Go Test Fuzz

In this tutorial, you'll learn about Go Test: Fuzzing. We cover key concepts, practical examples, and best practices.

Fuzz testing -- Use Go's built-in fuzz testing to automatically discover edge cases and bugs.

The Problem

Fuzz testing generates random inputs and feeds them to your function. Go 1.18+ has built-in fuzz support with the Fuzz testing function.

Wrong

func TestReverse(t *testing.T) {
    // Testing with known cases but missing edge cases
    tests := []struct{ in, want string }{
        {"hello", "olleh"},
        {"", ""},
    }
}

Output:

// May miss edge cases like unicode, control chars

Right

func FuzzReverse(f *testing.F) {
    testcases := []string{"hello", "world", " "}
    for _, tc := range testcases {
        f.Add(tc) // Seed corpus
    }
    f.Fuzz(func(t *testing.T, orig string) {
        rev := Reverse(orig)
        doubleRev := Reverse(rev)
        if orig != doubleRev {
            t.Errorf("Before: %q, after: %q", orig, doubleRev)
        }
    })
}

Output:

$ go test -fuzz=FuzzReverse
// Finds unicode issues automatically

Prevention

• Fuzz tests use FuzzXxx naming
• Add seed corpus with f.Add()
• Run with go test -fuzz=FuzzName
• Crashes saved to testdata/fuzz/
• Run continuously until cancelled (Ctrl+C)

Common Mistakes with test fuzz

1. Misunderstanding that String is [Char] with poor performance for large text operations
2. Using foldl instead of foldl' causing stack overflow on large lists
3. Forgetting deriving (Show, Eq) on custom data types needed for debugging

These mistakes appear frequently in real-world GO code. DodaTech's contributors have identified these patterns through analysis of open-source projects and production systems.

Practice Exercise

Write a pure function that safely divides two integers using Maybe, then test it with edge cases like division by zero and negative numbers.

This exercise reinforces the concepts covered in this guide. Try implementing it before checking online solutions.

FAQ

Built by the developers of Doda Browser, DodaZIP, and Durga Antivirus Pro. DodaTech tutorials help Go developers build production-ready software used by millions.