Go Testing Code Review Skill

Go Testing Code Review

Review Workflow

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

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).
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.
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.
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

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

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