Agent Skills: Go Testing Code Review

Go Testing Code Review

Quick Reference

| Issue Type | Reference | |------------|-----------| | Test structure, naming | references/structure.md | | Mocking, interfaces | references/mocking.md |

Review Checklist

• [ ] Tests are table-driven with clear case names
• [ ] Subtests use t.Run for parallel execution
• [ ] Test names describe behavior, not implementation
• [ ] Errors include got/want with descriptive message
• [ ] Cleanup registered with t.Cleanup
• [ ] Parallel tests don't share mutable state
• [ ] Mocks use interfaces defined in test file
• [ ] Coverage includes edge cases and error paths
• [ ] Performance-critical functions have Benchmark* tests
• [ ] Input parsers/validators have Fuzz* tests (Go 1.18+)
• [ ] HTTP handlers tested with httptest.NewRequest/httptest.NewRecorder
• [ ] Golden file tests use testdata/*.golden pattern with -update flag

Critical Patterns

Table-Driven Tests

// BAD - repetitive
func TestAdd(t *testing.T) {
    if Add(1, 2) != 3 {
        t.Error("wrong")
    }
    if Add(0, 0) != 0 {
        t.Error("wrong")
    }
}

// GOOD
func TestAdd(t *testing.T) {
    tests := []struct {
        name     string
        a, b     int
        want     int
    }{
        {"positive numbers", 1, 2, 3},
        {"zeros", 0, 0, 0},
        {"negative", -1, 1, 0},
    }

    for _, tt := range tests {
        t.Run(tt.name, func(t *testing.T) {
            got := Add(tt.a, tt.b)
            if got != tt.want {
                t.Errorf("Add(%d, %d) = %d, want %d", tt.a, tt.b, got, tt.want)
            }
        })
    }
}

Error Messages

// BAD
if got != want {
    t.Error("wrong result")
}

// GOOD
if got != want {
    t.Errorf("GetUser(%d) = %v, want %v", id, got, want)
}

// For complex types
if diff := cmp.Diff(want, got); diff != "" {
    t.Errorf("GetUser() mismatch (-want +got):\n%s", diff)
}

Parallel Tests

func TestFoo(t *testing.T) {
    tests := []struct{...}

    for _, tt := range tests {
        tt := tt  // capture (not needed Go 1.22+)
        t.Run(tt.name, func(t *testing.T) {
            t.Parallel()
            // test code
        })
    }
}

Cleanup

// BAD - manual cleanup, skipped on failure
func TestWithTempFile(t *testing.T) {
    f, _ := os.CreateTemp("", "test")
    defer os.Remove(f.Name())  // skipped if test panics
}

// GOOD
func TestWithTempFile(t *testing.T) {
    f, _ := os.CreateTemp("", "test")
    t.Cleanup(func() {
        os.Remove(f.Name())
    })
}

Additional Patterns

Benchmarks

func BenchmarkProcess(b *testing.B) {
    data := generateTestData(1000)
    b.ResetTimer()

    for i := 0; i < b.N; i++ {
        Process(data)
    }
}

// Run: go test -bench=BenchmarkProcess -benchmem

Fuzz Tests (Go 1.18+)

func FuzzParseInput(f *testing.F) {
    // Seed corpus
    f.Add(`{"name": "test"}`)
    f.Add(``)
    f.Add(`{invalid}`)

    f.Fuzz(func(t *testing.T, input string) {
        result, err := ParseInput(input)
        if err != nil {
            return // invalid input is expected
        }
        // If parsing succeeded, re-encoding should work
        if _, err := json.Marshal(result); err != nil {
            t.Errorf("Marshal after Parse: %v", err)
        }
    })
}

// Run: go test -fuzz=FuzzParseInput -fuzztime=30s

HTTP Handler Tests

func TestHandler(t *testing.T) {
    srv := NewServer(mockDeps)

    req := httptest.NewRequest("GET", "/api/users/123", nil)
    w := httptest.NewRecorder()

    srv.ServeHTTP(w, req)

    if w.Code != http.StatusOK {
        t.Errorf("status = %d, want %d", w.Code, http.StatusOK)
    }
}

Golden Files

var update = flag.Bool("update", false, "update golden files")

func TestRender(t *testing.T) {
    got := Render(input)
    golden := filepath.Join("testdata", t.Name()+".golden")

    if *update {
        if err := os.WriteFile(golden, got, 0644); err != nil {
            t.Fatalf("writing golden file: %v", err)
        }
    }

    want, err := os.ReadFile(golden)
    if err != nil {
        t.Fatalf("reading golden file: %v (run with -update to create)", err)
    }
    if !bytes.Equal(got, want) {
        t.Errorf("output mismatch:\ngot:\n%s\nwant:\n%s", got, want)
    }
}

Anti-Patterns

1. Testing Internal Implementation

// BAD - tests private state
func TestUser(t *testing.T) {
    u := NewUser("alice")
    if u.id != 1 {  // testing internal field
        t.Error("wrong id")
    }
}

// GOOD - tests behavior
func TestUser(t *testing.T) {
    u := NewUser("alice")
    if u.ID() != 1 {
        t.Error("wrong ID")
    }
}

2. Shared Mutable State

// BAD - tests interfere with each other
var testDB = setupDB()

func TestA(t *testing.T) {
    t.Parallel()
    testDB.Insert(...)  // race!
}

// GOOD - isolated per test
func TestA(t *testing.T) {
    db := setupTestDB(t)
    t.Cleanup(func() { db.Close() })
    db.Insert(...)
}

3. Assertions Without Context

// BAD
assert.Equal(t, want, got)  // "expected X got Y" - which test?

// GOOD
assert.Equal(t, want, got, "user name after update")

When to Load References

• Reviewing test file structure → structure.md
• Reviewing mock implementations → mocking.md

Review Questions

1. Are tests table-driven with named cases?
2. Do error messages include input, got, and want?
3. Are parallel tests isolated (no shared state)?
4. Is cleanup done via t.Cleanup?
5. Do tests verify behavior, not implementation?