go-testing-mistakes

name

go-testing-mistakes

description

Guides the agent to avoid common Go testing mistakes when writing or reviewing test code. Covers test categorization (build tags, env vars, short mode), race detection, test execution modes (parallel, shuffle), table-driven tests, avoiding sleeps in tests, handling the time API deterministically, using httptest/iotest utility packages, writing accurate benchmarks, and leveraging Go testing features (coverage, external test packages, utility functions, setup/teardown). Use when writing, reviewing, or refactoring Go tests, benchmarks, or test helpers, or when setting up CI test pipelines for Go projects.

Go Testing Mistakes

Rules and patterns for robust, accurate Go testing based on common mistakes #82-#90.

---

1. Categorize Tests (#82)

Rule: Always categorize tests so unit, integration, and long-running tests can be run independently. Use one or more of these three mechanisms.

Build tags (file-level)

Use //go:build tags to separate integration tests from unit tests.

// File: db_test.go
//go:build integration

package db

func TestInsert(t *testing.T) {
    // ...
}

• go test ./... runs only files without build tags.
• go test --tags=integration ./... includes files tagged integration.
• To run only integration tests, add //go:build !integration to unit test files.

Environment variables (test-level)

Prefer this when you want skipped tests to appear explicitly in output.

func TestInsert(t *testing.T) {
    if os.Getenv("INTEGRATION") != "true" {
        t.Skip("skipping integration test")
    }
    // ...
}

Short mode (speed-based)

Use testing.Short() to skip long-running tests during fast feedback loops.

func TestLongRunning(t *testing.T) {
    if testing.Short() {
        t.Skip("skipping long-running test")
    }
    // ...
}

Run with go test -short ./....

Combine approaches: Use build tags or env vars for test kind (unit vs integration) and short mode for test speed.

---

2. Enable the -race Flag (#83)

Rule: Always run go test -race ./... in local development and CI. The race detector finds real data races at runtime with zero false positives.

Overhead: Memory 5-10x, execution 2-20x. Do not enable in production binaries (canary releases excepted).

Key points:

• The race detector can produce false negatives. Loop concurrent test logic to increase detection probability:

func TestDataRace(t *testing.T) {
    for i := 0; i < 100; i++ {
        // concurrent logic under test
    }
}

• To exclude a file from race detection, use //go:build !race.

---

3. Use Test Execution Modes (#84)

Parallel execution

Mark independent tests with t.Parallel() to speed up long-running test suites.

func TestA(t *testing.T) {
    t.Parallel()
    // ...
}

• Sequential tests run first; parallel tests are paused then resumed together.
• Default max parallel = GOMAXPROCS. Override with go test -parallel 16 ./....

Shuffle mode

Use -shuffle=on to detect hidden inter-test dependencies.

go test -shuffle=on -v ./...

• Reproduce a specific order with the seed: go test -shuffle=1636399552801504000 -v ./...
• The seed is printed in verbose output.

---

4. Use Table-Driven Tests (#85)

Rule: When multiple tests share the same structure (call, compare, report), use table-driven tests with subtests.

func TestRemoveNewLineSuffix(t *testing.T) {
    tests := map[string]struct {
        input    string
        expected string
    }{
        "empty":                  {input: "", expected: ""},
        "ending with \\r\\n":     {input: "a\r\n", expected: "a"},
        "ending with \\n":        {input: "a\n", expected: "a"},
        "ending with multiple":   {input: "a\n\n\n", expected: "a"},
        "no newline":             {input: "a", expected: "a"},
    }

    for name, tt := range tests {
        t.Run(name, func(t *testing.T) {
            got := removeNewLineSuffixes(tt.input)
            if got != tt.expected {
                t.Errorf("got: %s, expected: %s", got, tt.expected)
            }
        })
    }
}

Parallel subtests: When combining table-driven tests with t.Parallel(), shadow the loop variable to avoid closure capture bugs:

for name, tt := range tests {
    tt := tt // shadow to capture current value
    t.Run(name, func(t *testing.T) {
        t.Parallel()
        // use tt safely
    })
}

Note: Go 1.22+ fixes the loop variable capture issue, but shadowing remains safe for backward compatibility.

