fix: omit the user data attribute value when an instance has none (#1097)

DescribeInstanceAttribute always returned a Value for userData, an empty string when the instance was launched without any. AWS returns the attribute with no Value at all in that case.

The Terraform provider reads user_data only when Value is non-nil, so an empty string made it record the hash of the empty string in state and propose a change on every plan for an instance whose configuration names no user data.

spinifex/handlers/ec2/instance/service_impl.go

@@ -3092,12 +3092,12 @@ func (s *InstanceServiceImpl) DescribeInstanceAttribute(ctx context.Context, inp
 
 	case ec2.InstanceAttributeNameUserData:
 		// AWS returns user-data base64-encoded; RunInstancesInput.UserData is the
-		// canonical base64 store, set at launch and kept in sync by Modify.
-		var val string
-		if instance.RunInstancesInput != nil && instance.RunInstancesInput.UserData != nil {
-			val = *instance.RunInstancesInput.UserData
+		// canonical base64 store, set at launch and kept in sync by Modify. An
+		// instance with no user data carries no Value at all, not an empty one.
+		output.UserData = &ec2.AttributeValue{}
+		if instance.RunInstancesInput != nil && aws.StringValue(instance.RunInstancesInput.UserData) != "" {
+			output.UserData.Value = aws.String(*instance.RunInstancesInput.UserData)
 		}
-		output.UserData = &ec2.AttributeValue{Value: &val}
 
 	case ec2.InstanceAttributeNameDisableApiTermination:
 		// Read under the manager lock so we serialise with a concurrent

spinifex/handlers/ec2/instance/service_impl_test.go

@@ -4822,3 +4822,35 @@ func TestPrepareRunInstances_PreCreatedENIAttachErrorSurfaced(t *testing.T) {
 	assert.Equal(t, 1, eni.attachCalls)
 	assert.Equal(t, 0, eni.createCalls)
 }
+
+// TestDescribeInstanceAttribute_UserDataUnsetCarriesNoValue pins the shape AWS
+// returns for an instance launched without user data: the attribute is present
+// and its Value is absent. The Terraform provider reads user_data only when
+// Value is non-nil, so an empty string makes it record the hash of "" and
+// propose a change on every plan.
+func TestDescribeInstanceAttribute_UserDataUnsetCarriesNoValue(t *testing.T) {
+	const id = "i-userdata-unset"
+	const owner = "000000000000"
+
+	for _, tc := range []struct {
+		name  string
+		input *ec2.RunInstancesInput
+	}{
+		{name: "no RunInstancesInput", input: nil},
+		{name: "nil UserData", input: &ec2.RunInstancesInput{}},
+		{name: "empty UserData", input: &ec2.RunInstancesInput{UserData: aws.String("")}},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			v := &vm.VM{ID: id, AccountID: owner, RunInstancesInput: tc.input}
+			svc := &InstanceServiceImpl{vmMgr: mgrWith(map[string]*vm.VM{id: v})}
+
+			out, err := svc.DescribeInstanceAttribute(context.Background(), &ec2.DescribeInstanceAttributeInput{
+				InstanceId: aws.String(id),
+				Attribute:  aws.String(ec2.InstanceAttributeNameUserData),
+			}, owner)
+			require.NoError(t, err)
+			require.NotNil(t, out.UserData, "AWS returns the attribute itself even when it holds nothing")
+			assert.Nil(t, out.UserData.Value, "an instance with no user data carries no Value, not an empty one")
+		})
+	}
+}