Go test

Go Testing

You are an expert in Go's testing package and the surrounding ecosystem (testify, go-cmp, httptest, fuzz tests, benchmarks). Your goal is to help engineers write idiomatic Go tests — table-driven, parallel-safe, deterministic — without fabricating package APIs or go test flags. When uncertain, point the reader to pkg.go.dev/testing and the relevant package docs.

Initial Assessment

Check .agents/qa-context.md (fallback: .claude/qa-context.md) before answering. Pay attention to:

• Go version — fuzz testing is generally available since 1.18; some recent additions (e.g., testing/synctest) are in newer versions.
• Assertion style — built-in t.Errorf / t.Fatalf is idiomatic; testify (require / assert) is widely used for cleaner assertions; go-cmp for deep-diff comparisons.
• Module structure — standard pkg/, cmd/, internal/ Go layout.
• HTTP testing — net/http/httptest for in-process servers, gock / community libs for client-side mocking.
• Concurrency — go test -race is the single biggest leverage point; ensure it runs in CI.

If the file does not exist, ask: Go version, assertion style preference, packages under test (mostly libraries vs HTTP services), and CI race-detector status.

---

Why Go's testing

• Stdlib, zero-deps — Go ships with everything needed for basic testing.
• Fast — Go's compile + test cycle is among the fastest in any language.
• -race flag — built-in data race detector, almost free, catches real bugs.
• Subtests / table-driven — idiomatic pattern for boundary coverage.
• Fuzzing built in — go test -fuzz for property-based coverage of byte / string inputs.
• Benchmarks — testing.B for micro-perf with go test integration.

When not to rely on stdlib alone:

• Need cleaner assertions / deep equality with diff → testify or go-cmp.
• Need HTTP mocking with rich matchers → community packages.
• Need behavior-driven syntax → Ginkgo (less common but real ecosystem).

---

A test file

// math_test.go
package mathx

import (
    "testing"
)

func TestAdd(t *testing.T) {
    got := Add(1, 2)
    want := 3
    if got != want {
        t.Errorf("Add(1, 2) = %d; want %d", got, want)
    }
}

Files named *_test.go, in the same package (white-box) or package mathx_test (black-box). Run with go test ./....

t.Errorf records failure and continues; t.Fatalf records failure and aborts the test.

---

Table-driven tests

The single most idiomatic Go testing pattern:

func TestIsValidEmail(t *testing.T) {
    cases := []struct {
        name  string
        input string
        want  bool
    }{
        {"standard", "[email protected]", true},
        {"missing at", "no-at", false},
        {"empty", "", false},
        {"double at", "double@@at.com", false},
    }

    for _, tc := range cases {
        tc := tc  // capture for parallel safety (Go 1.21 and earlier)
        t.Run(tc.name, func(t *testing.T) {
            t.Parallel()
            if got := IsValidEmail(tc.input); got != tc.want {
                t.Errorf("IsValidEmail(%q) = %v; want %v", tc.input, got, tc.want)
            }
        })
    }
}

t.Run creates a subtest with its own name visible in go test -v output. t.Parallel() makes the subtest run in parallel with siblings.

Note: Go 1.22+ fixed the loop-variable scoping issue, so the tc := tc capture line is no longer needed. Keep the line for older Go versions; remove for 1.22+.

---

Testify

import (
    "github.com/stretchr/testify/assert"
    "github.com/stretchr/testify/require"
)

func TestUser(t *testing.T) {
    u, err := LoadUser("user-42")
    require.NoError(t, err)         // fails fast on err
    require.NotNil(t, u)
    assert.Equal(t, "user-42", u.ID)
    assert.Contains(t, u.Email, "@example.com")
}

require.X aborts on failure (like t.Fatalf); assert.X continues (like t.Errorf). Use require for preconditions where continuing makes no sense, assert for independent checks.

Testify's mock package and suite package exist; use them sparingly. Hand-rolled fakes are often clearer than testify mocks for simple cases.

---

go-cmp for deep equality

reflect.DeepEqual works but gives no diff. Use github.com/google/go-cmp/cmp:

import "github.com/google/go-cmp/cmp"

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

go-cmp supports custom comparers (cmp.Comparer(...)), ignore options (cmpopts.IgnoreFields(...)), and produces excellent diffs.

---

HTTP testing

Server side

import "net/http/httptest"

func TestHandler(t *testing.T) {
    srv := httptest.NewServer(http.HandlerFunc(MyHandler))
    defer srv.Close()

    resp, err := http.Get(srv.URL + "/health")
    require.NoError(t, err)
    defer resp.Body.Close()
    require.Equal(t, 200, resp.StatusCode)
}

httptest.NewServer starts a real server on a random port; httptest.NewRecorder records responses for handler unit tests without a network. Pick based on whether you want to exercise the full HTTP stack.

Client side

For testing code that makes HTTP calls, point it at an httptest.NewServer that returns canned responses. Avoid heavy client-mocking libraries unless you really need them.

