core: Fix empty multi-variable type

Previously, interpolation of multi-variables was returning an empty
variable if the resource count was 0. The empty variable was defined as
TypeString, Value "". This means that empty resource counts fail type
checking for interpolation functions which operate on lists.

Instead, return an empty list if the count is 0. A context test tests
this against further regression. Also add a regression test covering the
case of a single count multi-variable.

In order to make the context testing framework deal with this change it
was necessary to special case empty lists in the test diff function.

Fixes #7002

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)}"
+}