Verify no local backup
+	if _, err := os.Stat(DefaultStateFilename + DefaultBackupExtension); err == nil {
+		t.Fatal("file should not exist")
+	}
+
+	{
+		// Verify existing environments exist
+		envPath := filepath.Join(backendlocal.DefaultEnvDir, "env2", backendlocal.DefaultStateFilename)
+		if _, err := os.Stat(envPath); err != nil {
+			t.Fatal("env should exist")
+		}
+	}
+
+	{
+		// Verify new environments exist
+		envPath := filepath.Join("envdir-new", "env2", backendlocal.DefaultStateFilename)
+		if _, err := os.Stat(envPath); err != nil {
+			t.Fatal("env should exist")
+		}
+	}
+}
+
 // Unsetting a saved backend
 func TestMetaBackend_configuredUnset(t *testing.T) {
 	// Create a temporary working directory that is empty

command/test-fixtures/backend-change-multi-default-to-single/.terraform/terraform.tfstate

@@ -0,0 +1,22 @@
+{
+    "version": 3,
+    "serial": 0,
+    "lineage": "666f9301-7e65-4b19-ae23-71184bb19b03",
+    "backend": {
+        "type": "local",
+        "config": {
+            "path": "local-state.tfstate"
+        },
+        "hash": 9073424445967744180
+    },
+    "modules": [
+        {
+            "path": [
+                "root"
+            ],
+            "outputs": {},
+            "resources": {},
+            "depends_on": []
+        }
+    ]
+}

command/test-fixtures/backend-change-multi-default-to-single/local-state.tfstate

@@ -0,0 +1,6 @@
+{
+    "version": 3,
+    "terraform_version": "0.8.2",
+    "serial": 7,
+    "lineage": "backend-change"
+}

command/test-fixtures/backend-change-multi-default-to-single/main.tf

@@ -0,0 +1,5 @@
+terraform {
+    backend "local-single" {
+        path = "local-state-2.tfstate"
+    }
+}

command/test-fixtures/backend-change-multi-to-multi/.terraform/terraform.tfstate

@@ -0,0 +1,22 @@
+{
+    "version": 3,
+    "serial": 0,
+    "lineage": "666f9301-7e65-4b19-ae23-71184bb19b03",
+    "backend": {
+        "type": "local",
+        "config": {
+            "path": "local-state.tfstate"
+        },
+        "hash": 9073424445967744180
+    },
+    "modules": [
+        {
+            "path": [
+                "root"
+            ],
+            "outputs": {},
+            "resources": {},
+            "depends_on": []
+        }
+    ]
+}

command/test-fixtures/backend-change-multi-to-multi/local-state.tfstate

@@ -0,0 +1,6 @@
+{
+    "version": 3,
+    "terraform_version": "0.8.2",
+    "serial": 7,
+    "lineage": "backend-change"
+}

command/test-fixtures/backend-change-multi-to-multi/main.tf

@@ -0,0 +1,5 @@
+terraform {
+    backend "local" {
+        environment_dir = "envdir-new"
+    }
+}

command/test-fixtures/backend-change-multi-to-multi/terraform.tfstate.d/env2/terraform.tfstate

@@ -0,0 +1,6 @@
+{
+    "version": 3,
+    "terraform_version": "0.8.2",
+    "serial": 7,
+    "lineage": "backend-change-env2"
+}

command/test-fixtures/backend-change-multi-to-single/.terraform/terraform.tfstate

@@ -0,0 +1,22 @@
+{
+    "version": 3,
+    "serial": 0,
+    "lineage": "666f9301-7e65-4b19-ae23-71184bb19b03",
+    "backend": {
+        "type": "local",
+        "config": {
+            "path": "local-state.tfstate"
+        },
+        "hash": 9073424445967744180
+    },
+    "modules": [
+        {
+            "path": [
+                "root"
+            ],
+            "outputs": {},
+            "resources": {},
+            "depends_on": []
+        }
+    ]
+}

command/test-fixtures/backend-change-multi-to-single/local-state.tfstate

@@ -0,0 +1,6 @@
+{
+    "version": 3,
+    "terraform_version": "0.8.2",
+    "serial": 7,
+    "lineage": "backend-change"
+}

command/test-fixtures/backend-change-multi-to-single/main.tf

@@ -0,0 +1,5 @@
+terraform {
+    backend "local-single" {
+        path = "local-state-2.tfstate"
+    }
+}

command/test-fixtures/backend-change-multi-to-single/terraform.tfstate.d/env2/terraform.tfstate

@@ -0,0 +1,6 @@
+{
+    "version": 3,
+    "terraform_version": "0.8.2",
+    "serial": 7,
+    "lineage": "backend-change-env2"
+}

command/test-fixtures/backend-change-single-to-single/.terraform/terraform.tfstate

@@ -0,0 +1,22 @@
+{
+    "version": 3,
+    "serial": 0,
+    "lineage": "666f9301-7e65-4b19-ae23-71184bb19b03",
+    "backend": {
+        "type": "local-single",
+        "config": {
+            "path": "local-state.tfstate"
+        },
+        "hash": 9073424445967744180
+    },
+    "modules": [
+        {
+            "path": [
+                "root"
+            ],
+            "outputs": {},
+            "resources": {},
+            "depends_on": []
+        }
+    ]
+}

command/test-fixtures/backend-change-single-to-single/local-state.tfstate

@@ -0,0 +1,6 @@
+{
+    "version": 3,
+    "terraform_version": "0.8.2",
+    "serial": 7,
+    "lineage": "backend-change"
+}

command/test-fixtures/backend-change-single-to-single/main.tf

@@ -0,0 +1,5 @@
+terraform {
+    backend "local-single" {
+        path = "local-state-2.tfstate"
+    }
+}

command/ui_input.go

@@ -20,6 +20,7 @@ import (
 var defaultInputReader io.Reader
 var defaultInputWriter io.Writer
 var testInputResponse []string
+var testInputResponseMap map[string]string
 
 // UIInput is an implementation of terraform.UIInput that asks the CLI
 // for input stdin.
@@ -65,13 +66,25 @@ func (i *UIInput) Input(opts *terraform.InputOpts) (string, error) {
 		return "", errors.New("interrupted")
 	}
 
-	// If we have test results, return those
+	// If we have test results, return those. testInputResponse is the
+	// "old" way of doing it and we should remove that.
 	if testInputResponse != nil {
 		v := testInputResponse[0]
 		testInputResponse = testInputResponse[1:]
 		return v, nil
 	}
 
+	// testInputResponseMap is the new way for test responses, based on
+	// the query ID.
+	if testInputResponseMap != nil {
+		v, ok := testInputResponseMap[opts.Id]
+		if !ok {
+			return "", fmt.Errorf("unexpected input request in test: %s", opts.Id)
+		}
+
+		return v, nil
+	}
+
 	log.Printf("[DEBUG] command: asking for input: %q", opts.Query)
 
 	// Listen for interrupts so we can cancel the input ask