docs: idiomatic Go patterns from stdlib + Kubernetes with source citations
This commit is contained in:
Rodin
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# Advanced Go Testing Patterns
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Patterns extracted from the Go standard library (`src/net/http/`, `src/encoding/json/`, `src/testing/`) and Kubernetes source code.
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---
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## 1. Table-Driven Tests
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The canonical Go test style. Every Go stdlib test file uses this pattern.
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### Pattern Name: Anonymous Struct Test Table
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**Source:** `/tmp/go-src/src/net/http/header_test.go` lines 17-108
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**What they do:** Define test cases as a slice of anonymous structs, iterate with a range loop.
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**Why:** Eliminates repetition, makes adding cases trivial, keeps the assertion logic in one place. Every test case gets the same verification path — no "special" cases hidden in different code paths.
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**Anti-pattern:** Writing individual assertions for each case, or copy-pasting test functions that differ by one input.
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**Code example (stdlib):**
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```go
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var headerWriteTests = []struct {
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h Header
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exclude map[string]bool
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expected string
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}{
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{Header{}, nil, ""},
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{
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Header{
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"Content-Type": {"text/html; charset=UTF-8"},
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"Content-Length": {"0"},
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},
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nil,
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"Content-Length: 0\r\nContent-Type: text/html; charset=UTF-8\r\n",
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},
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// ... more cases
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}
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func TestHeaderWrite(t *testing.T) {
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var buf strings.Builder
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for i, test := range headerWriteTests {
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test.h.WriteSubset(&buf, test.exclude)
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if buf.String() != test.expected {
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t.Errorf("#%d:\n got: %q\nwant: %q", i, buf.String(), test.expected)
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}
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buf.Reset()
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}
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}
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```
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---
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### Pattern Name: Named Table Tests with t.Run (Subtests)
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**Source:** `/tmp/go-src/src/encoding/json/encode_test.go` lines 285-320, `/tmp/go-src/src/encoding/json/scanner_test.go` lines 30-50
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**What they do:** Combine table-driven tests with `t.Run` for named subtests. Use a `CaseName` struct that captures file/line for error reporting.
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**Why:** Each case gets its own subtest name — visible in `go test -v`, filterable with `-run`, and individually re-runnable. The `CaseName`/`Where` pattern provides precise file:line for failures even in large test tables.
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**Anti-pattern:** Using index-only identification (hard to find which case failed), or creating separate `TestFoo_Case1`, `TestFoo_Case2` functions.
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**Code example (stdlib):**
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```go
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func TestValid(t *testing.T) {
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tests := []struct {
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CaseName
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data string
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ok bool
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}{
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{Name(""), `foo`, false},
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{Name(""), `}{`, false},
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{Name(""), `{}`, true},
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{Name("StringDoubleEscapes"), `{"foo":"bar"}`, true},
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}
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for _, tt := range tests {
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t.Run(tt.Name, func(t *testing.T) {
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if ok := Valid([]byte(tt.data)); ok != tt.ok {
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t.Errorf("%s: Valid(`%s`) = %v, want %v", tt.Where, tt.data, ok, tt.ok)
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}
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})
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}
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}
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```
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---
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### Pattern Name: CaseName with Caller Position Tracking
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**Source:** `/tmp/go-src/src/encoding/json/internal/jsontest/testcase.go` lines 18-37
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**What they do:** Create a helper type that captures the caller's file:line at the point of test case declaration, so error messages point back to the exact test case definition.
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**Why:** In a 1000-entry test table, `t.Errorf` points to the assertion line (same for all cases). CaseName makes failures point to the case definition.
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**Code example (stdlib):**
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```go
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type CaseName struct {
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Name string
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Where CasePos
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}
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func Name(s string) (c CaseName) {
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c.Name = s
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runtime.Callers(2, c.Where.pc[:])
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return c
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}
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type CasePos struct{ pc [1]uintptr }
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func (pos CasePos) String() string {
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frames := runtime.CallersFrames(pos.pc[:])
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frame, _ := frames.Next()
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return fmt.Sprintf("%s:%d", path.Base(frame.File), frame.Line)
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}
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```
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---
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## 2. Test Helper Patterns
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### Pattern Name: t.Helper() for Clean Stack Traces
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**Source:** `/tmp/go-src/src/testing/testing.go` lines 1415-1435
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**What they do:** Call `t.Helper()` as the first line in any test utility function. This marks the function as a helper, so test failure messages report the caller's line instead of the helper's line.
