terraform: call refresh before every diff

In the future, we'll make this a separate thing...

terraform/resource_provider.go

@@ -46,6 +46,10 @@ type ResourceProvider interface {
 	Diff(
 		*ResourceState,
 		*ResourceConfig) (*ResourceDiff, error)
+
+	// Refresh refreshes a resource and updates all of its attributes
+	// with the latest information.
+	Refresh(*ResourceState) (*ResourceState, error)
 }
 
 // ResourceConfig holds the configuration given for a resource. This is

terraform/resource_provider_mock.go

@@ -21,6 +21,11 @@ type MockResourceProvider struct {
 	DiffFn               func(*ResourceState, *ResourceConfig) (*ResourceDiff, error)
 	DiffReturn           *ResourceDiff
 	DiffReturnError      error
+	RefreshCalled        bool
+	RefreshState         *ResourceState
+	RefreshFn            func(*ResourceState) (*ResourceState, error)
+	RefreshReturn        *ResourceState
+	RefreshReturnError   error
 	ResourcesCalled      bool
 	ResourcesReturn      []ResourceType
 	ValidateCalled       bool
@@ -67,6 +72,18 @@ func (p *MockResourceProvider) Diff(
 	return p.DiffReturn, p.DiffReturnError
 }
 
+func (p *MockResourceProvider) Refresh(
+	s *ResourceState) (*ResourceState, error) {
+	p.RefreshCalled = true
+	p.RefreshState = s
+
+	if p.RefreshFn != nil {
+		return p.RefreshFn(s)
+	}
+
+	return p.RefreshReturn, p.RefreshReturnError
+}
+
 func (p *MockResourceProvider) Resources() []ResourceType {
 	p.ResourcesCalled = true
 	return p.ResourcesReturn

terraform/terraform.go

@@ -173,7 +173,14 @@ func (t *Terraform) diffWalkFn(
 	result.init()
 
 	cb := func(r *Resource) (map[string]string, error) {
-		diff, err := r.Provider.Diff(r.State, r.Config)
+		// Refresh the state so we're working with the latest resource info
+		newState, err := r.Provider.Refresh(r.State)
+		if err != nil {
+			return nil, err
+		}
+
+		// Get a diff from the newest state
+		diff, err := r.Provider.Diff(newState, r.Config)
 		if err != nil {
 			return nil, err
 		}

terraform/terraform_test.go

@@ -245,6 +245,14 @@ func TestTerraformDiff(t *testing.T) {
 	if actual != expected {
 		t.Fatalf("bad:\n%s", actual)
 	}
+
+	p := testProviderMock(testProvider(tf, "aws_instance.foo"))
+	if !p.RefreshCalled {
+		t.Fatal("refresh should be called")
+	}
+	if p.RefreshState == nil {
+		t.Fatal("refresh should have state")
+	}
 }
 
 func TestTerraformDiff_nil(t *testing.T) {
@@ -383,10 +391,15 @@ func testProviderFunc(n string, rs []string) ResourceProviderFactory {
 			return &diff, nil
 		}
 
+		refreshFn := func(s *ResourceState) (*ResourceState, error) {
+			return s, nil
+		}
+
 		result := &MockResourceProvider{
 			Meta:            n,
 			ApplyFn:         applyFn,
 			DiffFn:          diffFn,
+			RefreshFn:       refreshFn,
 			ResourcesReturn: resources,
 		}