Disambiguate backend configurations during migration

A user reported at hashicorp/terraform-provider-consul#275
that the message when migrating backend was not detailed enough to be sure what
was the previously configured backend, and what is the new one. This can make
migrating from one backend to another dangerous as there is no way to check the
configuration is correct when actually migrating to a new backend.

The message when using the consul backend (though the issue still stands with
other backends as well) was:

	Do you want to migrate all workspaces to "consul"?
	  Both the existing "consul" backend and the newly configured "consul" backend
	  support workspaces. When migrating between backends, Terraform will copy
	  all workspaces (with the same names). THIS WILL OVERWRITE any conflicting
	  states in the destination.

	  Terraform 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", Terraform will migrate all states. If you answer
	  "no", Terraform will abort.

I changed the Backend interface to add a String() method that returns an HCL
representation of the backend and used it to return more information when migrating.
The new message is:

	Do you want to copy existing state to the new backend?
	  Pre-existing state was found while migrating the previous "local" backend to the
	  newly configured "consul" backend. No existing state was found in the newly
	  configured "consul" backend.

	  The configuration for the previous "local" backend was:

	    backend "local" {
	      path          = ""
	      workspace_dir = ""
	    }

	  The configuration for the new "consul" backend is:

	    backend "consul" {
	      access_token = ""
	      address      = ""
	      ca_file      = ""
	      cert_file    = ""
	      datacenter   = ""
	      gzip         = false
	      http_auth    = ""
	      key_file     = ""
	      lock         = true
	      path         = "full/path"
	      scheme       = ""
	    }

	  Do you want to copy this state to the new "consul"
	  backend? Enter "yes" to copy and "no" to start with an empty state.

For the local and remote backends the implementation is straightforward, for all
the others the representation is generated from the backend's Schema and its
configuration.

backend/backend.go

@@ -112,6 +112,10 @@ type Backend interface {
 	// States returns a list of the names of all of the workspaces that exist
 	// in this backend.
 	Workspaces() ([]string, error)
+
+	// String returns a valid string representation of the backend as it would
+	// appear in the Terraform configuration
+	String() string
 }
 
 // Enhanced implements additional behavior on top of a normal backend.

backend/local/backend.go

@@ -9,8 +9,10 @@ import (
 	"os"
 	"path/filepath"
 	"sort"
+	"strings"
 	"sync"
 
+	"github.com/hashicorp/hcl/v2/hclwrite"
 	"github.com/hashicorp/terraform/backend"
 	"github.com/hashicorp/terraform/command/clistate"
 	"github.com/hashicorp/terraform/configs/configschema"
@@ -344,6 +346,16 @@ func (b *Local) Operation(ctx context.Context, op *backend.Operation) (*backend.
 	return runningOp, nil
 }
 
+func (b *Local) String() string {
+	f := hclwrite.NewFile()
+
+	body := f.Body().AppendNewBlock("backend", []string{"local"}).Body()
+	body.SetAttributeValue("path", cty.StringVal(b.StatePath))
+	body.SetAttributeValue("workspace_dir", cty.StringVal(b.StateWorkspaceDir))
+
+	return strings.TrimSpace(string(f.Bytes()))
+}
+
 // opWait waits for the operation to complete, and a stop signal or a
 // cancelation signal.
 func (b *Local) opWait(

backend/remote-state/artifactory/backend.go

@@ -13,6 +13,7 @@ import (
 
 func New() backend.Backend {
 	s := &schema.Backend{
+		Type: "artifactory",
 		Schema: map[string]*schema.Schema{
 			"username": &schema.Schema{
 				Type:        schema.TypeString,

backend/remote-state/azure/backend.go

@@ -11,6 +11,7 @@ import (
 // New creates a new backend for Azure remote state.
 func New() backend.Backend {
 	s := &schema.Backend{
+		Type: "azure",
 		Schema: map[string]*schema.Schema{
 			"storage_account_name": {
 				Type:        schema.TypeString,

backend/remote-state/consul/backend.go

@@ -14,6 +14,7 @@ import (
 // New creates a new backend for Consul remote state.
 func New() backend.Backend {
 	s := &schema.Backend{
+		Type: "consul",
 		Schema: map[string]*schema.Schema{
 			"path": &schema.Schema{
 				Type:        schema.TypeString,

backend/remote-state/cos/backend.go

@@ -42,6 +42,7 @@ type Backend struct {
 // New creates a new backend for TencentCloud cos remote state.
 func New() backend.Backend {
 	s := &schema.Backend{
+		Type: "cos",
 		Schema: map[string]*schema.Schema{
 			"secret_id": {
 				Type:        schema.TypeString,

