helper/schema: set the field to empty if it is a list and computed

helper/schema/resource_data.go

@@ -874,12 +874,19 @@ func (d *ResourceData) stateList(
 	schema *Schema) map[string]string {
 	countRaw := d.get(prefix, []string{"#"}, schema, d.stateSource(prefix))
 	if !countRaw.Exists {
-		return nil
+		if schema.Computed {
+			// If it is computed, then it always _exists_ in the state,
+			// it is just empty.
+			countRaw.Exists = true
+			countRaw.Value = 0
+		} else {
+			return nil
+		}
 	}
 	count := countRaw.Value.(int)
 
 	result := make(map[string]string)
-	if count > 0 {
+	if count > 0 || schema.Computed {
 		result[prefix+".#"] = strconv.FormatInt(int64(count), 10)
 	}
 	for i := 0; i < count; i++ {
@@ -974,7 +981,14 @@ func (d *ResourceData) stateSet(
 	schema *Schema) map[string]string {
 	raw := d.get(prefix, nil, schema, d.stateSource(prefix))
 	if !raw.Exists {
-		return nil
+		if schema.Computed {
+			// If it is computed, then it always _exists_ in the state,
+			// it is just empty.
+			raw.Exists = true
+			raw.Value = new(Set)
+		} else {
+			return nil
+		}
 	}
 
 	set := raw.Value.(*Set)

helper/schema/resource_data_test.go

@@ -1685,7 +1685,9 @@ func TestResourceDataState(t *testing.T) {
 			},
 
 			Result: &terraform.InstanceState{
-				Attributes: map[string]string{},
+				Attributes: map[string]string{
+					"config_vars.#": "0",
+				},
 			},
 		},
 
@@ -1834,6 +1836,40 @@ func TestResourceDataState(t *testing.T) {
 			},
 		},
 
+		{
+			Schema: map[string]*Schema{
+				"ports": &Schema{
+					Type:     TypeList,
+					Optional: true,
+					Computed: true,
+					Elem:     &Schema{Type: TypeInt},
+				},
+			},
+
+			State: nil,
+
+			Diff: &terraform.InstanceDiff{
+				Attributes: map[string]*terraform.ResourceAttrDiff{
+					"ports.#": &terraform.ResourceAttrDiff{
+						Old:         "",
+						NewComputed: true,
+					},
+				},
+			},
+
+			Partial: []string{},
+
+			Set: map[string]interface{}{
+				"ports": []interface{}{},
+			},
+
+			Result: &terraform.InstanceState{
+				Attributes: map[string]string{
+					"ports.#": "0",
+				},
+			},
+		},
+
 		// List of resources
 		{
 			Schema: map[string]*Schema{
@@ -1974,6 +2010,39 @@ func TestResourceDataState(t *testing.T) {
 				},
 			},
 		},
+
+		{
+			Schema: map[string]*Schema{
+				"ports": &Schema{
+					Type:     TypeSet,
+					Optional: true,
+					Computed: true,
+					Elem:     &Schema{Type: TypeInt},
+					Set: func(a interface{}) int {
+						return a.(int)
+					},
+				},
+			},
+
+			State: nil,
+
+			Diff: &terraform.InstanceDiff{
+				Attributes: map[string]*terraform.ResourceAttrDiff{
+					"ports.#": &terraform.ResourceAttrDiff{
+						Old:         "",
+						NewComputed: true,
+					},
+				},
+			},
+
+			Partial: []string{},
+
+			Result: &terraform.InstanceState{
+				Attributes: map[string]string{
+					"ports.#": "0",
+				},
+			},
+		},
 	}
 
 	for i, tc := range cases {

helper/schema/schema_test.go

@@ -667,6 +667,35 @@ func TestSchemaMap_Diff(t *testing.T) {
 			Err: false,
 		},
 
+		{
+			Schema: map[string]*Schema{
+				"ports": &Schema{
+					Type:     TypeSet,
+					Optional: true,
+					Computed: true,
+					Elem:     &Schema{Type: TypeInt},
+					Set: func(a interface{}) int {
+						return a.(int)
+					},
+				},
+			},
+
+			State: nil,
+
+			Config: nil,
+
+			Diff: &terraform.InstanceDiff{
+				Attributes: map[string]*terraform.ResourceAttrDiff{
+					"ports.#": &terraform.ResourceAttrDiff{
+						Old:         "",
+						NewComputed: true,
+					},
+				},
+			},
+
+			Err: false,
+		},
+
 		{
 			Schema: map[string]*Schema{
 				"ports": &Schema{