core: Allow refresh of local state with no resources

state/cache.go

@@ -64,8 +64,13 @@ func (s *CacheState) RefreshState() error {
 		// Cache is newer than remote. Not a big deal, user can just
 		// persist to get correct state.
 		s.refreshResult = CacheRefreshLocalNewer
-	case cached == nil && durable != nil:
+	case !cached.HasResources() && durable != nil:
 		// Cache should be updated since the remote is set but cache isn't
+		//
+		// If local is empty then we'll treat it as missing so that
+		// it can be overwritten by an already-existing remote. This
+		// allows the user to activate remote state for the first time
+		// against an already-existing remote state.
 		s.refreshResult = CacheRefreshUpdateLocal
 	case durable.Serial < cached.Serial:
 		// Cache is newer than remote. Not a big deal, user can just

state/cache_test.go

@@ -4,6 +4,8 @@ import (
 	"os"
 	"reflect"
 	"testing"
+
+	"github.com/hashicorp/terraform/terraform"
 )
 
 func TestCacheState(t *testing.T) {
@@ -50,6 +52,57 @@ func TestCacheState_persistDurable(t *testing.T) {
 	}
 }
 
+func TestCacheState_RefreshState(t *testing.T) {
+	for i, test := range []struct {
+		cacheModules []*terraform.ModuleState
+		expected     CacheRefreshResult
+	}{
+		{
+			cacheModules: nil,
+			expected:     CacheRefreshUpdateLocal,
+		},
+		{
+			cacheModules: []*terraform.ModuleState{},
+			expected:     CacheRefreshUpdateLocal,
+		},
+		{
+			cacheModules: []*terraform.ModuleState{
+				&terraform.ModuleState{
+					Path: terraform.RootModulePath,
+					Resources: map[string]*terraform.ResourceState{
+						"foo.foo": &terraform.ResourceState{},
+					},
+				},
+			},
+			expected: CacheRefreshLocalNewer,
+		},
+	} {
+		cache := testLocalState(t)
+		durable := testLocalState(t)
+		defer os.Remove(cache.Path)
+		defer os.Remove(durable.Path)
+
+		cs := &CacheState{
+			Cache:   cache,
+			Durable: durable,
+		}
+
+		state := cache.State()
+		state.Modules = test.cacheModules
+		if err := cs.WriteState(state); err != nil {
+			t.Fatalf("err: %s", err)
+		}
+
+		if err := cs.RefreshState(); err != nil {
+			t.Fatalf("err: %s", err)
+		}
+
+		if cs.RefreshResult() != test.expected {
+			t.Fatalf("bad %d: %v", i, cs.RefreshResult())
+		}
+	}
+}
+
 func TestCacheState_impl(t *testing.T) {
 	var _ StateReader = new(CacheState)
 	var _ StateWriter = new(CacheState)

terraform/state.go

@@ -232,6 +232,24 @@ func (s *State) Empty() bool {
 	return len(s.Modules) == 0
 }
 
+// HasResources returns true if the state contains any resources.
+//
+// This is similar to !s.Empty, but returns true also in the case where the
+// state has modules but all of them are devoid of resources.
+func (s *State) HasResources() bool {
+	if s.Empty() {
+		return false
+	}
+
+	for _, mod := range s.Modules {
+		if len(mod.Resources) > 0 {
+			return true
+		}
+	}
+
+	return false
+}
+
 // IsRemote returns true if State represents a state that exists and is
 // remote.
 func (s *State) IsRemote() bool {

terraform/state_test.go

@@ -1196,6 +1196,64 @@ func TestStateEmpty(t *testing.T) {
 	}
 }
 
+func TestStateHasResources(t *testing.T) {
+	cases := []struct {
+		In     *State
+		Result bool
+	}{
+		{
+			nil,
+			false,
+		},
+		{
+			&State{},
+			false,
+		},
+		{
+			&State{
+				Remote: &RemoteState{Type: "foo"},
+			},
+			false,
+		},
+		{
+			&State{
+				Modules: []*ModuleState{
+					&ModuleState{},
+				},
+			},
+			false,
+		},
+		{
+			&State{
+				Modules: []*ModuleState{
+					&ModuleState{},
+					&ModuleState{},
+				},
+			},
+			false,
+		},
+		{
+			&State{
+				Modules: []*ModuleState{
+					&ModuleState{},
+					&ModuleState{
+						Resources: map[string]*ResourceState{
+							"foo.foo": &ResourceState{},
+						},
+					},
+				},
+			},
+			true,
+		},
+	}
+
+	for i, tc := range cases {
+		if tc.In.HasResources() != tc.Result {
+			t.Fatalf("bad %d %#v:\n\n%#v", i, tc.Result, tc.In)
+		}
+	}
+}
+
 func TestStateFromFutureTerraform(t *testing.T) {
 	cases := []struct {
 		In     string