template/pkg/result/bench_test.go

package result_test

// Comparison benchmark: canonical Go error handling vs the result package.
//
// Scenario: a "process record" pipeline with five steps and varying call-stack
// depths. Four inputs exercise the happy path and failure at depths 3, 5, 10.
//
// Run:
//
//	go test -bench=. -benchmem ./pkg/result/
//
// What to look for:
//   - HappyPath: result always spawns a goroutine (result.Go); that dominates.
//   - FailDepthN: result pays goroutineID (runtime.Stack) + debug.Stack +
//     sync.Map.Store + runtime.Goexit; canonical just returns an error.
//   - Allocs: result error path allocates for the stack-trace string;
//     canonical error path is typically zero additional allocs.

import (
	"errors"
	"fmt"
	"strconv"
	"strings"
	"testing"

	"gitea.djmil.dev/go/template/pkg/result"
)

// ── Shared types ───────────────────────────────────────────────────────────────

type bHeader struct{ raw, id, name, val string }
type bFields struct {
	id        int
	name, val string
}
type bRecord struct {
	id    int
	name  string
	score float64
}
type bOutput struct{ line string }

// mockRsrc simulates a resource requiring cleanup (e.g. a file, DB conn).
type mockRsrc struct{ closed bool }

func (m *mockRsrc) close() { m.closed = true }

// ── Inputs ─────────────────────────────────────────────────────────────────────

const (
	happy  = "42|Alice|100" // all steps succeed
	fail3  = ""             // parseHeader fails at depth 3: empty input
	fail5  = "0|Alice|100"  // validateFields fails at depth 5: id ≤ 0
	fail10 = "42|Alice|-9"  // enrichRecord fails at depth 10: negative score
)

var (
	errEmpty    = errors.New("empty input")
	errNegScore = errors.New("negative score")
)

// ── Canonical: standard (value, error) error handling ──────────────────────────
//
// Each step returns (T, error); every call site does: if err != nil { return }.
// The entry point uses goRun, a thin generic helper that mirrors result.Go
// (spawns one goroutine, blocks, returns (T, error)) so both sides pay the
// same goroutine-spawn cost and the benchmark isolates the error-propagation
// mechanism only.
// //go:noinline on intermediate frames prevents the compiler from collapsing
// the declared stack depth, keeping the benchmark representative.

// goRun is the canonical counterpart of result.Go: spawns a goroutine, blocks
// until fn returns, and hands back (T, error). Both entry points are now
// structurally identical — the only measured difference is how errors travel
// through the call stack.
func goRun[T any](fn func() (T, error)) (T, error) {
	type ret struct {
		v   T
		err error
	}
	ch := make(chan ret, 1)
	go func() {
		v, err := fn()
		ch <- ret{v, err}
	}()
	r := <-ch
	return r.v, r.err
}

// parseHeader — 3 levels deep; fails on fail3 input.
//
//go:noinline
func c_parseHeader(raw string) (bHeader, error) { return c_parseHeader2(raw) }

//go:noinline
func c_parseHeader2(raw string) (bHeader, error) { return c_parseHeader3(raw) }
func c_parseHeader3(raw string) (bHeader, error) {
	if raw == "" {
		return bHeader{}, errEmpty
	}
	parts := strings.SplitN(raw, "|", 3)
	if len(parts) != 3 {
		return bHeader{}, fmt.Errorf("malformed record: %q", raw)
	}
	return bHeader{raw: raw, id: parts[0], name: parts[1], val: parts[2]}, nil
}

// validateFields — 5 levels deep; fails on fail5 input.
//
//go:noinline
func c_validate(h bHeader) (bFields, error) { return c_validate2(h) }

//go:noinline
func c_validate2(h bHeader) (bFields, error) { return c_validate3(h) }

//go:noinline
func c_validate3(h bHeader) (bFields, error) { return c_validate4(h) }

