How to Test `os.Exit` Scenarios in Go Without Disrupting the Test Suite?

Unit Testing Exit Scenarios in Go

Covering various test scenarios is crucial for robust software development. In Go, testing functions that call os.Exit presents a unique challenge. How can we test exit scenarios without interfering with the entire test suite?

Consider the following function, doomed:

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

To effectively test that invoking this function triggers an exit, you'll need to employ a strategy that confines the os.Exit call within the test and prevents it from affecting other tests.

Testing os.Exit

Andrew Gerrand, a core member of the Go team, provides an ingenious approach to this problem in his presentation. Let's implement this solution:

main.go (Program with function to be tested)

package main

import (
    "fmt"
    "os"
)

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

main_test.go (Test for Crasher function)

package main

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

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

    // Invoke the test again with "BE_CRASHER=1" to trigger the Crasher call.
    cmd := exec.Command(os.Args[0], "-test.run=TestCrasher")
    cmd.Env = append(os.Environ(), "BE_CRASHER=1")
    err := cmd.Run()

    // Validate exit code.
    if e, ok := err.(*exec.ExitError); ok && !e.Success() {
        return
    }
    t.Fatalf("process ran with err %v, want exit status 1", err)
}

Execution:

This solution works by:

• Invoking go test again in a separate process via exec.Command, limiting execution to the TestCrasher test.
• Passing a flag via an environment variable (BE_CRASHER=1) to control the behavior of the second invocation.
• Checking for the flag and, if set, calling Crasher and exiting immediately.
• Validating the exit code to ensure that the test passed as expected.

Using this approach, you can confidently test exit scenarios in Go without disrupting the rest of your test suite.

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