Dependency injection compared to global interfaces [closed] - unit-testing

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One of the reasons I see for preferring dependency injection over global variables is that it becomes difficult to mock a global during testing. But suppose I declare the global as an interface, I can avoid this problem.
package restclient
type HTTPClient interface {
Do(req *http.Request) (*http.Response, error)
}
var (
Client HTTPClient
)
func init() {
Client = &http.Client{}
}
In the above example, I am creating a global interface and initializing it to an http client. So whenever I want to make a REST call, I make use of this interface.
Now In my test cases, if I want to mock the http client, all I need to do is
restclient.Client = new(RestclientMock)
So wouldn't this approach of using global interfaces be much better than using dependency injection considering the simplicity?

Answer 1

The problem with this approach regarding testing starts to become clear when thinking of multiple tests running in parallel.
One test will set the global variable to be able to test with it and while testing, it gets overwritten by the next test, resulting in unpredictable behaviour for both tests. They now influence each other, using the same instance of the dependency.
Note: Actually, you can even experience the issue without enabling parallel execution of tests. Especially if the dependency has internal goroutines that could live beyond the execution of the test they were created for.