don't store prepared backend config

The backend gets to "prepare" the configuration before Configure is
called, in order to validate the values and insert defaults. We don't
want to store this value in the "config state", because it will often
not match the raw config after it is prepared, forcing unecessary
backend migrations during init.

Since PrepareConfig is always called before Configure, we can store the
config value directly, and assume that it will be prepared in the same
manner each time.

command/init_test.go

@@ -456,6 +456,52 @@ func TestInit_backendConfigKVReInit(t *testing.T) {
 	}
 }
 
+func TestInit_backendConfigKVReInitWithConfigDiff(t *testing.T) {
+	// Create a temporary working directory that is empty
+	td := tempDir(t)
+	copy.CopyDir(testFixturePath("init-backend"), td)
+	defer os.RemoveAll(td)
+	defer testChdir(t, td)()
+
+	ui := new(cli.MockUi)
+	c := &InitCommand{
+		Meta: Meta{
+			testingOverrides: metaOverridesForProvider(testProvider()),
+			Ui:               ui,
+		},
+	}
+
+	args := []string{"-input=false"}
+	if code := c.Run(args); code != 0 {
+		t.Fatalf("bad: \n%s", ui.ErrorWriter.String())
+	}
+
+	ui = new(cli.MockUi)
+	c = &InitCommand{
+		Meta: Meta{
+			testingOverrides: metaOverridesForProvider(testProvider()),
+			Ui:               ui,
+		},
+	}
+
+	// a second init with identical config should require no changes, nor
+	// should it change the backend.
+	args = []string{"-input=false", "-backend-config", "path=foo"}
+	if code := c.Run(args); code != 0 {
+		t.Fatalf("bad: \n%s", ui.ErrorWriter.String())
+	}
+
+	// make sure the backend is configured how we expect
+	configState := testDataStateRead(t, filepath.Join(DefaultDataDir, DefaultStateFilename))
+	cfg := map[string]interface{}{}
+	if err := json.Unmarshal(configState.Backend.ConfigRaw, &cfg); err != nil {
+		t.Fatal(err)
+	}
+	if cfg["path"] != "foo" {
+		t.Fatalf(`expected backend path="foo", got path="%v"`, cfg["foo"])
+	}
+}
+
 func TestInit_targetSubdir(t *testing.T) {
 	// Create a temporary working directory that is empty
 	td := tempDir(t)

command/meta_backend.go

@@ -185,9 +185,8 @@ func (m *Meta) BackendForPlan(settings plans.Backend) (backend.Enhanced, tfdiags
 	if validateDiags.HasErrors() {
 		return nil, diags
 	}
-	configVal = newVal
 
-	configureDiags := b.Configure(configVal)
+	configureDiags := b.Configure(newVal)
 	diags = diags.Append(configureDiags)
 
 	// If the backend supports CLI initialization, do it.
@@ -922,9 +921,8 @@ func (m *Meta) backend_C_r_S_unchanged(c *configs.Backend, cHash int, sMgr *stat
 	if validDiags.HasErrors() {
 		return nil, diags
 	}
-	configVal = newVal
 
-	configDiags := b.Configure(configVal)
+	configDiags := b.Configure(newVal)
 	diags = diags.Append(configDiags)
 	if configDiags.HasErrors() {
 		return nil, diags
@@ -1051,9 +1049,8 @@ func (m *Meta) backendInitFromConfig(c *configs.Backend) (backend.Backend, cty.V
 	if validateDiags.HasErrors() {
 		return nil, cty.NilVal, diags
 	}
-	configVal = newVal
 
-	configureDiags := b.Configure(configVal)
+	configureDiags := b.Configure(newVal)
 	diags = diags.Append(configureDiags.InConfigBody(c.Config))
 
 	return b, configVal, diags
@@ -1082,9 +1079,8 @@ func (m *Meta) backendInitFromSaved(s *terraform.BackendState) (backend.Backend,
 	if validateDiags.HasErrors() {
 		return nil, diags
 	}
-	configVal = newVal
 
-	configureDiags := b.Configure(configVal)
+	configureDiags := b.Configure(newVal)
 	diags = diags.Append(configureDiags)
 
 	return b, diags

command/meta_backend_migrate.go

@@ -98,7 +98,6 @@ func (m *Meta) backendMigrateState(opts *backendMigrateOpts) error {
 	// Multi-state to multi-state. We merge the states together (migrating
 	// each from the source to the destination one by one).
 	case !oneSingle && !twoSingle:
-		fmt.Printf("STATES: %q\n", oneStates)
 		// 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 {