no longer automatically attempt state migration

Add `init -migrate-state` flag to indicate automatic state migration is
desired. This flag will be implied by the `-force-copy` flag, since that
would indicate state migration is expected.

If `init` encounters a change to the stored backend configuration, it
will now always return an error when neither `-reconfigure` or
`-migrate-state` is supplied.

Turn the most common legacy output strings into diagnostics, removing
the "see above text" error output.

command/init.go

@@ -45,6 +45,7 @@ func (c *InitCommand) Run(args []string) int {
 	cmdFlags.BoolVar(&flagGet, "get", true, "")
 	cmdFlags.BoolVar(&c.forceInitCopy, "force-copy", false, "suppress prompts about copying state data")
 	cmdFlags.BoolVar(&c.reconfigure, "reconfigure", false, "reconfigure")
+	cmdFlags.BoolVar(&c.migrateState, "migrate-state", false, "migrate state")
 	cmdFlags.BoolVar(&flagUpgrade, "upgrade", false, "")
 	cmdFlags.Var(&flagPluginPath, "plugin-dir", "plugin directory")
 	cmdFlags.StringVar(&flagLockfile, "lockfile", "", "Set a dependency lockfile mode")
@@ -53,6 +54,12 @@ func (c *InitCommand) Run(args []string) int {
 		return 1
 	}
 
+	// Copying the state only happens during backend migration, so setting
+	// -force-copy implies -migrate-state
+	if c.forceInitCopy {
+		c.migrateState = true
+	}
+
 	var diags tfdiags.Diagnostics
 
 	if len(flagPluginPath) > 0 {

command/init_test.go

@@ -412,7 +412,7 @@ func TestInit_backendConfigFile(t *testing.T) {
 				View:             view,
 			},
 		}
-		args := []string{"-backend-config="}
+		args := []string{"-backend-config=", "-migrate-state"}
 		if code := c.Run(args); code != 0 {
 			t.Fatalf("bad: \n%s", ui.ErrorWriter.String())
 		}
@@ -555,7 +555,7 @@ func TestInit_backendConfigFileChange(t *testing.T) {
 		},
 	}
 
-	args := []string{"-backend-config", "input.config"}
+	args := []string{"-backend-config", "input.config", "-migrate-state"}
 	if code := c.Run(args); code != 0 {
 		t.Fatalf("bad: \n%s", ui.ErrorWriter.String())
 	}
@@ -644,7 +644,7 @@ func TestInit_backendConfigKVReInit(t *testing.T) {
 	}
 
 	// override the -backend-config options by settings
-	args = []string{"-input=false", "-backend-config", ""}
+	args = []string{"-input=false", "-backend-config", "", "-migrate-state"}
 	if code := c.Run(args); code != 0 {
 		t.Fatalf("bad: \n%s", ui.ErrorWriter.String())
 	}
@@ -906,7 +906,7 @@ func TestInit_inputFalse(t *testing.T) {
 		},
 	}
 
-	args = []string{"-input=false", "-backend-config=path=bar"}
+	args = []string{"-input=false", "-backend-config=path=bar", "-migrate-state"}
 	if code := c.Run(args); code == 0 {
 		t.Fatal("init should have failed", ui.OutputWriter)
 	}

command/meta.go

@@ -204,6 +204,9 @@ type Meta struct {
 	//
 	// reconfigure forces init to ignore any stored configuration.
 	//
+	// migrateState confirms the user wishes to migrate from the prior backend
+	// configuration to a new configuration.
+	//
 	// compactWarnings (-compact-warnings) selects a more compact presentation
 	// of warnings in the output when they are not accompanied by errors.
 	statePath        string
@@ -214,6 +217,7 @@ type Meta struct {
 	stateLockTimeout time.Duration
 	forceInitCopy    bool
 	reconfigure      bool
+	migrateState     bool
 	compactWarnings  bool
 
 	// Used with the import command to allow import of state when no matching config exists.

command/meta_backend.go

@@ -6,7 +6,6 @@ package command
 import (
 	"context"
 	"encoding/json"
-	"errors"
 	"fmt"
 	"log"
 	"path/filepath"
@@ -514,12 +513,19 @@ func (m *Meta) backendFromConfig(opts *BackendOpts) (backend.Backend, tfdiags.Di
 	// We're unsetting a backend (moving from backend => local)
 	case c == nil && !s.Backend.Empty():
 		log.Printf("[TRACE] Meta.Backend: previously-initialized %q backend is no longer present in config", s.Backend.Type)
+
+		initReason := fmt.Sprintf("Unsetting the previously set backend %q", s.Backend.Type)
 		if !opts.Init {
-			initReason := fmt.Sprintf(
-				"Unsetting the previously set backend %q",
-				s.Backend.Type)
-			m.backendInitRequired(initReason)
-			diags = diags.Append(errBackendInitRequired)
+			diags = diags.Append(tfdiags.Sourceless(
+				tfdiags.Error,
+				"Backend initialization required, please run \"terraform init\"",
+				fmt.Sprintf(strings.TrimSpace(errBackendInit), initReason),
+			))
+			return nil, diags
+		}
+
+		if !m.migrateState {
+			diags = diags.Append(migrateOrReconfigDiag)
 			return nil, diags
 		}
 
