How to test Go function containing log.Fatal()

Question

Say, I had the following code that prints some log messages. How would I go about testing that the correct messages have been logged? As log.Fatal calls o

Answer 1

I've combined answers from different sources to produce this:

import (
    "bufio"
    "bytes"
    "errors"
    "fmt"
    "io/ioutil"
    "log"
    "os"
    "os/exec"
    "os/user"
    "strings"
    "testing"

    "bou.ke/monkey"
    "github.com/stretchr/testify/assert"
    "github.com/stretchr/testify/mock"
    "github.com/stretchr/testify/require"
)


func TestCommandThatErrors(t *testing.T) {
    fakeExit := func(int) {
        panic("os.Exit called")
    }
    patch := monkey.Patch(os.Exit, fakeExit)
    defer patch.Unpatch()

    var buf bytes.Buffer
    log.SetOutput(&buf)

    for _, tc := range []struct {
        cliArgs       []string
        expectedError string
    }{
        {
            cliArgs:       []string{"dev", "api", "--dockerless"},
            expectedError: "Some services don't have dockerless variants implemented yet.",
        },
    } {
        t.Run(strings.Join(tc.cliArgs, " "), func(t *testing.T) {
            harness := createTestApp()
            for _, cmd := range commands {
                cmd(harness.app)
            }

            assert.Panics(t, func() { harness.app.run(tc.cliArgs) })
            assert.Contains(t, buf.String(), tc.expectedError)
            buf.Reset()
        })
    }
}

Works great :)