Golang Testing

---
name: golang-testing
description: Go testing patterns including table-driven tests, subtests, benchmarks, fuzzing, and test coverage. Follows TDD methodology with idiomatic Go practices.
---

# Go Testing Patterns

> **Size budget: 28 KB** — `token-budget.mjs --check`.

Comprehensive Go testing patterns for writing reliable, maintainable tests following TDD
methodology.

## When to Activate

- Writing new Go functions or methods
- Adding test coverage to existing code
- Creating benchmarks for performance-critical code
- Implementing fuzz tests for input validation
- Following TDD workflow in Go projects

## TDD Workflow for Go

### The RED-GREEN-REFACTOR Cycle

```text
RED     → Write a failing test first
GREEN   → Write minimal code to pass the test
REFACTOR → Improve code while keeping tests green
REPEAT  → Continue with next requirement
```

### Step-by-Step TDD in Go

```go
// Step 1: Define the interface/signature
// calculator.go
package calculator

func Add(a, b int) int {
    panic("not implemented") // Placeholder
}

// Step 2: Write failing test (RED)
// calculator_test.go
package calculator

import "testing"

func TestAdd(t *testing.T) {
    got := Add(2, 3)
    want := 5
    if got != want {
        t.Errorf("Add(2, 3) = %d; want %d", got, want)
    }
}

// Step 3: Run test - verify FAIL
// $ go test
// --- FAIL: TestAdd (0.00s)
// panic: not implemented

// Step 4: Implement minimal code (GREEN)
func Add(a, b int) int {
    return a + b
}

// Step 5: Run test - verify PASS
// $ go test
// PASS

// Step 6: Refactor if needed, verify tests still pass
```

## Table-Driven Tests

The standard pattern for Go tests. Enables comprehensive coverage with minimal code.

```go
func TestAdd(t *testing.T) {
    tests := []struct {
        name     string
        a, b     int
        expected int
    }{
        {"positive numbers", 2, 3, 5},
        {"negative numbers", -1, -2, -3},
        {"zero values", 0, 0, 0},
        {"mixed signs", -1, 1, 0},
        {"large numbers", 1000000, 2000000, 3000000},
    }

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

### Table-Driven Tests with Error Cases

```go
func TestParseConfig(t *testing.T) {
    tests := []struct {
        name    string
        input   string
        want    *Config
        wantErr bool
    }{
        {
            name:  "valid config",
            input: `{"host": "localhost", "port": 8080}`,
            want:  &Config{Host: "localhost", Port: 8080},
        },
        {
            name:    "invalid JSON",
            input:   `{invalid}`,
            wantErr: true,
        },
        {
            name:    "empty input",
            input:   "",
            wantErr: true,
        },
        {
            name:  "minimal config",
            input: `{}`,
            want:  &Config{}, // Zero value config
        },
    }

    for _, tt := range tests {
        t.Run(tt.name, func(t *testing.T) {
            got, err := ParseConfig(tt.input)

            if tt.wantErr {
                if err == nil {
                    t.Error("expected error, got nil")
                }
                return
            }

            if err != nil {
                t.Fatalf("unexpected error: %v", err)
            }

            if !reflect.DeepEqual(got, tt.want) {
                t.Errorf("got %+v; want %+v", got, tt.want)
            }
        })
    }
}
```

## Subtests and Sub-benchmarks

### Organizing Related Tests

```go
func TestUser(t *testing.T) {
    // Setup shared by all subtests
    db := setupTestDB(t)

    t.Run("Create", func(t *testing.T) {
        user := &User{Name: "Alice"}
        err := db.CreateUser(user)
        if err != nil {
            t.Fatalf("CreateUser failed: %v", err)
        }
        if user.ID == "" {
            t.Error("expected user ID to be set")
        }
    })

    t.Run("Get", func(t *testing.T) {
        user, err := db.GetUser("alice-id")
        if err != nil {
            t.Fatalf("GetUser failed: %v", err)
        }
        if user.Name != "Alice" {
            t.Errorf("got name %q; want %q", user.Name, "Alice")
        }
    })

    t.Run("Update", func(t *testing.T) {
        // ...
    })

    t.Run("Delete", func(t *testing.T) {
        // ...
    })
}
```

### Parallel Subtests

