command: meta.Backend is used for initializing the backend

This is a complex function that handles all the potential cases that can
happen with legacy remote state, new configurations, etc.

command/meta_backend_migrate.go

@@ -0,0 +1,194 @@
+package command
+
+import (
+	"fmt"
+	"io/ioutil"
+	"os"
+	"path/filepath"
+	"strings"
+
+	"github.com/hashicorp/terraform/state"
+	"github.com/hashicorp/terraform/terraform"
+)
+
+// 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.
+func (m *Meta) backendMigrateState(opts *backendMigrateOpts) error {
+	one := opts.One.State()
+	two := opts.Two.State()
+
+	var confirmFunc func(opts *backendMigrateOpts) (bool, error)
+	switch {
+	// No migration necessary
+	case one.Empty() && two.Empty():
+		return nil
+
+	// No migration necessary if we're inheriting state.
+	case one.Empty() && !two.Empty():
+		return nil
+
+	// We have existing state moving into no state. Ask the user if
+	// they'd like to do this.
+	case !one.Empty() && two.Empty():
+		confirmFunc = m.backendMigrateEmptyConfirm
+
+	// Both states are non-empty, meaning we need to determine which
+	// state should be used and update accordingly.
+	case !one.Empty() && !two.Empty():
+		confirmFunc = m.backendMigrateNonEmptyConfirm
+	}
+
+	if confirmFunc == nil {
+		panic("confirmFunc must not be nil")
+	}
+
+	// Confirm with the user whether we want to copy state over
+	confirm, err := confirmFunc(opts)
+	if err != nil {
+		return err
+	}
+	if !confirm {
+		return nil
+	}
+
+	// Confirmed! Write.
+	if err := opts.Two.WriteState(one); err != nil {
+		return fmt.Errorf(strings.TrimSpace(errBackendStateCopy),
+			opts.OneType, opts.TwoType, err)
+	}
+	if err := opts.Two.PersistState(); err != nil {
+		return fmt.Errorf(strings.TrimSpace(errBackendStateCopy),
+			opts.OneType, opts.TwoType, err)
+	}
+
+	// And we're done.
+	return nil
+}
+
+func (m *Meta) backendMigrateEmptyConfirm(opts *backendMigrateOpts) (bool, error) {
+	inputOpts := &terraform.InputOpts{
+		Id: "backend-migrate-to-backend",
+		Query: fmt.Sprintf(
+			"Do you want to copy state from %q to %q?",
+			opts.OneType, opts.TwoType),
+		Description: fmt.Sprintf(
+			strings.TrimSpace(inputBackendMigrateEmpty),
+			opts.OneType, opts.TwoType),
+	}
+
+	// Confirm with the user that the copy should occur
+	for {
+		v, err := m.UIInput().Input(inputOpts)
+		if err != nil {
+			return false, fmt.Errorf(
+				"Error asking for state copy action: %s", err)
+		}
+
+		switch strings.ToLower(v) {
+		case "no":
+			return false, nil
+
+		case "yes":
+			return true, nil
+		}
+	}
+}
+
+func (m *Meta) backendMigrateNonEmptyConfirm(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()
+
+	// Save both to a temporary
+	td, err := ioutil.TempDir("", "terraform")
+	if err != nil {
+		return false, fmt.Errorf("Error creating temporary directory: %s", err)
+	}
+	defer os.RemoveAll(td)
+
+	// Helper to write the state
+	saveHelper := func(n, path string, s *terraform.State) error {
+		f, err := os.Create(path)
+		if err != nil {
+			return err
+		}
+		defer f.Close()
+
+		return terraform.WriteState(s, f)
+	}
+
+	// Write the states
+	onePath := filepath.Join(td, fmt.Sprintf("1-%s.tfstate", opts.OneType))
+	twoPath := filepath.Join(td, fmt.Sprintf("2-%s.tfstate", opts.TwoType))
+	if err := saveHelper(opts.OneType, onePath, one); err != nil {
+		return false, fmt.Errorf("Error saving temporary state: %s", err)
+	}
+	if err := saveHelper(opts.TwoType, twoPath, two); err != nil {
+		return false, fmt.Errorf("Error saving temporary state: %s", err)
+	}
+
+	// Ask for confirmation
+	inputOpts := &terraform.InputOpts{
+		Id: "backend-migrate-to-backend",
+		Query: fmt.Sprintf(
+			"Do you want to copy state from %q to %q?",
+			opts.OneType, opts.TwoType),
+		Description: fmt.Sprintf(
+			strings.TrimSpace(inputBackendMigrateNonEmpty),
+			opts.OneType, opts.TwoType, onePath, twoPath),
+	}
+
+	// Confirm with the user that the copy should occur
+	for {
+		v, err := m.UIInput().Input(inputOpts)
+		if err != nil {
+			return false, fmt.Errorf(
+				"Error asking for state copy action: %s", err)
+		}
+
+		switch strings.ToLower(v) {
+		case "no":
+			return false, nil
+
+		case "yes":
+			return true, nil
+		}
+	}
+}
+
+type backendMigrateOpts struct {
+	OneType, TwoType string
+	One, Two         state.State
+}
+
+const errBackendStateCopy = `
+Error copying state from %q to %q: %s
+
+The state in %[1]q remains intact and unmodified. Please resolve the
+error above and try again.
+`
+
+const inputBackendMigrateEmpty = `
+Pre-existing state was found in %q while migrating to %q. No existing
+state was found in %[2]q. Do you want to copy the state from %[1]q to
+%[2]q? Enter "yes" to copy and "no" to start with an empty state.
+`
+
+const inputBackendMigrateNonEmpty = `
+Pre-existing state was found in %q while migrating to %q. An existing
+non-empty state exists in %[2]q. The two states have been saved to temporary
+files that will be removed after responding to this query.
+
+One (%[1]q): %[3]s
+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.
+`