How to Test Functions That Call os.Exit() in Go?

Testing os.Exit Scenarios in Go

When writing tests that involve calling functions which exit the program using os.Exit(), it becomes necessary to isolate their impact on the rest of the test suite. To address this challenge, we can leverage the following approach inspired by a presentation by Andrew Gerrand, a core Go team member.

Given a function that terminates the program via os.Exit():

package main

import (
    "fmt"
    "os"
)

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

Create a corresponding test case:

package main

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

func TestCrasher(t *testing.T) {
    // Check if the BE_CRASHER environment variable is set to 1.
    if os.Getenv("BE_CRASHER") == "1" {
        Crasher()
        return
    }

    // Construct a command to re-run the test binary, limiting its execution to TestCrasher.
    cmd := exec.Command(os.Args[0], "-test.run=TestCrasher")
    // Set additional environment variables.
    cmd.Env = append(os.Environ(), "BE_CRASHER=1")

    // Execute the command.
    err := cmd.Run()
    // Verify the exit status of the command.
    if e, ok := err.(*exec.ExitError); ok && !e.Success() {
        return
    }

    // Report failure if the command ran with an unexpected exit code.
    t.Fatalf("process ran with err %v, want exit status 1", err)
}

This test case re-invokes go test in a separate process, isolating the execution of TestCrasher from the rest of the suite. It also sets an environment variable (BE_CRASHER=1) that the invoked process checks for and, if present, calls the function under test. Thus, we avoid infinite loops and ensure that the correct exit code is validated.

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