How to mock GCP's storage in golang? - go

Question

I'm really new to mocking third-party library in go, I'm mocking cloud.google.com/go/storage right now
I'm using mockery. This is my current interface:
//Client storage client
type Client interface {
Bucket(name string) BucketHandle
Buckets(ctx context.Context, projectID string) BucketIterator
}
//BucketHandle storage's BucketHandle
type BucketHandle interface {
Attrs(context.Context) (*storage.BucketAttrs, error)
Objects(context.Context, *storage.Query) ObjectIterator
}
//ObjectIterator storage's ObjectIterator
type ObjectIterator interface {
Next() (*storage.ObjectAttrs, error)
}
//BucketIterator storage's BucketIterator
type BucketIterator interface {
Next() (*storage.BucketAttrs, error)
}
and this is how I use it in my function
//Runner runner for this module
type Runner struct {
StorageClient stiface.Client
}
.... function
//get storage client
client, err := storage.NewClient(ctx)
if err != nil {
return err
}
runner := Runner{
StorageClient: client,
}
.... rest of functions
However, I got this error:
cannot use client (type *"cloud.google.com/go/storage".Client) as type stiface.Client in field value:
*"cloud.google.com/go/storage".Client does not implement stiface.Client (wrong type for Bucket method)
have Bucket(string) *"cloud.google.com/go/storage".BucketHandle
want Bucket(string) stiface.BucketHandle
What have I done wrong here? Thanks!
Edit
here's one example of the code that I want to mock. I'd like to mock on bucketIterator.Next():
//GetBuckets get list of buckets
func GetBuckets(ctx context.Context, client *storage.Client, projectName string) []checker.Resource {
//Get bucket iterator based on a project
bucketIterator := client.Buckets(ctx, projectName)
//iterate over the buckets and store bucket details
buckets := make([]checker.Resource, 0)
for bucket, done := bucketIterator.Next(); done == nil; bucket, done = bucketIterator.Next() {
buckets = append(buckets, checker.Resource{
Name: bucket.Name,
Type: "Bucket",
})
}
return buckets
}

Answer 1

The error message is basically saying your stiface.Client defines an interface that *storage.Client does not implement. On first glance your code looks valid however the problem lies in your interface method signatures and because they have outputs as interfaces.
Go makes a difference between the statements:
This function returns a BucketHandle
and this function returns a *storage.BucketHandle that is a BucketHandle
Try changing your interface to return the *storage.BucketHandle. You can see a more complex example of similar behaviour in the mockery S3API example where the functions return the s3 types, not their own interfaces.

Answer 2

After some trial and error, the way you'd use stiface is as below
If you need to mock stiface.BucketIterator, you can create a mock as
type mockBucketIterator struct {
stiface.BucketIterator
}
and mock the Next accordingly
func (m mockBucketIterator) Next() (*storage.BucketAttrs, error) {
// mocks that you need this to return
return
}
You could use the same method to mock all the way up to satiface.Client and feed the mock client to your test.
For reference, a full example in my tests:
type clientMock struct {
stiface.Client
}
type bucketMock struct {
stiface.BucketHandle
}
type objectItMock struct {
stiface.ObjectIterator
i int
next []storage.ObjectAttrs
}
func (m clientMock) Bucket(name string) stiface.BucketHandle {
return bucketMock{}
}
and then the object iterator to return mocked iterator as well
func (it *objectItMock) Next() (a *storage.ObjectAttrs, err error) {
if it.i == len(it.next) {
err = iterator.Done
return
}
a = &it.next[it.i]
it.i += 1
return
}
func (m bucketMock) Objects(ctx context.Context, q *storage.Query) (it stiface.ObjectIterator) {
it = &objectItMock{
i: 0,
next: []storage.ObjectAttrs{
{Name: "abc"},
{Name: "def"},
{Name: "ghi"},
},
}
return
}