```go
func TestParallel(t *testing.T) {
    tests := []struct {
        name  string
        input string
    }{
        {"case1", "input1"},
        {"case2", "input2"},
        {"case3", "input3"},
    }

    for _, tt := range tests {
        tt := tt // Capture range variable
        t.Run(tt.name, func(t *testing.T) {
            t.Parallel() // Run subtests in parallel
            result := Process(tt.input)
            // assertions...
            _ = result
        })
    }
}
```

## Test Helpers

### Helper Functions

```go
func setupTestDB(t *testing.T) *sql.DB {
    t.Helper() // Marks this as a helper function

    db, err := sql.Open("sqlite3", ":memory:")
    if err != nil {
        t.Fatalf("failed to open database: %v", err)
    }

    // Cleanup when test finishes
    t.Cleanup(func() {
        db.Close()
    })

    // Run migrations
    if _, err := db.Exec(schema); err != nil {
        t.Fatalf("failed to create schema: %v", err)
    }

    return db
}

func assertNoError(t *testing.T, err error) {
    t.Helper()
    if err != nil {
        t.Fatalf("unexpected error: %v", err)
    }
}

// assertEqual is a generic test helper for comparable types.
func assertEqual[T comparable](
    t *testing.T, got, want T,
) {
    t.Helper()
    if got != want {
        t.Errorf("got %v; want %v", got, want)
    }
}
```

### Temporary Files and Directories

```go
func TestFileProcessing(t *testing.T) {
    // Create temp directory - automatically cleaned up
    tmpDir := t.TempDir()

    // Create test file
    testFile := filepath.Join(tmpDir, "test.txt")
    err := os.WriteFile(testFile, []byte("test content"), 0644)
    if err != nil {
        t.Fatalf("failed to create test file: %v", err)
    }

    // Run test
    result, err := ProcessFile(testFile)
    if err != nil {
        t.Fatalf("ProcessFile failed: %v", err)
    }

    // Assert...
    _ = result
}
```

## Golden Files

Testing against expected output files stored in `testdata/`.

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

func TestRender(t *testing.T) {
    tests := []struct {
        name  string
        input Template
    }{
        {"simple", Template{Name: "test"}},
        {"complex", Template{Name: "test", Items: []string{"a", "b"}}},
    }

    for _, tt := range tests {
        t.Run(tt.name, func(t *testing.T) {
            got := Render(tt.input)

            golden := filepath.Join("testdata", tt.name+".golden")

            if *update {
                // Update golden file: go test -update
                err := os.WriteFile(golden, got, 0644)
                if err != nil {
                    t.Fatalf("failed to update golden file: %v", err)
                }
            }

            want, err := os.ReadFile(golden)
            if err != nil {
                t.Fatalf("failed to read golden file: %v", err)
            }

            if !bytes.Equal(got, want) {
                t.Errorf("output mismatch:\ngot:\n%s\nwant:\n%s", got, want)
            }
        })
    }
}
```