backend/remote-state/etcdv2/backend.go

@@ -15,6 +15,7 @@ import (
 
 func New() backend.Backend {
 	s := &schema.Backend{
+		Type: "etcd",
 		Schema: map[string]*schema.Schema{
 			"path": &schema.Schema{
 				Type:        schema.TypeString,

backend/remote-state/etcdv3/backend.go

@@ -24,6 +24,7 @@ const (
 
 func New() backend.Backend {
 	s := &schema.Backend{
+		Type: "etcdv3",
 		Schema: map[string]*schema.Schema{
 			endpointsKey: &schema.Schema{
 				Type: schema.TypeList,

backend/remote-state/gcs/backend.go

@@ -36,6 +36,7 @@ type Backend struct {
 func New() backend.Backend {
 	b := &Backend{}
 	b.Backend = &schema.Backend{
+		Type:          "gcs",
 		ConfigureFunc: b.configure,
 		Schema: map[string]*schema.Schema{
 			"bucket": {

backend/remote-state/http/backend.go

@@ -18,6 +18,7 @@ import (
 
 func New() backend.Backend {
 	s := &schema.Backend{
+		Type: "http",
 		Schema: map[string]*schema.Schema{
 			"address": &schema.Schema{
 				Type:        schema.TypeString,

backend/remote-state/inmem/backend.go

@@ -44,6 +44,7 @@ func Reset() {
 func New() backend.Backend {
 	// Set the schema
 	s := &schema.Backend{
+		Type: "inmem",
 		Schema: map[string]*schema.Schema{
 			"lock_id": &schema.Schema{
 				Type:        schema.TypeString,

backend/remote-state/kubernetes/backend.go

@@ -26,7 +26,7 @@ import (
 const (
 	noConfigError = `
 
-[Kubernetes backend] Neither service_account nor load_config_file were set to true, 
+[Kubernetes backend] Neither service_account nor load_config_file were set to true,
 this could cause issues connecting to your Kubernetes cluster.
 `
 )
@@ -42,6 +42,7 @@ var (
 // New creates a new backend for kubernetes remote state.
 func New() backend.Backend {
 	s := &schema.Backend{
+		Type: "kubernetes",
 		Schema: map[string]*schema.Schema{
 			"secret_suffix": {
 				Type:        schema.TypeString,

backend/remote-state/manta/backend.go

@@ -19,6 +19,7 @@ import (
 
 func New() backend.Backend {
 	s := &schema.Backend{
+		Type: "manta",
 		Schema: map[string]*schema.Schema{
 			"account": {
 				Type:        schema.TypeString,

backend/remote-state/oss/backend.go

@@ -34,6 +34,7 @@ import (
 // New creates a new backend for OSS remote state.
 func New() backend.Backend {
 	s := &schema.Backend{
+		Type: "oss",
 		Schema: map[string]*schema.Schema{
 			"access_key": &schema.Schema{
 				Type:        schema.TypeString,

backend/remote-state/pg/backend.go

@@ -18,6 +18,7 @@ const (
 // New creates a new backend for Postgres remote state.
 func New() backend.Backend {
 	s := &schema.Backend{
+		Type: "pg",
 		Schema: map[string]*schema.Schema{
 			"conn_str": {
 				Type:        schema.TypeString,

backend/remote-state/s3/backend.go

@@ -20,6 +20,7 @@ import (
 // New creates a new backend for S3 remote state.
 func New() backend.Backend {
 	s := &schema.Backend{
+		Type: "s3",
 		Schema: map[string]*schema.Schema{
 			"bucket": {
 				Type:        schema.TypeString,

backend/remote-state/swift/backend.go

@@ -25,6 +25,7 @@ type Config struct {
 // New creates a new backend for Swift remote state.
 func New() backend.Backend {
 	s := &schema.Backend{
+		Type: "swift",
 		Schema: map[string]*schema.Schema{
 			"auth_url": {
 				Type:        schema.TypeString,

backend/remote/backend.go

@@ -14,6 +14,8 @@ import (
 
 	tfe "github.com/hashicorp/go-tfe"
 	version "github.com/hashicorp/go-version"
+	"github.com/hashicorp/hcl/v2/hclsyntax"
+	"github.com/hashicorp/hcl/v2/hclwrite"
 	svchost "github.com/hashicorp/terraform-svchost"
 	"github.com/hashicorp/terraform-svchost/disco"
 	"github.com/hashicorp/terraform/backend"
@@ -792,6 +794,25 @@ func (b *Remote) Operation(ctx context.Context, op *backend.Operation) (*backend
 	return runningOp, nil
 }
 
