terraform: PruneProviderTransfomer

terraform/test-fixtures/transform-provider-prune/main.tf

@@ -0,0 +1,2 @@
+provider "aws" {}
+resource "foo_instance" "web" {}

terraform/transform_provider.go

@@ -71,6 +71,27 @@ func (t *MissingProviderTransformer) Transform(g *Graph) error {
 	return nil
 }
 
+// PruneProviderTransformer is a GraphTransformer that prunes all the
+// providers that aren't needed from the graph. A provider is unneeded if
+// no resource or module is using that provider.
+type PruneProviderTransformer struct{}
+
+func (t *PruneProviderTransformer) Transform(g *Graph) error {
+	for _, v := range g.Vertices() {
+		// We only care about the providers
+		if _, ok := v.(GraphNodeProvider); !ok {
+			continue
+		}
+
+		// Does anything depend on this? If not, then prune it.
+		if s := g.UpEdges(v); s.Len() == 0 {
+			g.Remove(v)
+		}
+	}
+
+	return nil
+}
+
 type graphNodeMissingProvider struct {
 	ProviderNameValue string
 }

terraform/transform_provider_test.go

@@ -53,6 +53,45 @@ func TestMissingProviderTransformer(t *testing.T) {
 	}
 }
 
+func TestPruneProviderTransformer(t *testing.T) {
+	mod := testModule(t, "transform-provider-prune")
+
+	g := Graph{Path: RootModulePath}
+	{
+		tf := &ConfigTransformer{Module: mod}
+		if err := tf.Transform(&g); err != nil {
+			t.Fatalf("err: %s", err)
+		}
+	}
+
+	{
+		transform := &MissingProviderTransformer{Providers: []string{"foo"}}
+		if err := transform.Transform(&g); err != nil {
+			t.Fatalf("err: %s", err)
+		}
+	}
+
+	{
+		transform := &ProviderTransformer{}
+		if err := transform.Transform(&g); err != nil {
+			t.Fatalf("err: %s", err)
+		}
+	}
+
+	{
+		transform := &PruneProviderTransformer{}
+		if err := transform.Transform(&g); err != nil {
+			t.Fatalf("err: %s", err)
+		}
+	}
+
+	actual := strings.TrimSpace(g.String())
+	expected := strings.TrimSpace(testTransformPruneProviderBasicStr)
+	if actual != expected {
+		t.Fatalf("bad:\n\n%s", actual)
+	}
+}
+
 func TestGraphNodeMissingProvider_impl(t *testing.T) {
 	var _ dag.Vertex = new(graphNodeMissingProvider)
 	var _ dag.NamedVertex = new(graphNodeMissingProvider)
@@ -77,3 +116,9 @@ aws_instance.web
 provider.aws
 provider.foo
 `
+
+const testTransformPruneProviderBasicStr = `
+foo_instance.web
+  provider.foo
+provider.foo
+`