helper/schema: don't say that a set is computed if it exists

helper/schema/resource_data.go

@@ -304,6 +304,7 @@ func (d *ResourceData) getSet(
 			return d.getList(k, parts, schema, source)
 		}
 
+		result.Exists = raw.Exists
 		return result
 	}
 

helper/schema/schema_test.go

@@ -641,6 +641,32 @@ func TestSchemaMap_Diff(t *testing.T) {
 			Err: false,
 		},
 
+		{
+			Schema: map[string]*Schema{
+				"ports": &Schema{
+					Type:     TypeSet,
+					Computed: true,
+					Required: true,
+					Elem:     &Schema{Type: TypeInt},
+					Set: func(a interface{}) int {
+						return a.(int)
+					},
+				},
+			},
+
+			State: &terraform.InstanceState{
+				Attributes: map[string]string{
+					"ports.#": "0",
+				},
+			},
+
+			Config: nil,
+
+			Diff: nil,
+
+			Err: false,
+		},
+
 		{
 			Schema: map[string]*Schema{
 				"ports": &Schema{