go-testing-code-review

name	go-testing-code-review
description	Reviews Go test code for proper table-driven tests, assertions, and coverage patterns. Use when reviewing *_test.go files.

Go Testing Code Review

Review Workflow

Follow this sequence in order. Do not emit findings until every Pass below is satisfied.

1. Baseline go.mod — Open go.mod for the module under review and read the go directive.
   Pass: You can state the exact go X.YY value (in the review preamble or working notes). Apply version-gated advice only when it matches this baseline (e.g. fuzz tests Go 1.18+, loop-variable capture pre-Go 1.22).

2. Read surrounding tests — For each *_test.go (or benchmark/fuzz file) in scope, read full test functions and any table struct{...} / helpers they use, not only the diff hunk.
   Pass: At least one full func Test... / func Benchmark... / func Fuzz... (or helper it calls) containing the change was read per in-scope file.

3. Scope the checklist — Decide which Review Checklist rows apply (table-driven structure, parallelism, HTTP, golden files, mocks). Open references/structure.md and/or references/mocking.md for those topics; skip rows N/A to the diff with a one-line reason (e.g. “no t.Parallel in change”).
   Pass: The review (or working notes) lists which checklist themes you applied, or marks themes N/A with a diff-tied reason.

4. Pre-report verification — Load and follow review-verification-protocol.
   Pass: The protocol’s Pre-Report Verification Checklist is satisfied for each finding you will report (actual test code read, surrounding context checked, “wrong” vs “different style” distinguished, etc.).

Hard gates (same sequence, shorter)

Step	Objective pass condition
1	go X.YY from go.mod is recorded before version-specific test advice.
2	Full enclosing test (or helper it uses) read per in-scope test file, not diff-only.
3	In-scope checklist themes listed or N/A with diff-tied reason; references opened as needed.
4	review-verification-protocol completed for every reported issue.

Output Format

Report findings as:

[FILE:LINE] ISSUE_TITLE
Severity: Critical | Major | Minor | Informational
Description of the issue and why it matters.

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?