//go:noinline
func c_validate4(h bHeader) (bFields, error) { return c_validate5(h) }
func c_validate5(h bHeader) (bFields, error) {
	id, err := strconv.Atoi(h.id)
	if err != nil {
		return bFields{}, fmt.Errorf("parse id %q: %w", h.id, err)
	}
	if id <= 0 {
		return bFields{}, fmt.Errorf("id %d: must be > 0", id)
	}
	return bFields{id: id, name: h.name, val: h.val}, nil
}

// transformData — 5 levels deep; does not fail on any benchmark input.
//
//go:noinline
func c_transform(f bFields) (bRecord, error) { return c_transform2(f) }

//go:noinline
func c_transform2(f bFields) (bRecord, error) { return c_transform3(f) }

//go:noinline
func c_transform3(f bFields) (bRecord, error) { return c_transform4(f) }

//go:noinline
func c_transform4(f bFields) (bRecord, error) { return c_transform5(f) }
func c_transform5(f bFields) (bRecord, error) {
	v, err := strconv.ParseFloat(f.val, 64)
	if err != nil {
		return bRecord{}, fmt.Errorf("parse value %q: %w", f.val, err)
	}
	return bRecord{id: f.id, name: f.name, score: v * 1.5}, nil
}

// enrichRecord — 10 levels deep; 1 defer at level 1; fails on fail10 input.
//
//go:noinline
func c_enrich(r bRecord) (bRecord, error) {
	res := &mockRsrc{}
	defer res.close()
	return c_enrich2(r)
}

//go:noinline
func c_enrich2(r bRecord) (bRecord, error) { return c_enrich3(r) }

//go:noinline
func c_enrich3(r bRecord) (bRecord, error) { return c_enrich4(r) }

//go:noinline
func c_enrich4(r bRecord) (bRecord, error) { return c_enrich5(r) }

//go:noinline
func c_enrich5(r bRecord) (bRecord, error) { return c_enrich6(r) }

//go:noinline
func c_enrich6(r bRecord) (bRecord, error) { return c_enrich7(r) }

//go:noinline
func c_enrich7(r bRecord) (bRecord, error) { return c_enrich8(r) }

//go:noinline
func c_enrich8(r bRecord) (bRecord, error) { return c_enrich9(r) }

//go:noinline
func c_enrich9(r bRecord) (bRecord, error) { return c_enrich10(r) }
func c_enrich10(r bRecord) (bRecord, error) {
	if r.score < 0 {
		return bRecord{}, errNegScore
	}
	return bRecord{id: r.id, name: r.name, score: r.score + 10.0}, nil
}

// formatOutput — 3 levels deep; does not fail on any benchmark input.
//
//go:noinline
func c_format(r bRecord) (bOutput, error) { return c_format2(r) }

//go:noinline
func c_format2(r bRecord) (bOutput, error) { return c_format3(r) }
func c_format3(r bRecord) (bOutput, error) {
	return bOutput{line: fmt.Sprintf("%d|%s|%.2f", r.id, r.name, r.score)}, nil
}

// canonicalProcess is the pipeline entry point using canonical error handling.
func canonicalProcess(raw string) (bOutput, error) {
	return goRun(func() (bOutput, error) {
		h, err := c_parseHeader(raw)
		if err != nil {
			return bOutput{}, fmt.Errorf("parse header: %w", err)
		}
		f, err := c_validate(h)
		if err != nil {
			return bOutput{}, fmt.Errorf("validate fields: %w", err)
		}
		r, err := c_transform(f)
		if err != nil {
			return bOutput{}, fmt.Errorf("transform: %w", err)
		}
		r, err = c_enrich(r)
		if err != nil {
			return bOutput{}, fmt.Errorf("enrich: %w", err)
		}
		out, err := c_format(r)
		if err != nil {
			return bOutput{}, fmt.Errorf("format: %w", err)
		}
		return out, nil
	})
}

// ── Result: happy-path-oriented error handling ─────────────────────────────────
//
// Each step returns result.Expect[T]; the call site chains .Expect("ctx").
// The entry point wraps everything in result.Go, which spawns one goroutine
// and collects any Expect failure as a normal Go error.