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**Why:** Without `t.Helper()`, every failure in a helper function points to the helper itself, not the test case that triggered the failure. Makes debugging test failures require reading the full stack.
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**Anti-pattern:** Writing test utilities that call `t.Fatal`/`t.Error` without marking themselves as helpers.
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**Code example (stdlib):**
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```go
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// From net/http/clientserver_test.go lines 100-131
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func run[T TBRun[T]](t T, f func(t T, mode testMode), opts ...any) {
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t.Helper()
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modes := []testMode{http1Mode, http2Mode, http3Mode}
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parallel := true
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for _, opt := range opts {
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switch opt := opt.(type) {
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case []testMode:
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modes = opt
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case testNotParallelOpt:
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parallel = false
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default:
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t.Fatalf("unknown option type %T", opt)
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}
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}
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// ...
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for _, mode := range modes {
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t.Run(string(mode), func(t T) {
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t.Helper()
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// ...
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f(t, mode)
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})
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}
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}
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```
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---
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### Pattern Name: *testing.T as First Argument to Helpers
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**Source:** `/tmp/go-src/src/net/http/serve_test.go` lines 4555-4580
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**What they do:** Pass `*testing.T` (or `testing.TB`) as the first argument to test helper functions, making the dependency on the test context explicit.
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**Why:** The test object provides `Fatal`, `Error`, `Log`, `Helper`, `Cleanup` — everything a helper needs for reporting. Accepting it as a parameter (rather than capturing it in a closure) makes helpers reusable across tests.
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**Code example (stdlib):**
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```go
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mustGet := func(url string, headers ...string) {
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t.Helper()
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req, err := NewRequest("GET", url, nil)
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if err != nil {
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t.Fatal(err)
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}
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for len(headers) > 0 {
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req.Header.Add(headers[0], headers[1])
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headers = headers[2:]
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}
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res, err := c.Do(req)
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if err != nil {
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t.Errorf("Error fetching %s: %v", url, err)
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return
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}
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_, err = io.ReadAll(res.Body)
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defer res.Body.Close()
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}
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```
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---
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## 3. t.Cleanup vs defer
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### Pattern Name: t.Cleanup for Test-Scoped Resources
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**Source:** `/tmp/go-src/src/testing/testing.go` lines 1439-1468, `/tmp/go-src/src/net/http/clientserver_test.go` lines 120-127
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**What they do:** Use `t.Cleanup(fn)` instead of `defer` for resource cleanup in tests.
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**Why:**
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1. `defer` runs at the end of the *function*, not the *test*. In subtests launched with `t.Run`, a `defer` in a helper function runs when the helper returns — not when the subtest completes.
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2. `t.Cleanup` runs after the test AND all its subtests finish — guaranteeing resources are available for the full test lifetime.
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3. `t.Cleanup` is called in reverse order (LIFO), matching `defer` semantics but scoped to the test.
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**Anti-pattern:** Using `defer` for cleanup in test setup functions that return before the test finishes, or in subtests where timing matters.
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**Code example (stdlib):**
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```go
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// From net/http/clientserver_test.go
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func run[T TBRun[T]](t T, f func(t T, mode testMode), opts ...any) {
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// ...
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for _, mode := range modes {
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t.Run(string(mode), func(t T) {
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t.Cleanup(func() {
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afterTest(t) // Goroutine leak detection — runs AFTER subtest body completes
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})
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f(t, mode)
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})
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}
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}
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```
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---
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## 4. testdata/ Directory Pattern
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### Pattern Name: testdata/ for Test Fixtures
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**Source:** `/tmp/go-src/src/net/http/testdata/` (contains `file`, `index.html`, `style.css`), `/tmp/go-src/src/net/http/fs_test.go` line 38
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**What they do:** Store test fixtures in a `testdata/` directory adjacent to the test files. Reference them with relative paths like `"testdata/file"`.
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**Why:**
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1. `go build` ignores `testdata/` directories — they never end up in production binaries.
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2. `go test` runs with the package directory as CWD — relative paths to `testdata/` work reliably.
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3. Fixtures are version-controlled alongside the code they test.
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4. Separates test data from test logic.
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**Anti-pattern:** Embedding large test fixtures as string literals in test files, or referencing absolute paths.