## Mocking with Interfaces

### Interface-Based Mocking

```go
// Define interface for dependencies
type UserRepository interface {
    GetUser(id string) (*User, error)
    SaveUser(user *User) error
}

// Production implementation
type PostgresUserRepository struct {
    db *sql.DB
}

func (r *PostgresUserRepository) GetUser(id string) (*User, error) {
    // Real database query
}

// Mock implementation for tests
type MockUserRepository struct {
    GetUserFunc  func(id string) (*User, error)
    SaveUserFunc func(user *User) error
}

func (m *MockUserRepository) GetUser(id string) (*User, error) {
    return m.GetUserFunc(id)
}

func (m *MockUserRepository) SaveUser(user *User) error {
    return m.SaveUserFunc(user)
}

// Test using mock
func TestUserService(t *testing.T) {
    mock := &MockUserRepository{
        GetUserFunc: func(id string) (*User, error) {
            if id == "123" {
                return &User{ID: "123", Name: "Alice"}, nil
            }
            return nil, ErrNotFound
        },
    }

    service := NewUserService(mock)

    user, err := service.GetUserProfile("123")
    if err != nil {
        t.Fatalf("unexpected error: %v", err)
    }
    if user.Name != "Alice" {
        t.Errorf("got name %q; want %q", user.Name, "Alice")
    }
}
```

## Benchmarks

### Basic Benchmarks

```go
func BenchmarkProcess(b *testing.B) {
    data := generateTestData(1000)
    b.ResetTimer() // Don't count setup time

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

// Run: go test -bench=BenchmarkProcess -benchmem
// Output: BenchmarkProcess-8   10000   105234 ns/op   4096 B/op   10 allocs/op
```

### Benchmark with Different Sizes

```go
func BenchmarkSort(b *testing.B) {
    sizes := []int{100, 1000, 10000, 100000}

    for _, size := range sizes {
        b.Run(fmt.Sprintf("size=%d", size), func(b *testing.B) {
            data := generateRandomSlice(size)
            b.ResetTimer()

            for i := 0; i < b.N; i++ {
                // Make a copy to avoid sorting already sorted data
                tmp := make([]int, len(data))
                copy(tmp, data)
                sort.Ints(tmp)
            }
        })
    }
}
```

### Memory Allocation Benchmarks

```go
func BenchmarkStringConcat(b *testing.B) {
    parts := []string{"hello", "world", "foo", "bar", "baz"}

    b.Run("plus", func(b *testing.B) {
        for i := 0; i < b.N; i++ {
            var s string
            for _, p := range parts {
                s += p
            }
            _ = s
        }
    })

    b.Run("builder", func(b *testing.B) {
        for i := 0; i < b.N; i++ {
            var sb strings.Builder
            for _, p := range parts {
                sb.WriteString(p)
            }
            _ = sb.String()
        }
    })

    b.Run("join", func(b *testing.B) {
        for i := 0; i < b.N; i++ {
            _ = strings.Join(parts, "")
        }
    })
}
```

## Fuzzing (Go 1.18+)

### Basic Fuzz Test

```go
func FuzzParseJSON(f *testing.F) {
    // Add seed corpus
    f.Add(`{"name": "test"}`)
    f.Add(`{"count": 123}`)
    f.Add(`[]`)
    f.Add(`""`)

    f.Fuzz(func(t *testing.T, input string) {
        var result map[string]interface{}
        err := json.Unmarshal([]byte(input), &result)

        if err != nil {
            // Invalid JSON is expected for random input
            return
        }

        // If parsing succeeded, re-encoding should work
        _, err = json.Marshal(result)
        if err != nil {
            t.Errorf("Marshal failed after successful Unmarshal: %v", err)
        }
    })
}

// Run: go test -fuzz=FuzzParseJSON -fuzztime=30s
```

### Fuzz Test with Multiple Inputs

```go
func FuzzCompare(f *testing.F) {
    f.Add("hello", "world")
    f.Add("", "")
    f.Add("abc", "abc")

    f.Fuzz(func(t *testing.T, a, b string) {
        result := Compare(a, b)

        // Property: Compare(a, a) should always equal 0
        if a == b && result != 0 {
            t.Errorf("Compare(%q, %q) = %d; want 0", a, b, result)
        }

        // Property: Compare(a, b) and Compare(b, a) should have opposite signs
        reverse := Compare(b, a)
        if (result > 0 && reverse >= 0) || (result < 0 && reverse <= 0) {
            if result != 0 || reverse != 0 {
                t.Errorf("Compare(%q, %q) = %d, Compare(%q, %q) = %d; inconsistent",
                    a, b, result, b, a, reverse)
            }
        }
    })
}
```

