Sort interpolated lists by index

Interpolated lists should be sorted by index number, rather than
lexicographic key order.

terraform/interpolate.go

@@ -620,6 +620,33 @@ func (i *Interpolater) computeResourceMultiVariable(
 	return &variable, err
 }
 
+type indexKeys []string
+
+// we need to separate the index integer from the ID, and sort numerically
+func (i indexKeys) Less(j, k int) bool {
+	jDot := strings.LastIndex(i[j], ".")
+	kDot := strings.LastIndex(i[j], ".")
+
+	// These should all be properly formatted, but check the indexes and return
+	// a safe value just in case.
+	if jDot < 0 || kDot < 0 {
+		return i[j] < i[k]
+	}
+
+	jIdx, _ := strconv.Atoi(i[j][jDot+1:])
+	kIdx, _ := strconv.Atoi(i[k][kDot+1:])
+
+	return jIdx < kIdx
+}
+
+func (i indexKeys) Swap(j, k int) {
+	i[j], i[k] = i[k], i[j]
+}
+
+func (i indexKeys) Len() int {
+	return len(i)
+}
+
 func (i *Interpolater) interpolateComplexTypeAttribute(
 	resourceID string,
 	attributes map[string]string) (ast.Variable, error) {
@@ -648,7 +675,9 @@ func (i *Interpolater) interpolateComplexTypeAttribute(
 				keys = append(keys, id)
 			}
 		}
-		sort.Strings(keys)
+
+		// sort the keys by their index number, rather than lexicographically by the key
+		sort.Sort(indexKeys(keys))
 
 		var members []string
 		for _, key := range keys {

terraform/interpolate_test.go

@@ -4,6 +4,7 @@ import (
 	"fmt"
 	"os"
 	"reflect"
+	"sort"
 	"sync"
 	"testing"
 
@@ -674,6 +675,68 @@ func TestInterpolater_selfVarWithoutResource(t *testing.T) {
 	}
 }
 
+// Verify sorting by key index number
+func TestInterpolator_indexKeySort(t *testing.T) {
+	keys := []string{"a.1", "a.2", "a.10", "a.20", "a.3"}
+	sorted := []string{"a.1", "a.2", "a.3", "a.10", "a.20"}
+
+	sort.Sort(indexKeys(keys))
+	for i := range keys {
+		if keys[i] != sorted[i] {
+			t.Fatalf("indexes out of order\nexpected: %q\ngot: %q", sorted, keys)
+		}
+	}
+}
+
+func TestInterpolator_interpolatedListOrder(t *testing.T) {
+	state := &State{
+		Modules: []*ModuleState{
+			&ModuleState{
+				Path: rootModulePath,
+				Resources: map[string]*ResourceState{
+					"aws_route53_zone.list": &ResourceState{
+						Type:         "aws_route53_zone",
+						Dependencies: []string{},
+						Primary: &InstanceState{
+							ID: "null",
+							Attributes: map[string]string{
+								"foo.#":  "12",
+								"foo.0":  "a",
+								"foo.1":  "b",
+								"foo.2":  "c",
+								"foo.3":  "d",
+								"foo.4":  "e",
+								"foo.5":  "f",
+								"foo.6":  "g",
+								"foo.7":  "h",
+								"foo.8":  "i",
+								"foo.9":  "j",
+								"foo.10": "k",
+								"foo.11": "l",
+							},
+						},
+					},
+				},
+			},
+		},
+	}
+
+	i := &Interpolater{
+		Module:    testModule(t, "interpolate-multi-vars"),
+		StateLock: new(sync.RWMutex),
+		State:     state,
+	}
+
+	scope := &InterpolationScope{
+		Path: rootModulePath,
+	}
+
+	list := []interface{}{"a", "b", "c", "d", "e", "f", "g", "h", "i", "j", "k", "l"}
+
+	testInterpolate(t, i, scope, "aws_route53_zone.list.foo",
+		interfaceToVariableSwallowError(list))
+}
+
 func getInterpolaterFixture(t *testing.T) *Interpolater {
 	lock := new(sync.RWMutex)
 	state := &State{