How to test os.exit scenarios in Go

Question

Given this code

func doomed() {
  os.Exit(1)
}

How do I properly test that calling this function will result in an exit using go test? This needs to occur within a suite of tests, in other words the os.Exit() call cannot impact the other tests and should be trapped.

Answer 1

There's a presentation by Andrew Gerrand (one of the core members of the Go team) where he shows how to do it.

Given a function (in main.go)

package main

import (
    "fmt"
    "os"
)

func Crasher() {
    fmt.Println("Going down in flames!")
    os.Exit(1)
}

here's how you would test it (through main_test.go):

package main

import (
    "os"
    "os/exec"
    "testing"
)

func TestCrasher(t *testing.T) {
    if os.Getenv("BE_CRASHER") == "1" {
        Crasher()
        return
    }
    cmd := exec.Command(os.Args[0], "-test.run=TestCrasher")
    cmd.Env = append(os.Environ(), "BE_CRASHER=1")
    err := cmd.Run()
    if e, ok := err.(*exec.ExitError); ok && !e.Success() {
        return
    }
    t.Fatalf("process ran with err %v, want exit status 1", err)
}

What the code does is invoke go test again in a separate process through exec.Command, limiting execution to the TestCrasher test (via the -test.run=TestCrasher switch). It also passes in a flag via an environment variable (BE_CRASHER=1) which the second invocation checks for and, if set, calls the system-under-test, returning immediately afterwards to prevent running into an infinite loop. Thus, we are being dropped back into our original call site and may now validate the actual exit code.

Source: Slide 23 of Andrew's presentation. The second slide contains a link to the presentation's video as well. He talks about subprocess tests at 47:09