## Test Coverage

### Running Coverage

```bash
# Basic coverage
go test -cover ./...

# Generate coverage profile
go test -coverprofile=coverage.out ./...

# View coverage in browser
go tool cover -html=coverage.out

# View coverage by function
go tool cover -func=coverage.out

# Coverage with race detection
go test -race -coverprofile=coverage.out ./...
```

### Coverage Targets

| Code Type | Target |
|-----------|--------|
| Critical business logic | 100% |
| Public APIs | 90%+ |
| General code | 70%+ |
| Generated code | Exclude |

### Excluding Generated Code from Coverage

```go
//go:generate mockgen -source=interface.go -destination=mock_interface.go

// In coverage profile, exclude with build tags:
// go test -cover -tags=!generate ./...
```

## HTTP Handler Testing

```go
func TestHealthHandler(t *testing.T) {
    // Create request
    req := httptest.NewRequest(http.MethodGet, "/health", nil)
    w := httptest.NewRecorder()

    // Call handler
    HealthHandler(w, req)

    // Check response
    resp := w.Result()
    defer resp.Body.Close()

    if resp.StatusCode != http.StatusOK {
        t.Errorf("got status %d; want %d", resp.StatusCode, http.StatusOK)
    }

    body, _ := io.ReadAll(resp.Body)
    if string(body) != "OK" {
        t.Errorf("got body %q; want %q", body, "OK")
    }
}

func TestAPIHandler(t *testing.T) {
    tests := []struct {
        name       string
        method     string
        path       string
        body       string
        wantStatus int
        wantBody   string
    }{
        {
            name:       "get user",
            method:     http.MethodGet,
            path:       "/users/123",
            wantStatus: http.StatusOK,
            wantBody:   `{"id":"123","name":"Alice"}`,
        },
        {
            name:       "not found",
            method:     http.MethodGet,
            path:       "/users/999",
            wantStatus: http.StatusNotFound,
        },
        {
            name:       "create user",
            method:     http.MethodPost,
            path:       "/users",
            body:       `{"name":"Bob"}`,
            wantStatus: http.StatusCreated,
        },
    }

    handler := NewAPIHandler()

    for _, tt := range tests {
        t.Run(tt.name, func(t *testing.T) {
            var body io.Reader
            if tt.body != "" {
                body = strings.NewReader(tt.body)
            }

            req := httptest.NewRequest(tt.method, tt.path, body)
            req.Header.Set("Content-Type", "application/json")
            w := httptest.NewRecorder()

            handler.ServeHTTP(w, req)

            if w.Code != tt.wantStatus {
                t.Errorf("got status %d; want %d", w.Code, tt.wantStatus)
            }

            if tt.wantBody != "" && w.Body.String() != tt.wantBody {
                t.Errorf("got body %q; want %q", w.Body.String(), tt.wantBody)
            }
        })
    }
}
```

## Testing Commands

```bash
# Run all tests
go test ./...

# Run tests with verbose output
go test -v ./...

# Run specific test
go test -run TestAdd ./...

# Run tests matching pattern
go test -run "TestUser/Create" ./...

# Run tests with race detector
go test -race ./...

# Run tests with coverage
go test -cover -coverprofile=coverage.out ./...

# Run short tests only
go test -short ./...

# Run tests with timeout
go test -timeout 30s ./...

# Run benchmarks
go test -bench=. -benchmem ./...

# Run fuzzing
go test -fuzz=FuzzParse -fuzztime=30s ./...

# Count test runs (for flaky test detection)
go test -count=10 ./...
```

## Best Practices

**DO:**

- Write tests FIRST (TDD)
- Use table-driven tests for comprehensive coverage
- Test behavior, not implementation
- Use `t.Helper()` in helper functions
- Use `t.Parallel()` for independent tests
- Clean up resources with `t.Cleanup()`
- Use meaningful test names that describe the scenario