---

5. Never Sleep in Unit Tests (#86)

Rule: Never use time.Sleep as a synchronization mechanism in tests. It causes flakiness.

Option A: Retry with polling (acceptable)

func assertEventually(t *testing.T, assertion func() bool, maxRetries int, waitTime time.Duration) {
    t.Helper()
    for i := 0; i < maxRetries; i++ {
        if assertion() {
            return
        }
        time.Sleep(waitTime)
    }
    t.Fatal("assertion never became true")
}

Use short intervals with many retries instead of a single long sleep. Libraries like testify provide Eventually.

Option B: Channel synchronization (preferred)

type publisherMock struct {
    ch chan []Foo
}

func (p *publisherMock) Publish(got []Foo) {
    p.ch <- got
}

func TestGetBestFoo(t *testing.T) {
    mock := publisherMock{ch: make(chan []Foo)}
    defer close(mock.ch)
    h := Handler{publisher: &mock, n: 2}

    foo := h.getBestFoo(42)
    // check foo ...

    select {
    case published := <-mock.ch:
        if len(published) != 2 {
            t.Fatalf("expected 2, got %d", len(published))
        }
    case <-time.After(time.Second):
        t.Fatal("timed out waiting for publish")
    }
}

Prefer synchronization over retries. If synchronization is impossible, reconsider the design.

---

6. Handle the time API Deterministically (#87)

Rule: Do not call time.Now() directly inside code under test. Inject the time dependency so tests are deterministic.

Option A: Inject a now function dependency

type now func() time.Time

type Cache struct {
    mu     sync.RWMutex
    events []Event
    now    now
}

func NewCache() *Cache {
    return &Cache{now: time.Now}
}

In tests, inject a fixed time:

cache := &Cache{now: func() time.Time {
    return parseTime(t, "2020-01-01T12:00:00.06Z")
}}

Option B: Accept time as a parameter (simpler, more limited)

func (c *Cache) TrimOlderThan(t time.Time) {
    // no internal time.Now() call
}

Caller computes the time; test passes a deterministic value.

Do not use a global var now = time.Now -- it prevents parallel test execution due to shared mutable state.

---

7. Use Testing Utility Packages (#88)

httptest

For testing HTTP handlers -- avoid real HTTP transport:

func TestHandler(t *testing.T) {
    req := httptest.NewRequest(http.MethodGet, "http://localhost", strings.NewReader("foo"))
    w := httptest.NewRecorder()
    Handler(w, req)

    if got := w.Result().Header.Get("X-API-VERSION"); got != "1.0" {
        t.Errorf("expected 1.0, got %s", got)
    }
    body, _ := io.ReadAll(w.Result().Body)
    if string(body) != "hello foo" {
        t.Errorf("unexpected body: %s", body)
    }
}

For testing HTTP clients -- use httptest.NewServer to spin up a local server with a stub handler, then pass srv.URL to the client under test. Call defer srv.Close(). Also available: httptest.NewTLSServer, httptest.NewUnstartedServer.

iotest

Test custom io.Reader correctness with iotest.TestReader(reader, expected).

Test resilience to I/O errors using wrappers:

Function	Behavior
iotest.ErrReader(err)	Always returns the given error
iotest.HalfReader(r)	Reads half as many bytes as requested
iotest.OneByteReader(r)	Reads one byte at a time
iotest.TimeoutReader(r)	Fails on second read, then recovers
iotest.TruncateWriter(w, n)	Silently stops after n bytes

Wrap readers/writers with these to verify error tolerance.

---

8. Write Accurate Benchmarks (#89)

Reset or pause the timer for setup

// One-time setup: reset before the loop
func BenchmarkFoo(b *testing.B) {
    expensiveSetup()
    b.ResetTimer()
    for i := 0; i < b.N; i++ {
        functionUnderTest()
    }
}

// Per-iteration setup: stop/start around it
func BenchmarkBar(b *testing.B) {
    for i := 0; i < b.N; i++ {
        b.StopTimer()
        data := expensiveSetup()
        b.StartTimer()
        functionUnderTest(data)
    }
}

Caveat: If the function under test is very fast relative to setup, the benchmark may take excessively long. Decrease -benchtime to mitigate.

