How to test the ioutil.ReadFile and os.Stat? - unit-testing

Question

I have the following function:
func GetDataFromFile(path string) ([]byte, error) {
_, err := os.Stat(path)
if err != nil {
return nil, err
}
data, err := ioutil.ReadFile(path)
if err != nil {
return nil, err
}
return data, nil
}
I want to do tests for functions ioutil.ReadFile and os.Stat(path) when they throw errors.
I know that I can create non-exist path for os.Stat(path), but how to test such kind functions without "workarounds" and guessing how functions are working?
Regards.

Answer 1

I agree with abhink here, I would not expect you to test this particular function. But in practice, similar situation happens often.
My best solution is to use a factory to create GetDataFromFile. In this case, you inject the dependencies.
main.go
package main
import (
"io/ioutil"
"os"
)
func getDataFromFileFactory(
stat func(filename string) (os.FileInfo, error),
readFile func(filename string) ([]byte, error),
) func(path string) ([]byte, error) {
return func(path string) ([]byte, error) {
_, err := stat(path)
if err != nil {
return nil, err
}
data, err := readFile(path)
if err != nil {
return nil, err
}
return data, nil
}
}
var GetDataFromFile = getDataFromFileFactory(os.Stat, ioutil.ReadFile)
func main() {}
main_test.go
package main
import (
"errors"
"os"
"testing"
)
func TestGetDataFromFile(t *testing.T) {
stat := func(filename string) (os.FileInfo, error) {
return nil, errors.New("err msg")
}
readfile := func(filename string) ([]byte, error) {
t.Error("should not call this function")
return nil, nil
}
getDataFromFile := getDataFromFileFactory(stat, readfile)
if _, err := getDataFromFile("foo"); err.Error() != "err msg" {
t.Error("expected an error to be thrown")
}
}

Answer 2

Is there any specific reason you want to test these library functions?
You should only concern yourself with testing your own code and logic. Libraries are tested by those who create and maintain them and you should use them fully expecting to perform as per their documentation. Any genuine deviation from expected behavior should be reported to the authors/maintainers of the library.
As for testing GetDataFromFile, it would be perfectly legitimate to test it by supplying incorrect path. For more specific errors you can also read up on the input parameters that would cause them and call the functions with those arguments.