**DON'T:**

- Test private functions directly (test through public API)
- Use `time.Sleep()` in tests (use channels or conditions)
- Ignore flaky tests (fix or remove them)
- Mock everything (prefer integration tests when possible)
- Skip error path testing

## Integration with CI/CD

```yaml
# GitHub Actions example
test:
  runs-on: ubuntu-latest
  steps:
    - uses: actions/checkout@v4
    - uses: actions/setup-go@v5
      with:
        go-version: '1.22'

    - name: Run tests
      run: go test -race -coverprofile=coverage.out ./...

    - name: Check coverage
      run: |
        go tool cover -func=coverage.out | grep total | awk '{print $3}' | \
        awk -F'%' '{if ($1 < 80) exit 1}'
```

**Remember**: Tests are documentation. They show how your code is meant to be used. Write them
clearly and keep them up to date.

## Purpose

Principal-level Go test methodology: table-driven tests + subtests, parallel execution
(`t.Parallel()`), race detection, benchmarks (`testing.B`), fuzz testing (Go 1.18+), httptest +
testcontainers, coverage with branch tracking, go-cmp for deep equality.

**Negative scope** (NOT what this skill covers):

- Go language idioms — see `golang-patterns`
- Generic test taxonomy — see `testing.md` rule
- TDD methodology — see `tdd-workflow`
- Performance profiling — defer to project-specific
- Mock generation strategies — use `gomock` / `mockery` per project

## When NOT to use

- Pure CLI tools with simple I/O (lighter testing models work)
- Code-generation tools where snapshot tests beat unit tests
- BDD-style requirement tests (use Ginkgo if mandated)

## Standards Cited

- **Go Specification + testing package** (`pkg.go.dev/testing`) — canonical
- **Go Code Review Comments** (`go.dev/wiki/CodeReviewComments`) — test naming + organisation
- **Effective Go** (`golang.org/doc/effective_go`) — idioms applied to tests
- **github.com/google/go-cmp** — deep-equality helper
- **github.com/stretchr/testify** — assertion + suite helpers (use sparingly)
- **github.com/golang/mock** + **go.uber.org/mock** — mock generation
- **Testcontainers Go** (`golang.testcontainers.org`) — ephemeral DB / Redis / Kafka
- **Go 1.18+ Fuzz Testing** (`go.dev/security/fuzz/`)

## Anti-Patterns

| Pattern | Why bad | Correct alternative |
| --- | --- | --- |
| `TestFoo_Bar` (underscore in test name) | Sonar S100 violation; non-idiomatic Go | `func TestFoo(t *testing.T) { t.Run("Bar", func(t *testing.T) { ... }) }` |
| Test depends on order of execution | Brittle | Each `t.Run` self-contained; use `t.Parallel()` to surface order coupling |
| `go test ./...` without `-race` | Data races missed | CI always runs `-race`; locally for concurrent code |
| Mocking everything via `testify/mock` | Tests the mock, not the code | Prefer interface fakes + dependency injection |
| `reflect.DeepEqual(got, want)` | No diff output on failure | `cmp.Diff(want, got)` from `go-cmp` |
| `time.Sleep` for goroutine completion | Flaky | `WaitGroup` / channel-based sync |
| `for _, tc := range cases { ... }` (capture-in-loop bug pre-Go-1.22) | All iterations see last `tc` | `tc := tc` at top of body OR use Go 1.22+ semantics (per-iter scoping) |
| `t.Fatal()` deep in helper | Without `t.Helper()`, line points to helper | `t.Helper()` at top of helper functions |
| Hardcoded `time.Now()` in code-under-test | Tests can't control clock | Inject `Clock` interface (per `no-ambient-globals.md`) |
| Hardcoded ports / global state | Parallel tests collide | Random ports via `httptest.NewServer` |

