How Do You Test `os.Exit` Behavior in Go Test Suites?

Testing os.Exit Scenarios in Go

When testing Go functions that invoke os.Exit(), a unique approach is required to prevent these exits from interfering with subsequent tests.

To address this, consider the following code:

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

How to Properly Test os.Exit Behavior Using Go Test

To test the exit behavior of this function, we can leverage a technique outlined by Andrew Gerrand, a core member of the Go team:

1. Create a Separate Process for Testing:
   Invoke go test again in a separate process using exec.Command. Limit execution to the test that triggers os.Exit.
2. Pass an Environment Variable:
   Set an environment variable (e.g., BE_CRASHER=1) in the subprocess. The tested function can check for this variable and call os.Exit only if set.
3. Validate the Exit Code:
   Once the subprocess exits, validate the exit code in the original test process. If it matches the expected value (e.g., 1), the test has succeeded.

Example Test Code:

package main

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

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

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)
}

By separating testing processes and using environment variables to control execution, you can effectively test os.Exit scenarios in your Go test suites without affecting other tests.

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