// parseHeader — 3 levels deep; fails on fail3 input.
//
//go:noinline
func r_parseHeader(raw string) result.Expect[bHeader] { return r_parseHeader2(raw) }

//go:noinline
func r_parseHeader2(raw string) result.Expect[bHeader] { return r_parseHeader3(raw) }
func r_parseHeader3(raw string) result.Expect[bHeader] {
	if raw == "" {
		return result.Fail[bHeader](errEmpty)
	}
	parts := strings.SplitN(raw, "|", 3)
	if len(parts) != 3 {
		return result.Fail[bHeader](fmt.Errorf("malformed record: %q", raw))
	}
	return result.Ok(bHeader{raw: raw, id: parts[0], name: parts[1], val: parts[2]})
}

// validateFields — 5 levels deep; fails on fail5 input.
//
//go:noinline
func r_validate(h bHeader) result.Expect[bFields] { return r_validate2(h) }

//go:noinline
func r_validate2(h bHeader) result.Expect[bFields] { return r_validate3(h) }

//go:noinline
func r_validate3(h bHeader) result.Expect[bFields] { return r_validate4(h) }

//go:noinline
func r_validate4(h bHeader) result.Expect[bFields] { return r_validate5(h) }
func r_validate5(h bHeader) result.Expect[bFields] {
	id, err := strconv.Atoi(h.id)
	if err != nil {
		return result.Fail[bFields](fmt.Errorf("parse id %q: %w", h.id, err))
	}
	if id <= 0 {
		return result.Fail[bFields](fmt.Errorf("id %d: must be > 0", id))
	}
	return result.Ok(bFields{id: id, name: h.name, val: h.val})
}

// transformData — 5 levels deep; does not fail on any benchmark input.
//
//go:noinline
func r_transform(f bFields) result.Expect[bRecord] { return r_transform2(f) }

//go:noinline
func r_transform2(f bFields) result.Expect[bRecord] { return r_transform3(f) }

//go:noinline
func r_transform3(f bFields) result.Expect[bRecord] { return r_transform4(f) }

//go:noinline
func r_transform4(f bFields) result.Expect[bRecord] { return r_transform5(f) }
func r_transform5(f bFields) result.Expect[bRecord] {
	v, err := strconv.ParseFloat(f.val, 64)
	if err != nil {
		return result.Fail[bRecord](fmt.Errorf("parse value %q: %w", f.val, err))
	}
	return result.Ok(bRecord{id: f.id, name: f.name, score: v * 1.5})
}

// enrichRecord — 10 levels deep; 1 defer at level 1; fails on fail10 input.
//
//go:noinline
func r_enrich(r bRecord) result.Expect[bRecord] {
	res := &mockRsrc{}
	defer res.close()
	return r_enrich2(r)
}

//go:noinline
func r_enrich2(r bRecord) result.Expect[bRecord] { return r_enrich3(r) }

//go:noinline
func r_enrich3(r bRecord) result.Expect[bRecord] { return r_enrich4(r) }

//go:noinline
func r_enrich4(r bRecord) result.Expect[bRecord] { return r_enrich5(r) }

//go:noinline
func r_enrich5(r bRecord) result.Expect[bRecord] { return r_enrich6(r) }

//go:noinline
func r_enrich6(r bRecord) result.Expect[bRecord] { return r_enrich7(r) }

//go:noinline
func r_enrich7(r bRecord) result.Expect[bRecord] { return r_enrich8(r) }

//go:noinline
func r_enrich8(r bRecord) result.Expect[bRecord] { return r_enrich9(r) }

//go:noinline
func r_enrich9(r bRecord) result.Expect[bRecord] { return r_enrich10(r) }
func r_enrich10(r bRecord) result.Expect[bRecord] {
	if r.score < 0 {
		return result.Fail[bRecord](errNegScore)
	}
	return result.Ok(bRecord{id: r.id, name: r.name, score: r.score + 10.0})
}

// formatOutput — 3 levels deep; does not fail on any benchmark input.
//
//go:noinline
func r_format(r bRecord) result.Expect[bOutput] { return r_format2(r) }