+func (b *Remote) String() string {
+	f := hclwrite.NewFile()
+
+	body := f.Body().AppendNewBlock("backend", []string{"remote"}).Body()
+	body.SetAttributeValue("hostname", cty.StringVal(b.hostname))
+	body.SetAttributeValue("organization", cty.StringVal(b.organization))
+	body.SetAttributeRaw("token", hclwrite.Tokens{&hclwrite.Token{
+		Type:  hclsyntax.TokenIdent,
+		Bytes: []byte("(sensitive)"),
+	}})
+	body.AppendNewline()
+
+	body = body.AppendNewBlock("workspaces", []string{}).Body()
+	body.SetAttributeValue("name", cty.StringVal(b.workspace))
+	body.SetAttributeValue("prefix", cty.StringVal(b.prefix))
+
+	return strings.TrimSpace(string(f.Bytes()))
+}
+
 func (b *Remote) cancel(cancelCtx context.Context, op *backend.Operation, r *tfe.Run) error {
 	if r.Actions.IsCancelable {
 		// Only ask if the remote operation should be canceled

builtin/providers/terraform/data_source_state_test.go

@@ -369,3 +369,7 @@ func (b backendFailsConfigure) DeleteWorkspace(name string) error {
 func (b backendFailsConfigure) Workspaces() ([]string, error) {
 	return nil, fmt.Errorf("Workspaces not implemented")
 }
+
+func (b backendFailsConfigure) String() string {
+	return ""
+}

command/meta_backend_migrate.go

@@ -13,6 +13,7 @@ import (
 
 	"github.com/hashicorp/terraform/backend"
 	"github.com/hashicorp/terraform/command/clistate"
+	"github.com/hashicorp/terraform/internal/logging"
 	"github.com/hashicorp/terraform/states"
 	"github.com/hashicorp/terraform/states/statemgr"
 	"github.com/hashicorp/terraform/terraform"
@@ -412,7 +413,8 @@ func (m *Meta) backendMigrateEmptyConfirm(one, two statemgr.Full, opts *backendM
 		Query: "Do you want to copy existing state to the new backend?",
 		Description: fmt.Sprintf(
 			strings.TrimSpace(inputBackendMigrateEmpty),
-			opts.OneType, opts.TwoType),
+			opts.OneType, opts.TwoType,
+			logging.Indent(opts.One.String()), logging.Indent(opts.Two.String())),
 	}
 
 	return m.confirm(inputOpts)
@@ -453,7 +455,8 @@ func (m *Meta) backendMigrateNonEmptyConfirm(
 		Query: "Do you want to copy existing state to the new backend?",
 		Description: fmt.Sprintf(
 			strings.TrimSpace(inputBackendMigrateNonEmpty),
-			opts.OneType, opts.TwoType, onePath, twoPath),
+			opts.OneType, opts.TwoType, onePath, twoPath,
+			logging.Indent(opts.One.String()), logging.Indent(opts.Two.String())),
 	}
 
 	// Confirm with the user that the copy should occur
@@ -496,7 +499,7 @@ This will attempt to copy (with permission) all workspaces again.
 `
 
 const errBackendStateCopy = `
-Error copying state from the previous %q backend to the newly configured 
+Error copying state from the previous %q backend to the newly configured
 %q backend:
     %s
 
@@ -507,7 +510,17 @@ the error above and try again.
 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
+configured %[2]q backend.
+
+The configuration for the previous %[1]q backend was:
+
+%[3]s
+
+The configuration for the new %[2]q backend is:
+
+%[4]s
+
+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.
 `
 
@@ -520,6 +533,14 @@ removed after responding to this query.
 Previous (type %[1]q): %[3]s
 New      (type %[2]q): %[4]s
 
+The configuration for the previous %[1]q backend was:
+
+%[5]s
+
+The configuration for the new %[2]q backend is:
+
+%[6]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.
@@ -550,7 +571,7 @@ If you answer "yes", Terraform will migrate all states. If you answer
 
 const inputBackendNewWorkspaceName = `
 Please provide a new workspace name (e.g. dev, test) that will be used
-to migrate the existing default workspace. 
+to migrate the existing default workspace.
 `
 
 const inputBackendSelectWorkspace = `