在函数中附加要切片的项不会更改原始切片

Question

Can someone explain why the two aren't equivalent? The latter does build, but doesn't work as expected. I thought slices would be changed automatically, as contain a pointer to the array.

// Working spec

func TestProcessRecords(t *testing.T) {
    var messageSent []*sqs.SendMessageInput
    w := &SQSWriter{
        queueURL: aws.String("aQueueURL"),
        service: &mock.SQS{
            SendMessageStub: func(input *sqs.SendMessageInput) (*sqs.SendMessageOutput, error) {
                messageSent = append(messageSent, input)
                return nil, nil
            },
        },
    }
    inputEvent := readFirehoseEventFromFile(t, "../../../../testdata/firehose_event.json")
    processRecords(inputEvent.Records, w)
    assert.Equal(t, 2, len(inputEvent.Records))
    assert.Equal(t, 1, len(messageSent))
}

Attempted refactoring, as the mockedWriter will be used across specs

// Not Working spec

func mockWriter(messageSent []*sqs.SendMessageInput) *SQSWriter{
    return &SQSWriter{
        queueURL: aws.String("aQueueURL"),
        service: &mock.SQS{
            SendMessageStub: func(input *sqs.SendMessageInput) (*sqs.SendMessageOutput, error) {
                messageSent = append(messageSent, input)
                return nil, nil
            },
        },
    }
}

func TestProcessRecords(t *testing.T) {
    messageSent := []*sqs.SendMessageInput{}
    inputEvent := readFirehoseEventFromFile(t, "../../../../testdata/firehose_event.json")
    processRecords(inputEvent.Records, mockWriter(messageSent))
    assert.Equal(t, 2, len(inputEvent.Records))
    assert.Equal(t, 1, len(messageSent))
}

I should mention that I'm coming from a background in JS/Ruby/Python, and it is taking a bit of time to get a firmer grasp of go fundamentals.

Thanks in advance

Answer 1

This was the answer, all credits to @mkopriva.

func mockWriter(messageSent *[]*sqs.SendMessageInput) *SQSWriter{
    return &SQSWriter{
        queueURL: aws.String("aQueueURL"),
        service: &mock.SQS{
            SendMessageStub: func(input *sqs.SendMessageInput) (*sqs.SendMessageOutput, error) {
                *messageSent = *append(messageSent, input)
                return nil, nil
            },
        },
    }
}

func TestProcessRecords(t *testing.T) {
    messageSent := []*sqs.SendMessageInput{}
    inputEvent := readFirehoseEventFromFile(t, "../../../../testdata/firehose_event.json")
    processRecords(inputEvent.Records, mockWriter(&messageSent))
    assert.Equal(t, 2, len(inputEvent.Records))
    assert.Equal(t, 1, len(messageSent))
}