//go:noinline
func r_format2(r bRecord) result.Expect[bOutput] { return r_format3(r) }
func r_format3(r bRecord) result.Expect[bOutput] {
	return result.Ok(bOutput{line: fmt.Sprintf("%d|%s|%.2f", r.id, r.name, r.score)})
}

// resultProcess is the pipeline entry point using result-package error handling.
func resultProcess(raw string) (bOutput, error) {
	return result.Go(func() bOutput {
		h := r_parseHeader(raw).Expect("parse header")
		f := r_validate(h).Expect("validate fields")
		r := r_transform(f).Expect("transform")
		r = r_enrich(r).Expect("enrich")
		return r_format(r).Expect("format")
	}).Unwrap()
}

// ── Benchmarks ─────────────────────────────────────────────────────────────────

var sinkOutput bOutput // prevents the compiler from eliminating pipeline work

func BenchmarkCanonical_HappyPath(b *testing.B) {
	b.ReportAllocs()
	for b.Loop() {
		out, err := canonicalProcess(happy)
		if err != nil {
			b.Fatal(err)
		}
		sinkOutput = out
	}
}

func BenchmarkResult_HappyPath(b *testing.B) {
	b.ReportAllocs()
	for b.Loop() {
		out, err := resultProcess(happy)
		if err != nil {
			b.Fatal(err)
		}
		sinkOutput = out
	}
}

func BenchmarkCanonical_FailDepth3(b *testing.B) {
	b.ReportAllocs()
	for b.Loop() {
		_, err := canonicalProcess(fail3)
		if err == nil {
			b.Fatal("expected error")
		}
	}
}

func BenchmarkResult_FailDepth3(b *testing.B) {
	b.ReportAllocs()
	for b.Loop() {
		_, err := resultProcess(fail3)
		if err == nil {
			b.Fatal("expected error")
		}
	}
}

func BenchmarkCanonical_FailDepth5(b *testing.B) {
	b.ReportAllocs()
	for b.Loop() {
		_, err := canonicalProcess(fail5)
		if err == nil {
			b.Fatal("expected error")
		}
	}
}

func BenchmarkResult_FailDepth5(b *testing.B) {
	b.ReportAllocs()
	for b.Loop() {
		_, err := resultProcess(fail5)
		if err == nil {
			b.Fatal("expected error")
		}
	}
}

func BenchmarkCanonical_FailDepth10(b *testing.B) {
	b.ReportAllocs()
	for b.Loop() {
		_, err := canonicalProcess(fail10)
		if err == nil {
			b.Fatal("expected error")
		}
	}
}

func BenchmarkResult_FailDepth10(b *testing.B) {
	b.ReportAllocs()
	for b.Loop() {
		_, err := resultProcess(fail10)
		if err == nil {
			b.Fatal("expected error")
		}
	}
}

// ── CaptureStack=false variants ────────────────────────────────────────────────
//
// These show the floor: goroutine spawn + Goexit mechanism with no stack
// capture. Set result.CaptureStack=false at startup to reach this level in
// production. The happy-path cost is unchanged (CaptureStack is only
// consulted on error paths).

func BenchmarkResult_NoStack_HappyPath(b *testing.B) {
	result.CaptureStack = false
	defer func() { result.CaptureStack = true }()
	b.ReportAllocs()
	for b.Loop() {
		out, err := resultProcess(happy)
		if err != nil {
			b.Fatal(err)
		}
		sinkOutput = out
	}
}

func BenchmarkResult_NoStack_FailDepth3(b *testing.B) {
	result.CaptureStack = false
	defer func() { result.CaptureStack = true }()
	b.ReportAllocs()
	for b.Loop() {
		_, err := resultProcess(fail3)
		if err == nil {
			b.Fatal("expected error")
		}
	}
}

func BenchmarkResult_NoStack_FailDepth10(b *testing.B) {
	result.CaptureStack = false
	defer func() { result.CaptureStack = true }()
	b.ReportAllocs()
	for b.Loop() {
		_, err := resultProcess(fail10)
		if err == nil {
			b.Fatal("expected error")
		}
	}
}