Using mockgen in reflect mode

Question

I am trying to write some tests for an existing library but cannot get gomock to work

I would like to test behaviour if rand.Read fails. Here is an example of what I would like to test. I would like to see the log.Error line executing in test

import (
     "crypto/rand"
)
    func GetRandomBytes(n int) ([]byte, error) {

        b := make([]byte, n)
        _, err := rand.Read(b)

        if err != nil {
            log.Error("Failed to get entropy from system", err)
            return nil, err
        }

        return b, nil
    }

The gomock system should let me force the "rand.Read" call to do the right thing

However, I cannot get the mockgen tool to work in "reflect" mode

$ $GOPATH/bin/mockgen 'crypto/rand' Read
# command-line-arguments
./prog.go:22:28: invalid indirect of "crypto/rand".Read (type func([]byte) (int, error))
2018/01/21 11:20:30 Loading input failed: exit status 2

I'm using go version go1.9.2 linux/amd64 on Ubuntu 14.04

genmock -prog_only 'crypto/rand' Read works fine, but the code it generates doesn't look useful for what I need to do

I don't think mockgen can be used to mock a global scope function. — leaf bebop, Jan 21 '18 at 12:20

score 2 · Accepted Answer · answered Jan 21 '18 at 22:25

I’ve had the same task recently. Also tried to mock, but finally make in another way.

You can make a function accepting Read function and returning your target function GetRandomBytes. Like:

func MakeRandomBytesGetter(fn func([]byte) (int,error)) func(int)([]bute,error) {
    return func(n int) ([]byte, error) {

        b := make([]byte, n)
        _, err := fn(b)

        if err != nil {
            log.Error("Failed to get entropy from system", err)
            return nil, err
        }

        return b, nil
    }
}

var (
    GetRamdomBytes = MakeRandomBytesGetter(rand.Read)
)

For tests you can use different functions with the same signature but another behavior. Like:

func FailRead(n int) ([]byte, error) {
    return []byte{}, fmt.Errorf("Read failed")
}

And check you function behavior when it gets such errors.

Looks good, I'll have to inform the owners of the function being checked. Once I've implemented the test I'll accept the answer, upvote etc etc — Vorsprung, Jan 22 '18 at 11:13

Using mockgen in reflect mode

1 Answers1