How to test os.exit scenarios in Go

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.

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

I do this by using bouk/monkey:

func TestDoomed(t *testing.T) {
  fakeExit := func(int) {
    panic("os.Exit called")      
  }
  patch := monkey.Patch(os.Exit, fakeExit)
  defer patch.Unpatch()
  assert.PanicsWithValue(t, "os.Exit called", doomed, "os.Exit was not called")
}

monkey is super-powerful when it comes to this sort of work, and for fault injection and other difficult tasks. It does come with some caveats.

I don't think you can test the actual os.Exit without simulating testing from the outside (using exec.Command) process.

That said, you might be able to accomplish your goal by creating an interface or function type and then use a noop implementation in your tests:

Go Playground

package main

import "os"
import "fmt"

type exiter func (code int)

func main() {
	doExit(func(code int){})
	fmt.Println("got here")
	doExit(func(code int){ os.Exit(code)})
}

func doExit(exit exiter) {
	exit(1)
}

You can't, you would have to use exec.Command and test the returned value.

Code for testing:

package main
import "os"

var my_private_exit_function func(code int) = os.Exit

func main() {
	MyAbstractFunctionAndExit(1)
}

func MyAbstractFunctionAndExit(exit int) {
	my_private_exit_function(exit)
}

Testing code:

package main

import (
    "os"
    "testing"
)

func TestMyAbstractFunctionAndExit(t *testing.T) {
	var ok bool = false // The default value can be omitted :)

	// Prepare testing
	my_private_exit_function = func(c int) {
		ok = true
	}
	// Run function
	MyAbstractFunctionAndExit(1)
	// Check
	if ok == false {
		t.Errorf("Error in AbstractFunction()")
	}
	// Restore if need
	my_private_exit_function = os.Exit
}

To test the os.Exit like scenarios we can use the https://github.com/undefinedlabs/go-mpatch along with the below code. This ensures that your code remains clean as well as readable and maintainable.

type PatchedOSExit struct {
	Called     bool
	CalledWith int
	patchFunc  *mpatch.Patch
}

func PatchOSExit(t *testing.T, mockOSExitImpl func(int)) *PatchedOSExit {
	patchedExit := &PatchedOSExit{Called: false}

	patchFunc, err := mpatch.PatchMethod(os.Exit, func(code int) {
		patchedExit.Called = true
		patchedExit.CalledWith = code

		mockOSExitImpl(code)
	})

	if err != nil {
		t.Errorf("Failed to patch os.Exit due to an error: %v", err)

		return nil
	}

	patchedExit.patchFunc = patchFunc

	return patchedExit
}

func (p *PatchedOSExit) Unpatch() {
	_ = p.patchFunc.Unpatch()
}

You can consume the above code as follows:

func NewSampleApplication() {
	os.Exit(101)
}

func Test_NewSampleApplication_OSExit(t *testing.T) {
	// Prepare mock setup
	fakeExit := func(int) {}

	p := PatchOSExit(t, fakeExit)
	defer p.Unpatch()

	// Call the application code
	NewSampleApplication()

	// Assert that os.Exit gets called
	if p.Called == false {
		t.Errorf("Expected os.Exit to be called but it was not called")
		return
	}

	// Also, Assert that os.Exit gets called with the correct code
	expectedCalledWith := 101

	if p.CalledWith != expectedCalledWith {
		t.Errorf("Expected os.Exit to be called with %d but it was called with %d", expectedCalledWith, p.CalledWith)
		return
	}
}

I've also added a link to Playground: https://go.dev/play/p/FA0dcwVDOm7

In my code I've just used

func doomedOrNot() int {
  if (doomed) {
    return 1
  }
  return 0
}

then calling it like:

if exitCode := doomedOrNot(); exitCode != 0 {
  os.Exit(exitCode)
}

This way doomedOrNot can be tested easily.