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**Code example (stdlib):**
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```go
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// From net/http/fs_test.go line 38
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const testFile = "testdata/file"
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// Usage in test:
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ServeFile(w, r, "testdata/file")
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```
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---
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## 5. Golden File Testing
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### Pattern Name: Golden Files with -update Flag
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**Source:** `/tmp/go-src/src/cmd/gofmt/gofmt_test.go` lines 18, 113-138
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**What they do:** Compare test output against `.golden` files. Provide a `-update` flag that regenerates golden files from current output when behavior intentionally changes.
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**Why:**
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1. Tests complex output (formatted code, generated HTML, serialized data) without embedding it in test code.
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2. The `-update` flag makes intentional changes easy: run `go test -update`, review the diff, commit.
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3. Golden files serve as documentation of expected behavior.
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4. Reviewers can see exactly what output changed in diffs.
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**Anti-pattern:** Comparing against inline expected strings that span 50+ lines, or manually constructing expected output.
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**Code example (stdlib):**
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```go
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var update = flag.Bool("update", false, "update .golden files")
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func runTest(t *testing.T, in, out string) {
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// ... produce actual output ...
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expected, err := os.ReadFile(out)
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if err != nil {
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t.Error(err)
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return
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}
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if got := buf.Bytes(); !bytes.Equal(got, expected) {
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if *update {
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if in != out {
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if err := os.WriteFile(out, got, 0666); err != nil {
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t.Error(err)
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}
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return
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}
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}
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t.Errorf("(gofmt %s) != %s\n%s", in, out,
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diff.Diff("expected", expected, "got", got))
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}
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}
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func TestRewrite(t *testing.T) {
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match, _ := filepath.Glob("testdata/*.input")
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for _, in := range match {
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name := filepath.Base(in)
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t.Run(name, func(t *testing.T) {
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out := in[:len(in)-len(".input")] + ".golden"
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runTest(t, in, out)
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})
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}
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}
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```
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---
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## 6. httptest Patterns
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### Pattern Name: httptest.NewRecorder for Unit-Testing Handlers
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**Source:** `/tmp/go-src/src/net/http/serve_test.go` lines 387-393
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**What they do:** Use `httptest.NewRecorder()` to test HTTP handlers without starting a server. Captures status code, headers, and body.
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**Why:** Fast, no network, no port allocation, no goroutines. Perfect for unit testing individual handlers in isolation.
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**Anti-pattern:** Spinning up a full server to test handler logic that doesn't need networking.
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**Code example (stdlib):**
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```go
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func TestServeMuxHandler(t *testing.T) {
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mux := NewServeMux()
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for _, e := range serveMuxRegister {
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mux.Handle(e.pattern, e.h)
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}
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for _, tt := range serveMuxTests {
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r := &Request{Method: tt.method, Host: tt.host, URL: &url.URL{Path: tt.path}}
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h, pattern := mux.Handler(r)
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rr := httptest.NewRecorder()
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h.ServeHTTP(rr, r)
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if pattern != tt.pattern || rr.Code != tt.code {
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t.Errorf("%s %s %s = %d, %q, want %d, %q",
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tt.method, tt.host, tt.path, rr.Code, pattern, tt.code, tt.pattern)
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}
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}
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}
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```
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---
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### Pattern Name: httptest.NewServer for Integration-Style Tests
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**Source:** `/tmp/go-src/src/net/http/clientserver_test.go` lines 203-280
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**What they do:** Use `httptest.NewServer` / `httptest.NewUnstartedServer` for end-to-end HTTP testing with a real TCP listener on localhost.
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**Why:** Tests the full HTTP stack including transport, TLS, connection pooling, timeouts. The `clientServerTest` helper in the stdlib runs each test across HTTP/1.1, HTTP/2, and HTTP/3 modes.
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**Code example (stdlib):**
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```go
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func newClientServerTest(t testing.TB, mode testMode, h Handler, opts ...any) *clientServerTest {
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cst := &clientServerTest{t: t, h2: mode == http2Mode, h: h}
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cst.ts = httptest.NewUnstartedServer(h)
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// ... configure based on mode ...