@@ -529,11 +535,12 @@ func (m *Meta) backendFromConfig(opts *BackendOpts) (backend.Backend, tfdiags.Di
 	case c != nil && s.Backend.Empty():
 		log.Printf("[TRACE] Meta.Backend: moving from default local state only to %q backend", c.Type)
 		if !opts.Init {
-			initReason := fmt.Sprintf(
-				"Initial configuration of the requested backend %q",
-				c.Type)
-			m.backendInitRequired(initReason)
-			diags = diags.Append(errBackendInitRequired)
+			initReason := fmt.Sprintf("Initial configuration of the requested backend %q", c.Type)
+			diags = diags.Append(tfdiags.Sourceless(
+				tfdiags.Error,
+				"Backend initialization required, please run \"terraform init\"",
+				fmt.Sprintf(strings.TrimSpace(errBackendInit), initReason),
+			))
 			return nil, diags
 		}
 
@@ -558,12 +565,22 @@ func (m *Meta) backendFromConfig(opts *BackendOpts) (backend.Backend, tfdiags.Di
 		}
 		log.Printf("[TRACE] Meta.Backend: backend configuration has changed (from type %q to type %q)", s.Backend.Type, c.Type)
 
+		initReason := fmt.Sprintf("Backend configuration changed for %q", c.Type)
+		if s.Backend.Type != c.Type {
+			initReason = fmt.Sprintf("Backend configuration changed from %q to %q", s.Backend.Type, c.Type)
+		}
+
 		if !opts.Init {
-			initReason := fmt.Sprintf(
-				"Backend configuration changed for %q",
-				c.Type)
-			m.backendInitRequired(initReason)
-			diags = diags.Append(errBackendInitRequired)
+			diags = diags.Append(tfdiags.Sourceless(
+				tfdiags.Error,
+				"Backend initialization required, please run \"terraform init\"",
+				fmt.Sprintf(strings.TrimSpace(errBackendInit), initReason),
+			))
+			return nil, diags
+		}
+
+		if !m.migrateState {
+			diags = diags.Append(migrateOrReconfigDiag)
 			return nil, diags
 		}
 
@@ -1138,11 +1155,6 @@ func (m *Meta) remoteBackendVersionCheck(b backend.Backend, workspace string) tf
 // Output constants and initialization code
 //-------------------------------------------------------------------
 
-// errBackendInitRequired is the final error message shown when reinit
-// is required for some reason. The error message includes the reason.
-var errBackendInitRequired = errors.New(
-	"Initialization required. Please see the error message above.")
-
 const errBackendLocalRead = `
 Error reading local state: %s
 
@@ -1205,8 +1217,7 @@ and try again.
 `
 
 const errBackendInit = `
-[reset][bold][yellow]Backend reinitialization required. Please run "terraform init".[reset]
-[yellow]Reason: %s
+Reason: %s
 
 The "backend" is the interface that Terraform uses to store state,
 perform operations, etc. If this message is showing up, it means that the
@@ -1214,8 +1225,9 @@ Terraform configuration you're using is using a custom configuration for
 the Terraform backend.
 
 Changes to backend configurations require reinitialization. This allows
-Terraform to set up the new configuration, copy existing state, etc. This is
-only done during "terraform init". Please run that command now then try again.
+Terraform to set up the new configuration, copy existing state, etc. Please run
+"terraform init" with either the "-reconfigure" or "-migrate-state" flags to
+use the current configuration.
 
 If the change reason above is incorrect, please verify your configuration
 hasn't changed and try again. At this point, no changes to your existing
@@ -1254,3 +1266,10 @@ const successBackendSet = `
 Successfully configured the backend %q! Terraform will automatically
 use this backend unless the backend configuration changes.
 `
+
+var migrateOrReconfigDiag = tfdiags.Sourceless(
+	tfdiags.Error,
+	"Backend configuration changed",
+	"A change in the backend configuration has been detected, which may require migrating existing state.\n\n"+
+		"If you wish to attempt automatic migration of the state, use \"terraform init -migrate-state\".\n"+
+		`If you wish to store the current configuration with no changes to the state, use "terraform init -reconfigure".`)

command/meta_backend_test.go

@@ -314,6 +314,10 @@ func TestMetaBackend_configureNewWithState(t *testing.T) {
 	// Setup the meta
 	m := testMetaBackend(t, nil)
 
+	// This combination should not require the extra -migrate-state flag, since
+	// there is no existing backend config
+	m.migrateState = false
+
 	// Get the backend
 	b, diags := m.Backend(&BackendOpts{Init: true})
 	if diags.HasErrors() {
@@ -1884,5 +1888,8 @@ func testMetaBackend(t *testing.T, args []string) *Meta {
 		t.Fatalf("unexpected error: %s", err)
 	}
 
+	// metaBackend tests are verifying migrate actions
+	m.migrateState = true
+
 	return &m
 }

command/state_rm_test.go

@@ -400,7 +400,7 @@ func TestStateRm_needsInit(t *testing.T) {
 		t.Fatalf("expected error output, got:\n%s", ui.OutputWriter.String())
 	}
 
-	if !strings.Contains(ui.ErrorWriter.String(), "Initialization") {
+	if !strings.Contains(ui.ErrorWriter.String(), "Backend initialization") {
 		t.Fatalf("expected initialization error, got:\n%s", ui.ErrorWriter.String())
 	}
 }