Do not trust single micro-benchmark runs

• Micro-benchmarks are affected by machine activity, thermal scaling, cache alignment, and instruction ordering.
• Use -count=10 and benchstat for statistical comparison:

go test -bench=. -count=10 | tee stats.txt
benchstat stats.txt

• Use tools like perflock to limit CPU contention from background processes.

Prevent compiler optimizations from eliminating your benchmark

The compiler may inline and then dead-code-eliminate calls with no observable side effects.

// BAD: compiler may optimize away the call entirely
func BenchmarkPopcnt(b *testing.B) {
    for i := 0; i < b.N; i++ {
        popcnt(uint64(i))
    }
}

// GOOD: assign to local, then to global to prevent elimination
var globalResult uint64

func BenchmarkPopcnt(b *testing.B) {
    var v uint64
    for i := 0; i < b.N; i++ {
        v = popcnt(uint64(i))
    }
    globalResult = v
}

Pattern: (1) assign result to a local variable each iteration, (2) assign final value to a package-level global after the loop. Do not write directly to a global each iteration (heap writes are slower).

Beware the observer effect

Reusing the same data across benchmark iterations lets CPU caches warm up, skewing results for CPU-bound functions.

// BAD: matrix cached in L1 after first iterations
func BenchmarkCalcSum(b *testing.B) {
    s := createMatrix(rows)
    b.ResetTimer()
    for i := 0; i < b.N; i++ {
        calculateSum(s)
    }
}

// GOOD: fresh data each iteration prevents cache bias
func BenchmarkCalcSum(b *testing.B) {
    for i := 0; i < b.N; i++ {
        b.StopTimer()
        s := createMatrix(rows)
        b.StartTimer()
        calculateSum(s)
    }
}

---

9. Leverage All Go Testing Features (#90)

Code coverage

go test -coverprofile=coverage.out ./...
go tool cover -html=coverage.out

To include cross-package coverage (e.g., foo.go tested by bar_test.go):

go test -coverpkg=./... -coverprofile=coverage.out ./...

Do not chase 100% coverage mechanically -- reason about what your tests actually cover.

Test from an external package

Use package foo_test to enforce black-box testing that only exercises the public API. This prevents tests from depending on unexported internals.

Utility functions with *testing.T

Pass *testing.T to test helpers and call t.Fatal on error instead of returning errors. Mark helpers with t.Helper() so failure locations point to the caller.

func createCustomer(t *testing.T, name string) Customer {
    t.Helper()
    c, err := newCustomer(name)
    if err != nil {
        t.Fatal(err)
    }
    return c
}

Setup and teardown

Per test: Call setup directly, teardown via defer or t.Cleanup.

func TestMySQL(t *testing.T) {
    db := createConnection(t, dsn)
    // t.Cleanup registered inside createConnection closes db
    // ...
}

func createConnection(t *testing.T, dsn string) *sql.DB {
    t.Helper()
    db, err := sql.Open("mysql", dsn)
    if err != nil {
        t.FailNow()
    }
    t.Cleanup(func() { _ = db.Close() })
    return db
}

Multiple t.Cleanup calls execute LIFO (like defer).

Per package: Use TestMain.

func TestMain(m *testing.M) {
    setup()
    code := m.Run()
    teardown()
    os.Exit(code)
}

---

Quick Reference

#	Mistake	Rule
82	Not categorizing tests	Use build tags, env vars, and/or -short to separate test kinds and speeds
83	No -race flag	Always go test -race in dev and CI
84	Ignoring execution modes	Use t.Parallel() for speed; -shuffle=on to detect hidden dependencies
85	No table-driven tests	Use map/slice + t.Run subtests; shadow loop vars for parallel subtests
86	Sleeping in tests	Use channel sync (preferred) or retry/poll; never bare time.Sleep
87	Brittle time API usage	Inject now as a dependency or accept time.Time as parameter
88	Not using httptest/iotest	httptest for handler/client tests; iotest for reader correctness and error resilience
89	Inaccurate benchmarks	Reset timer for setup; use -count+benchstat; prevent compiler elision; avoid observer effect
90	Missing testing features	Use -coverpkg, _test packages, t.Helper, t.Cleanup, TestMain