How does Go's built-in fuzzing work and when should you use property-based testing?

Go 1.18 added native fuzz testing via go test -fuzz. Fuzzing automatically generates inputs that exercise edge cases your hand-written tests miss — particularly effective for parsers, serialisers, and cryptographic code.

// Fuzz test — finds inputs that cause a panic or incorrect result
func FuzzParseURL(f *testing.F) {
    // Seed corpus: known interesting inputs
    f.Add("https://example.com/path?q=1")
    f.Add("http://user:pass@host:8080/")
    f.Add("")

    f.Fuzz(func(t *testing.T, s string) {
        // Property: parsing should never panic
        u, err := url.Parse(s)
        if err != nil { return } // error is ok, panic is not

        // Property: round-trip should be stable
        // parsing the string representation should give the same URL
        u2, err := url.Parse(u.String())
        if err != nil {
            t.Errorf("round-trip parse failed: %v", err)
        }
        if u.String() != u2.String() {
            t.Errorf("round-trip changed URL: %q → %q",
                u.String(), u2.String())
        }
    })
}

// Another fuzz example: JSON encode/decode round-trip
func FuzzJSONRoundTrip(f *testing.F) {
    f.Add(`{"name":"Alice","age":30}`)

    f.Fuzz(func(t *testing.T, data []byte) {
        var v map[string]any
        if err := json.Unmarshal(data, &v); err != nil { return }

        encoded, err := json.Marshal(v)
        if err != nil {
            t.Errorf("marshal failed after successful unmarshal: %v", err)
        }
        var v2 map[string]any
        if err := json.Unmarshal(encoded, &v2); err != nil {
            t.Errorf("second unmarshal failed: %v", err)
        }
    })
}

// Run fuzzing:
// go test -fuzz=FuzzParseURL                    # fuzz until stopped
// go test -fuzz=FuzzParseURL -fuzztime=60s      # fuzz for 60 seconds
// go test                                        # replays corpus only (CI)

When to use fuzzing: parsers (JSON, YAML, protobuf), network protocol handlers, cryptographic code, regular expression engines, any function that accepts arbitrary byte/string input. Fuzzing found critical bugs in Go's own standard library.

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