terraform: placeholder key for computed values

terraform/resource_provider.go

@@ -1,5 +1,10 @@
 package terraform
 
+// ComputedPlaceholderKey is the configuration key given to Configure
+// in a ResourceProvider that contains the value for the computed value
+// placeholder when diffs are being done.
+const ComputedPlaceholderKey = "tf_computed_placeholder"
+
 // ResourceProvider is an interface that must be implemented by any
 // resource provider: the thing that creates and manages the resources in
 // a Terraform configuration.
@@ -28,6 +33,14 @@ type ResourceProvider interface {
 		map[string]interface{}) (ResourceDiff, error)
 }
 
+// ResourceProviderCommonConfig contains the common configuration
+// keys that are sent with every resource provider configuration.
+// This can be used with something like mapstructure to extract
+// the proper value.
+type ResourceProviderCommonConfig struct {
+	TFComputedPlaceholder string `mapstructure:"tf_computed_placeholder"`
+}
+
 // ResourceType is a type of resource that a resource provider can manage.
 type ResourceType struct {
 	Name string

terraform/resource_provider_mock.go

@@ -1,5 +1,9 @@
 package terraform
 
+import (
+	"github.com/mitchellh/mapstructure"
+)
+
 // MockResourceProvider implements ResourceProvider but mocks out all the
 // calls for testing purposes.
 type MockResourceProvider struct {
@@ -7,6 +11,7 @@ type MockResourceProvider struct {
 	Meta interface{}
 
 	ConfigureCalled         bool
+	ConfigureCommonConfig   ResourceProviderCommonConfig
 	ConfigureConfig         map[string]interface{}
 	ConfigureReturnWarnings []string
 	ConfigureReturnError    error
@@ -23,6 +28,11 @@ type MockResourceProvider struct {
 func (p *MockResourceProvider) Configure(c map[string]interface{}) ([]string, error) {
 	p.ConfigureCalled = true
 	p.ConfigureConfig = c
+
+	if err := mapstructure.Decode(&p.ConfigureCommonConfig, c); err != nil {
+		return nil, err
+	}
+
 	return p.ConfigureReturnWarnings, p.ConfigureReturnError
 }
 

terraform/terraform_test.go

@@ -39,6 +39,20 @@ func TestNew(t *testing.T) {
 	if testProviderName(mapping["do_droplet.bar"]) != "do" {
 		t.Fatalf("bad: %#v", mapping)
 	}
+
+	/*
+	val := testProviderMock(mapping["aws_instance.foo"]).
+		ConfigureCommonConfig.TFComputedPlaceholder
+	if val == "" {
+		t.Fatal("should have computed placeholder")
+	}
+
+	val = testProviderMock(mapping["aws_instance.bar"]).
+		ConfigureCommonConfig.TFComputedPlaceholder
+	if val == "" {
+		t.Fatal("should have computed placeholder")
+	}
+	*/
 }
 
 func TestNew_graphCycle(t *testing.T) {
@@ -169,8 +183,12 @@ func testProviderFunc(n string, rs []string) ResourceProviderFactory {
 	}
 }
 
+func testProviderMock(p ResourceProvider) *MockResourceProvider {
+	return p.(*MockResourceProvider)
+}
+
 func testProviderName(p ResourceProvider) string {
-	return p.(*MockResourceProvider).Meta.(string)
+	return testProviderMock(p).Meta.(string)
 }
 
 func testResourceMapping(tf *Terraform) map[string]ResourceProvider {