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switch mode {
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case http1Mode:
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cst.ts.Start()
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case http2Mode:
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cst.ts.EnableHTTP2 = true
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cst.ts.StartTLS()
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}
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cst.c = cst.ts.Client()
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t.Cleanup(cst.close)
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return cst
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}
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```
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---
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## 7. Benchmark Patterns
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### Pattern Name: b.ReportAllocs + b.RunParallel + b.SetBytes
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**Source:** `/tmp/go-src/src/encoding/json/bench_test.go` lines 85-101
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**What they do:** Combine `b.ReportAllocs()` for allocation reporting, `b.RunParallel` for concurrent benchmarks, and `b.SetBytes` for throughput metrics.
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**Why:**
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- `b.ReportAllocs()` shows allocations/op — critical for hot paths.
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- `b.RunParallel` measures performance under contention (real-world server behavior).
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- `b.SetBytes` converts to MB/s throughput — meaningful for serialization benchmarks.
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**Anti-pattern:** Benchmarks that only measure wall time without allocation tracking, or sequential benchmarks for concurrent code.
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**Code example (stdlib):**
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```go
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func BenchmarkCodeEncoder(b *testing.B) {
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b.ReportAllocs()
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if codeJSON == nil {
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b.StopTimer()
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codeInit()
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b.StartTimer()
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}
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b.RunParallel(func(pb *testing.PB) {
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enc := NewEncoder(io.Discard)
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for pb.Next() {
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if err := enc.Encode(&codeStruct); err != nil {
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b.Fatalf("Encode error: %v", err)
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}
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}
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})
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b.SetBytes(int64(len(codeJSON)))
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}
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```
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---
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## 8. Integration Test Separation
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### Pattern Name: testing.Short() for Expensive Tests
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**Source:** `/tmp/go-src/src/net/http/serve_test.go` lines 800, 1000, 2212, 2581
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**What they do:** Skip slow/flaky/network-dependent tests with `testing.Short()`. The Go CI runs with `-short` in fast mode, full tests in thorough mode.
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**Why:** Fast feedback loop for development (`go test -short`), full validation in CI. No custom build tags needed.
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**Anti-pattern:** Separate `_integration_test.go` files with build tags (Go stdlib doesn't do this), or always-slow tests that can't be skipped.
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**Code example (stdlib):**
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```go
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func TestServerTimeouts(t *testing.T) {
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if testing.Short() {
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t.Skip("skipping in short mode")
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}
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// ... expensive test with real timeouts ...
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}
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```
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---
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## 9. No Assertion Libraries in Stdlib
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### Pattern Name: Plain if/t.Errorf Over Assertion Frameworks
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**Source:** Every test file in `/tmp/go-src/src/` (zero imports of `testify`, `gomega`, or any assertion library)
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**What they do:** Use plain Go: `if got != want { t.Errorf(...) }`. Never import assertion libraries.
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**Why:**
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1. No implicit control flow — `t.Errorf` continues execution, so you see ALL failures at once.
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2. No magic — the test reads like regular Go code.
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3. Error messages are custom-crafted for each assertion, providing context that generic `assert.Equal` cannot.
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4. One less dependency.
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**Anti-pattern (Kubernetes uses this, stdlib does NOT):**
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```go
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// Kubernetes style (not stdlib):
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assert.Equal(t, expected, actual)
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require.NoError(t, err)
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```
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**Stdlib style:**
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||||
```go
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if got := v.Elem().Interface(); !reflect.DeepEqual(got, tt.out) {
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t.Fatalf("%s: Decode:\n\tgot: %#v\n\twant: %#v", tt.Where, got, tt.out)
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||||
}
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```
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||||
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||||
---
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||||
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||||
## 10. Goroutine Leak Detection
|
||||
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||||
### Pattern Name: TestMain + afterTest Goroutine Checking
|
||||
|
||||
**Source:** `/tmp/go-src/src/net/http/main_test.go` (entire file)
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||||
|
||||
**What they do:** `TestMain` runs the test suite and checks for leaked goroutines after all tests complete. `afterTest` checks for goroutine leaks after each individual test.
|
||||
|
||||
**Why:** HTTP code spawns goroutines for connections, background reads, etc. Leaked goroutines indicate resource leaks (connections not closed, servers not shut down). Catching them prevents production OOMs.
