add Rehash to terraform.BackendState

This method mirrors that of config.Backend, so we can compare the
configration of a backend read from a config vs that of a backend read
from a state. This will prevent init from reinitializing when using
`-backend-config` options that match the existing state.

command/init_test.go

@@ -406,6 +406,64 @@ func TestInit_copyBackendDst(t *testing.T) {
 	}
 }
 
+func TestInit_backendReinitWithExtra(t *testing.T) {
+	td := tempDir(t)
+	copy.CopyDir(testFixturePath("init-backend-empty"), td)
+	defer os.RemoveAll(td)
+	defer testChdir(t, td)()
+
+	m := testMetaBackend(t, nil)
+	opts := &BackendOpts{
+		ConfigExtra: map[string]interface{}{"path": "hello"},
+		Init:        true,
+	}
+
+	b, err := m.backendConfig(opts)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	ui := new(cli.MockUi)
+	c := &InitCommand{
+		Meta: Meta{
+			ContextOpts: testCtxConfig(testProvider()),
+			Ui:          ui,
+		},
+	}
+
+	args := []string{"-backend-config", "path=hello"}
+	if code := c.Run(args); code != 0 {
+		t.Fatalf("bad: \n%s", ui.ErrorWriter.String())
+	}
+
+	// Read our saved backend config and verify we have our settings
+	state := testStateRead(t, filepath.Join(DefaultDataDir, DefaultStateFilename))
+	if v := state.Backend.Config["path"]; v != "hello" {
+		t.Fatalf("bad: %#v", v)
+	}
+
+	if state.Backend.Hash != b.Hash {
+		t.Fatal("mismatched state and config backend hashes")
+	}
+
+	if state.Backend.Rehash() != b.Rehash() {
+		t.Fatal("mismatched state and config re-hashes")
+	}
+
+	// init again and make sure nothing changes
+	if code := c.Run(args); code != 0 {
+		t.Fatalf("bad: \n%s", ui.ErrorWriter.String())
+	}
+	state = testStateRead(t, filepath.Join(DefaultDataDir, DefaultStateFilename))
+	if v := state.Backend.Config["path"]; v != "hello" {
+		t.Fatalf("bad: %#v", v)
+	}
+
+	if state.Backend.Hash != b.Hash {
+		t.Fatal("mismatched state and config backend hashes")
+	}
+}
+
 /*
 func TestInit_remoteState(t *testing.T) {
 	tmp, cwd := testCwd(t)

command/meta_backend.go

@@ -415,9 +415,17 @@ func (m *Meta) backendFromConfig(opts *BackendOpts) (backend.Backend, error) {
 	case c != nil && s.Remote.Empty() && !s.Backend.Empty():
 		// If our configuration is the same, then we're just initializing
 		// a previously configured remote backend.
-		if !s.Backend.Empty() && s.Backend.Hash == cHash {
+		if !s.Backend.Empty() {
+			hash := s.Backend.Hash
+			// on init we need an updated hash containing any extra options
+			// that were added after merging.
+			if opts.Init {
+				hash = s.Backend.Rehash()
+			}
+			if hash == cHash {
 				return m.backend_C_r_S_unchanged(c, sMgr)
 			}
+		}
 
 		if !opts.Init {
 			initReason := fmt.Sprintf(
@@ -451,7 +459,11 @@ func (m *Meta) backendFromConfig(opts *BackendOpts) (backend.Backend, error) {
 	case c != nil && !s.Remote.Empty() && !s.Backend.Empty():
 		// If the hashes are the same, we have a legacy remote state with
 		// an unchanged stored backend state.
-		if s.Backend.Hash == cHash {
+		hash := s.Backend.Hash
+		if opts.Init {
+			hash = s.Backend.Rehash()
+		}
+		if hash == cHash {
 			if !opts.Init {
 				initReason := fmt.Sprintf(
 					"Legacy remote state found with configured backend %q",

command/test-fixtures/init-backend-empty/main.tf

@@ -0,0 +1,4 @@
+terraform {
+    backend "local" {
+    }
+}

config/config_terraform.go

@@ -72,7 +72,7 @@ type Backend struct {
 	Hash uint64
 }
 
-// Hash returns a unique content hash for this backend's configuration
+// Rehash returns a unique content hash for this backend's configuration
 // as a uint64 value.
 func (b *Backend) Rehash() uint64 {
 	// If we have no backend, the value is zero

terraform/state.go

@@ -20,6 +20,7 @@ import (
 	"github.com/hashicorp/go-version"
 	"github.com/hashicorp/terraform/config"
 	"github.com/mitchellh/copystructure"
+	"github.com/mitchellh/hashstructure"
 	"github.com/satori/go.uuid"
 )
 
@@ -801,6 +802,32 @@ func (s *BackendState) Empty() bool {
 	return s == nil || s.Type == ""
 }
 
+// Rehash returns a unique content hash for this backend's configuration
+// as a uint64 value.
+// The Hash stored in the backend state needs to match the config itself, but
+// we need to compare the backend config after it has been combined with all
+// options.
+// This function must match the implementation used by config.Backend.
+func (s *BackendState) Rehash() uint64 {
+	if s == nil {
+		return 0
+	}
+
+	// Use hashstructure to hash only our type with the config.
+	code, err := hashstructure.Hash(map[string]interface{}{
+		"type":   s.Type,
+		"config": s.Config,
+	}, nil)
+
+	// This should never happen since we have just some basic primitives
+	// so panic if there is an error.
+	if err != nil {
+		panic(err)
+	}
+
+	return code
+}
+
 // RemoteState is used to track the information about a remote
 // state store that we push/pull state to.
 type RemoteState struct {