Mock external dependencies in golang - unit-testing

Question

I have a program in go that connects to AWS S3 and gets a file.
I'd like to write some tests for it, but I'd like to know, more generally, how to do these mocks in Golang. I know there are some libraries to create mocks but if I remember correctly I read someone suggesting using only standard libraries for unit tests was the best way to go.
So, how would you test a function like this?
func (s S3Input) Sample(key string) ([]byte, error) {
var buf []byte
waBuf := aws.NewWriteAtBuffer(buf)
_, err := s.Downloader.Download(
waBuf,
&s3.GetObjectInput{
Bucket: aws.String(s.Bucket),
Key: aws.String(key),
},
)
if err != nil {
return nil, err
}
return buf, nil
}
Thank you!

Answer 1

One way to do it is to inject the dependencies in your structure, like such:
type S3Inputer interface {
NewWriteAtBuffer(buf []byte) *aws.WriteAtBuffer
String(v string) *string
}
type S3Input struct {
newWriteAtBufferFunc func(buf []byte) *aws.WriteAtBuffer
stringFunc func(v string) *string
}
func (s *S3Input) NewWriteAtBuffer(buf []byte) *WriteAtBuffer {
return s.newWriteAtBufferFunc(buf)
}
func (s *S3Input) String(v string) *string {
return s.stringFunc(v)
}
func (s S3Input) Sample(key string) ([]byte, error) {
var buf []byte
waBuf := s.NewWriteAtBuffer(buf)
_, err := s.Downloader.Download(
waBuf,
&s3.GetObjectInput{
Bucket: s.String(s.Bucket),
Key: s.String(key),
},
)
if err != nil {
return nil, err
}
return buf, nil
}
func main() {
s := &S3Input{
StringFunc: aws.String,
NewWriteAtBufferFunc: aws.NewWriteAtBuffer,
}
// ...
}
This allows you to replace those functions with whatever you want for testing, without the need of any testing framework.
Then, the testing function would look something like this:
func (s S3Input) TestSample(t *testing.T) {
s3Mock := &S3Input{
StringFunc: (func (v string) *string {
return nil
}),
NewWriteAtBufferFunc: (func (buf []byte) *aws.WriteAtBuffer {
return nil
}),
}
res, err := s3Mock.Sample(...) //
// asserts & error checks
}
You could improve it by creating a S3InputMock type instead of reusing the base one, both would implement the S3Inputer interface and your mock could have attributes allowing it to help you with testing. For example, it could count the number of times a function is called, store the arguments it received, have its methods behave differently depending on the attributes you set for easier testing, etc.