## Verification Checklist

- [ ] `go test -race ./...` runs in CI
- [ ] `-coverprofile=coverage.out` with `go tool cover -func` checked; ≥ 90% touched
- [ ] Subtests via `t.Run("desc", func(t *testing.T) { ... })`
- [ ] `t.Helper()` at top of every test helper
- [ ] `t.Parallel()` used on independent subtests
- [ ] Fuzz targets for parsers / serializers (`go test -fuzz=FuzzFoo`)
- [ ] Benchmarks via `testing.B` with `b.ReportAllocs()`
- [ ] No `time.Sleep`; sync via channels / WaitGroup
- [ ] HTTP servers via `httptest.NewServer` (random port)
- [ ] Diff failures via `cmp.Diff(want, got)` from go-cmp
- [ ] Go 1.22+ per-iteration loop scoping OR explicit `tc := tc`

## Cross-References

- `~/.claude/skills/golang-patterns/SKILL.md` — code idioms under test
- `~/.claude/skills/tdd-workflow/SKILL.md` — RED-GREEN-REFACTOR
- `~/.claude/rules-library/common/testing.md` — coverage thresholds
- `~/.claude/rules-library/common/no-ambient-globals.md` — Clock / RNG injection
- `~/.claude/rules-library/golang/no-discards.md` — Go pattern hooks
- `~/.claude/rules-library/golang/testing.md` — Go-specific testing rule
- `~/.claude/agents/go-reviewer.md` — Go code review delegate
- `~/.claude/agents/tdd-guide.md`

## Why this skill exists

Go test idioms have a deceptive cleanliness: table-driven tests look simple but capture-in-loop bugs
(pre-Go-1.22), missing `t.Helper()`, and lack of `-race` produce hard-to-debug failures. The
patterns above codify the production-ready posture: subtests for navigation, parallel-where-safe for
speed, go-cmp for diff output, fuzz for parser invariants, testcontainers for integration. Following
them produces test suites that run in seconds and catch real bugs (not vibes).

## Compliance & Standards Mapping

- **ISO/IEC 25010:2011 §6** — Product quality model (Functional
  Suitability, Reliability, Performance Efficiency, Usability,
  Security, Maintainability, Portability, Compatibility)
- **ISO/IEC/IEEE 12207:2017 §6.4** — Software construction +
  verification + validation processes
- **NIST SP 800-218 SSDF §PW** — Produce Well-Secured Software
  (applies to every code-authoring skill)
- **NIST SP 800-53 Rev 5 §SA-11** — Developer testing +
  evaluation
- **OWASP ASVS 4.0.3 §V1.1** — Secure SDLC requirements
- **OWASP ASVS 4.0.3 §V14.2** — Dependency lifecycle
- **CWE Top 25 (2026)** — Weakness classes the patterns in this
  skill prevent
- **SLSA Framework v1.0 Build L2+** — Provenance + integrity

## Learning hooks

Per `~/.claude/rules/common/continuous-learning-mandate.md`:

**Signals to watch**:

- Test name uses `_` separator (`TestFoo_Bar`) instead of `t.Run("sub test", ...)` (S100 violation)
- Table-driven test entry not bound by index (sister `golang/no-discards.md` rule 2)
- `testify` mock used where a fake / stub would be simpler (over-mocking)
- `t.Fatal` used instead of `t.Errorf` when subsequent assertions should still run
- Race detector not enabled in CI (`-race` flag missing)
- Coverage gate below 80% on a package with active development
- Benchmarks added without `b.ResetTimer()` before measurement block
- Fuzz test missing on a parser / decoder / state machine
- `testify` `assert` used where `require` was needed (test continues with nil receiver, panics
  later)
- `t.Parallel()` skipped where tests are genuinely independent (slow suite)

**Refinement candidates**:

- New testing-package row when a new Go testing facility ships (synctest, fuzzing improvements)
- Tightening of the coverage floor on packages flagged as critical
- New cross-reference when a sister rule (golang/no-discards, testing) adds a per-language
  verification
- New table-driven idiom when generics improve subtest dispatch patterns