How Can I Achieve Accurate Test Coverage for Go Code Using `os.Exit()`?

Testing os.Exit Scenarios in Go with Coverage Information (coveralls.io/Goveralls)

This question pertains to testing scenarios involving os.Exit() in Go, particularly how to capture coverage information for these scenarios. The provided sample code demonstrates a re-exec method, but it suffers from limitations with coverage tooling and fragility.

Coverage Challenges

One difficulty is that coverage frameworks such as coveralls.io may not record the coverage of tests that evoke os.Exit(). This is because the coverage instrumentation is not applied when the test binary is re-executed.

Modified Re-exec Method

To address this issue, a modified re-exec method can be employed. The original code is refactored to extract os.Exit as a variable named osExit. The test code then replaces osExit with a mock function that records the exit code. This allows coverage information for osExit calls to be captured.

Coverage of External Calls

The same technique can be applied to cover calls to functions like log.Fatalf(), which indirectly invoke os.Exit. The logFatalf function is replaced with a mock function that records the message format and arguments passed to it. This ensures coverage of scenarios where os.Exit is called from external functions.

Example

The modified code and test are as follows:

// bar.go
package foo

import (
    "fmt"
    "os"
)

var osExit = os.Exit

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

// bar_test.go
package foo

import (
    "reflect"
    "testing"
)

var logFatalf = log.Fatalf

func TestCrasher(t *testing.T) {
    // Save original functions and restore at the end
    oldOsExit := osExit
    oldLogFatalf := logFatalf
    defer func() {
        osExit = oldOsExit
        logFatalf = oldLogFatalf
    }()

    // Mock osExit
    var got int
    myExit := func(code int) {
        got = code
    }
    osExit = myExit

    // Mock logFatalf
    var gotFormat string
    var gotV []interface{}
    myFatalf := func(format string, v ...interface{}) {
        gotFormat, gotV = format, v
    }
    logFatalf = myFatalf

    // Run test
    Crasher()

    // Check results
    exp := 1
    if got != exp {
        t.Errorf("Expected exit code: %d, got: %d", exp, got)
    }

    expFormat, expV := "Exiting with code: %d", []interface{}{1}
    if gotFormat != expFormat || !reflect.DeepEqual(gotV, expV) {
        t.Error("Something went wrong")
    }
}

By isolating the exit-related functionality and using mock functions, both os.Exit and external function calls that trigger os.Exit can be covered in Go tests. This approach ensures accurate coverage reporting for scenarios involving process termination.

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