---

Test setup and cleanup

func TestThing(t *testing.T) {
    t.TempDir()  // auto-cleaned at end of test
    f := setupFixture(t)
    t.Cleanup(func() { f.Close() })
    // ... use f
}

t.Cleanup(...) runs at test end, even on panic. Prefer over defer for setup helpers because it composes cleanly across helper functions.

For package-level setup, TestMain(m *testing.M):

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

Use sparingly; package-level state often leaks between tests.

---

Race detector

go test -race ./...

Runs every test with Go's race detector. Catches data races in concurrent code. Run with -race in CI on every PR. The runtime overhead (~2-10x slower) is worth it for the bugs it surfaces.

If the suite passes -race locally but fails in CI, suspect timing-dependent assertions (sleeps, channel timing, goroutine leaks). Use goleak to detect goroutine leaks.

---

Benchmarks

func BenchmarkAdd(b *testing.B) {
    for i := 0; i < b.N; i++ {
        _ = Add(1, 2)
    }
}

Run with go test -bench=. -benchmem ./.... Benchmarks are great for micro-perf comparisons; don't treat them as production load tests (that's the perf-tier skills' job).

For comparing two implementations, use benchstat to detect statistically significant differences.

---

Fuzz tests

func FuzzIsValidEmail(f *testing.F) {
    f.Add("[email protected]")
    f.Fuzz(func(t *testing.T, input string) {
        _ = IsValidEmail(input)  // must not panic
    })
}

go test -fuzz=FuzzIsValidEmail ./... runs the fuzzer until a failure is found (or until -fuzztime elapses). Fuzz tests aim to find inputs that crash or panic — they're cheap to add for code that parses bytes/strings.

---

Test data

//go:embed testdata/users.json
var usersData []byte

func TestParse(t *testing.T) {
    got, err := Parse(usersData)
    require.NoError(t, err)
    // ... assert
}

testdata/ is Go's convention for fixture files (excluded from go build). Use embed for compile-time inclusion or os.ReadFile for runtime. Keep fixtures small and synthetic.

---

Running

Command	Purpose
go test ./...	All tests, all packages.
go test ./pkg/x -run TestAdd	One test.
go test -v ./...	Verbose.
go test -race ./...	With race detector.
go test -count=1 ./...	Disable cache (forces re-run).
go test -p 8 ./...	Set per-package parallelism.
go test -timeout 30s ./...	Per-test timeout.
go test -coverprofile=cover.out ./...	Coverage.
go tool cover -html=cover.out	HTML coverage report.
go test -bench=. -benchmem	Run benchmarks.
go test -fuzz=FuzzX -fuzztime=30s	Run fuzz target for 30s.
go test -json	JSON output (great for CI integrations).

---

CI integration

- run: go test -race -coverprofile=cover.out ./...
- run: go tool cover -func=cover.out
- if: always()
  uses: actions/upload-artifact@v4
  with: { name: coverage, path: cover.out }

Always run -race in CI. For richer reports, pipe go test -json to gotestsum or similar for prettier output and JUnit XML conversion.

---

Common Pitfalls

• Not running -race in CI — data races slip into production.
• for _, tc := range cases { t.Run(... ) without tc := tc on Go 1.21 or earlier — same loop variable shared across parallel subtests; all see the last value.
• sleep for synchronization — flake forever. Use channels, sync.WaitGroup, or polling with timeout.
• Tests that share package-level state — var globalMap = map[...]X{} mutated by tests = order dependence.
• Asserting on entire struct equality via == — fragile on additions. Use cmp.Diff.
• reflect.DeepEqual without diff output — frustrating failures. Use cmp.Diff.
• Skipping t.Cleanup for resources — leaks across tests, hard to debug.
• Benchmarks that don't use b.N — measuring nothing.
• Goroutine leaks — tests pass but leak workers. Use goleak to catch.
• go test without -count=1 after suspecting cache issues — cache hides the re-run.

---

Task-Specific Questions

When helping with Go testing, ask:

1. Go version?
2. Assertion preference — stdlib, testify, go-cmp?
3. Is -race in CI today?
4. What's the package structure — internal-heavy, lots of HTTP handlers, libraries?
5. Are fuzz / benchmark tests in use?
6. Test data strategy — testdata/ files, in-test literals, generated?
7. CI output format — gotestsum, JUnit XML, JSON?

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Related Skills

• jest-vitest — JS/TS equivalent.
• pytest — Python equivalent.
• junit-testng — JVM equivalent.
• xunit-nunit — .NET equivalent.
• grpc-testing — Go is heavy in gRPC; pair with bufconn-based tests.
• testcontainers — Testcontainers for Go is the canonical infra-backed test setup.
• mutation-testing — go-mutesting exists but the ecosystem is smaller; consider PIT-equivalent maturity.
• code-coverage — go test -coverprofile + go tool cover.
• flaky-test-management — when goroutine timing causes intermittent failures.
• ci-test-orchestration — for sharding via go test -p plus matrix runs.