|
||||
|
||||
**Code example (stdlib):**
|
||||
```go
|
||||
func TestMain(m *testing.M) {
|
||||
v := m.Run()
|
||||
if v == 0 && goroutineLeaked() {
|
||||
os.Exit(1)
|
||||
}
|
||||
os.Exit(v)
|
||||
}
|
||||
|
||||
func goroutineLeaked() bool {
|
||||
for i := 0; i < 5; i++ {
|
||||
gs := interestingGoroutines()
|
||||
if len(gs) == 0 {
|
||||
return false
|
||||
}
|
||||
time.Sleep(100 * time.Millisecond)
|
||||
}
|
||||
// Report leaked goroutines
|
||||
return true
|
||||
}
|
||||
|
||||
func afterTest(t testing.TB) {
|
||||
http.DefaultTransport.(*http.Transport).CloseIdleConnections()
|
||||
// Check for leaked goroutines from this specific test...
|
||||
}
|
||||
```
|
||||
|
||||
---
|
||||
|
||||
## 11. export_test.go Pattern
|
||||
|
||||
### Pattern Name: Bridge File for Internal Testing
|
||||
|
||||
**Source:** `/tmp/go-src/src/net/http/export_test.go` lines 1-50
|
||||
|
||||
**What they do:** Create an `export_test.go` file in the package itself (package `http`, not `http_test`) that exports internal symbols to external test packages. Only compiled during testing.
|
||||
|
||||
**Why:** Allows `http_test` (external test package) to access internals needed for white-box testing without polluting the public API. The `_test.go` suffix means it's never included in production builds.
|
||||
|
||||
**Code example (stdlib):**
|
||||
```go
|
||||
// export_test.go — package http (not http_test!)
|
||||
package http
|
||||
|
||||
var (
|
||||
DefaultUserAgent = defaultUserAgent
|
||||
ExportRefererForURL = refererForURL
|
||||
ExportServerNewConn = (*Server).newConn
|
||||
ExportErrRequestCanceled = errRequestCanceled
|
||||
)
|
||||
```
|
||||
|
||||
---
|
||||
|
||||
## 12. Multi-Mode Test Runner
|
||||
|
||||
### Pattern Name: Generic Test Runner Across Protocol Modes
|
||||
|
||||
**Source:** `/tmp/go-src/src/net/http/clientserver_test.go` lines 100-134
|
||||
|
||||
**What they do:** A generic `run[T]` function that executes every client/server test in HTTP/1.1, HTTP/2, and HTTP/3 modes automatically. Tests opt into specific modes via options.
|
||||
|
||||
**Why:** Ensures behavioral consistency across protocol versions. A single test function covers all modes — no duplication. Bugs in one protocol version are caught immediately.
|
||||
|
||||
**Code example (stdlib):**
|
||||
```go
|
||||
// Test declaration (one line runs across 3 protocols):
|
||||
func TestServerTimeouts(t *testing.T) { run(t, testServerTimeouts, []testMode{http1Mode}) }
|
||||
|
||||
// The runner:
|
||||
func run[T TBRun[T]](t T, f func(t T, mode testMode), opts ...any) {
|
||||
t.Helper()
|
||||
modes := []testMode{http1Mode, http2Mode, http3Mode}
|
||||
for _, mode := range modes {
|
||||
t.Run(string(mode), func(t T) {
|
||||
t.Helper()
|
||||
t.Cleanup(func() { afterTest(t) })
|
||||
f(t, mode)
|
||||
})
|
||||
}
|
||||
}
|
||||
```
|
||||
|
||||
---
|
||||
|
||||
## 13. testLogWriter — Routing Server Logs to Test Output
|
||||
|
||||
### Pattern Name: io.Writer Adapter for *testing.T
|
||||
|
||||
**Source:** `/tmp/go-src/src/net/http/clientserver_test.go` lines 337-345
|
||||
|
||||
**What they do:** Implement `io.Writer` backed by `t.Logf`, so server error logs appear in test output (visible with `-v`, suppressed otherwise).
|
||||
|
||||
**Why:** Server logs are crucial for debugging test failures but shouldn't clutter passing output. `t.Log` gives you both: silent on pass, verbose on fail.
|
||||
|
||||
**Code example (stdlib):**
|
||||
```go
|
||||
type testLogWriter struct {
|
||||
t testing.TB
|
||||
}
|
||||
|
||||
func (w testLogWriter) Write(b []byte) (int, error) {
|
||||
w.t.Logf("server log: %v", strings.TrimSpace(string(b)))
|
||||
return len(b), nil
|
||||
}
|
||||
|
||||
// Usage:
|
||||
cst.ts.Config.ErrorLog = log.New(testLogWriter{t}, "", 0)
|
||||
```
|
||||
Reference in New Issue
Block a user