Merge pull request #7135 from hashicorp/b-empty-multi-variables

core: Fix empty multi-variable type

terraform/context_apply_test.go

@@ -48,6 +48,69 @@ func TestContext2Apply(t *testing.T) {
 	}
 }
 
+func TestContext2Apply_resourceCountOneList(t *testing.T) {
+	m := testModule(t, "apply-resource-count-one-list")
+	p := testProvider("null")
+	p.ApplyFn = testApplyFn
+	p.DiffFn = testDiffFn
+	ctx := testContext2(t, &ContextOpts{
+		Module: m,
+		Providers: map[string]ResourceProviderFactory{
+			"null": testProviderFuncFixed(p),
+		},
+	})
+
+	if _, err := ctx.Plan(); err != nil {
+		t.Fatalf("err: %s", err)
+	}
+
+	state, err := ctx.Apply()
+	if err != nil {
+		t.Fatalf("err: %s", err)
+	}
+
+	actual := strings.TrimSpace(state.String())
+	expected := strings.TrimSpace(`null_resource.foo:
+  ID = foo
+
+Outputs:
+
+test = [foo]`)
+	if actual != expected {
+		t.Fatalf("expected: \n%s\n\ngot: \n%s\n", expected, actual)
+	}
+}
+func TestContext2Apply_resourceCountZeroList(t *testing.T) {
+	m := testModule(t, "apply-resource-count-zero-list")
+	p := testProvider("null")
+	p.ApplyFn = testApplyFn
+	p.DiffFn = testDiffFn
+	ctx := testContext2(t, &ContextOpts{
+		Module: m,
+		Providers: map[string]ResourceProviderFactory{
+			"null": testProviderFuncFixed(p),
+		},
+	})
+
+	if _, err := ctx.Plan(); err != nil {
+		t.Fatalf("err: %s", err)
+	}
+
+	state, err := ctx.Apply()
+	if err != nil {
+		t.Fatalf("err: %s", err)
+	}
+
+	actual := strings.TrimSpace(state.String())
+	expected := strings.TrimSpace(`<no state>
+Outputs:
+
+test = []`)
+	if actual != expected {
+		t.Fatalf("expected: \n%s\n\ngot: \n%s\n", expected, actual)
+	}
+}
+
 func TestContext2Apply_mapVarBetweenModules(t *testing.T) {
 	m := testModule(t, "apply-map-var-through-module")
 	p := testProvider("null")

terraform/context_test.go

@@ -2,6 +2,7 @@ package terraform
 
 import (
 	"fmt"
+	"reflect"
 	"strings"
 	"testing"
 	"time"
@@ -166,7 +167,12 @@ func testDiffFn(
 
 		attrDiff := &ResourceAttrDiff{
 			Old: "",
-			New: v.(string),
+		}
+
+		if reflect.DeepEqual(v, []interface{}{}) {
+			attrDiff.New = ""
+		} else {
+			attrDiff.New = v.(string)
 		}
 
 		if k == "require_new" {

terraform/interpolate.go

@@ -487,7 +487,7 @@ func (i *Interpolater) computeResourceMultiVariable(
 
 	// If we have no module in the state yet or count, return empty
 	if module == nil || len(module.Resources) == 0 || count == 0 {
-		return &ast.Variable{Type: ast.TypeString, Value: ""}, nil
+		return &ast.Variable{Type: ast.TypeList, Value: []ast.Variable{}}, nil
 	}
 
 	var values []string

terraform/test-fixtures/apply-resource-count-one-list/main.tf

@@ -0,0 +1,7 @@
+resource "null_resource" "foo" {
+  count = 1
+}
+
+output "test" {
+  value = "${sort(null_resource.foo.*.id)}"
+}

terraform/test-fixtures/apply-resource-count-zero-list/main.tf

@@ -0,0 +1,7 @@
+resource "null_resource" "foo" {
+  count = 0
+}
+
+output "test" {
+  value = "${sort(null_resource.foo.*.id)}"
+}