template/pkg/result/result.go

package result

import (
	"errors"
	"fmt"
	"runtime"
	"strings"
	"sync"
)

// Expect holds either a value of type T or an error.
type Expect[T any] struct {
	value T
	err   error
}

// gErrors stores errors set by Expect/Expectf before calling runtime.Goexit,
// keyed by goroutine ID. Entries are consumed by the enclosing Go or Run call.
var gErrors sync.Map

// CaptureStack controls whether Expect, Expectf, and Must capture a stack
// trace at the failure site. Defaults to true. Set to false at program startup
// (before spawning goroutines) to cut ~1.5 KB of allocation and most of the
// error-path overhead; errors are still collected and propagated, just without
// a trace. StackTrace will return an empty string for errors captured while
// this is false.
var CaptureStack = true

// callers returns the program counters of the call stack starting at the
// caller's caller, skipping skip frames above runtime.Callers itself.
// Returns nil when CaptureStack is false.
func callers(skip int) []uintptr {
	if !CaptureStack {
		return nil
	}
	var pcs [32]uintptr
	n := runtime.Callers(skip, pcs[:])
	if n == 0 {
		return nil
	}
	cp := make([]uintptr, n)
	copy(cp, pcs[:n])
	return cp
}

// goroutineID returns the current goroutine's numeric ID by parsing the first
// line of runtime.Stack output ("goroutine NNN [...]"). Called only on error
// paths so the runtime.Stack overhead is acceptable.
func goroutineID() uint64 {
	var buf [64]byte
	n := runtime.Stack(buf[:], false)
	var id uint64
	for _, b := range buf[10:n] { // skip "goroutine "
		if b < '0' || b > '9' {
			break
		}
		id = id*10 + uint64(b-'0')
	}
	return id
}

// Ok wraps a successful value in an Expect.
func Ok[T any](v T) Expect[T] {
	return Expect[T]{value: v}
}

// Fail wraps an error in an Expect.
func Fail[T any](err error) Expect[T] {
	return Expect[T]{err: err}
}

// Of is a convenience constructor that bridges standard Go (value, error)
// return signatures:
//
//	result.Of(os.Open("file.txt")).Expect("open config")
func Of[T any](v T, err error) Expect[T] {
	return Expect[T]{value: v, err: err}
}

// Err returns the wrapped error, or nil on success.
func (r Expect[T]) Err() error {
	return r.err
}

// Must returns the value or panics with the wrapped error and a stack trace.
// Prefer [Expect.Expect] — it exits via runtime.Goexit and is collected
// cleanly by [Go] or [Run]. Must is for call sites where a genuine unrecoverable
// condition warrants an immediate crash.
func (r Expect[T]) Must() T {
	if r.err != nil {
		panic(&stackError{err: r.err, stack: callers(3)})
	}
	return r.value
}

// Expect returns the value or exits the current goroutine via runtime.Goexit,
// storing the annotated error for collection by the enclosing [Go] or [Run]
// call. The stack trace is captured at this call site.
//
//	data := Parse(raw).Expect("parse user input")
func (r Expect[T]) Expect(msg string) T {
	if r.err != nil {
		gErrors.Store(goroutineID(), &stackError{
			err:   fmt.Errorf("%s: %w", msg, r.err),
			stack: callers(3),
		})
		runtime.Goexit()
	}
	return r.value
}

// Expectf is like [Expect.Expect] but accepts a fmt.Sprintf-style format string
// for the context message. The wrapped error is always appended as ": <err>".
//
//	data := Parse(raw).Expectf("parse user input id=%d", id)
func (r Expect[T]) Expectf(format string, args ...any) T {
	if r.err != nil {
		gErrors.Store(goroutineID(), &stackError{
			err:   fmt.Errorf("%s: %w", fmt.Sprintf(format, args...), r.err),
			stack: callers(3),
		})
		runtime.Goexit()
	}
	return r.value
}

// Unwrap returns the value and error in the standard Go (value, error) form.
// Useful at the boundary where you want to re-join normal error-return code.
func (r Expect[T]) Unwrap() (T, error) {
	return r.value, r.err
}

// Async runs fn in a new goroutine and returns a channel that will receive
// exactly one Expect[T] when the goroutine finishes.  The channel is buffered
// so the goroutine never blocks even if the caller is busy with other work.
//
// Use Async when you want to run several operations concurrently and collect
// their results later:
//
//	aCh := result.Async(func() *A { return buildA().Expect("build A") })
//	bCh := result.Async(func() *B { return buildB().Expect("build B") })
//	a := (<-aCh).Expect("A")
//	b := (<-bCh).Expect("B")
//
// Genuine runtime panics (nil-pointer dereferences, etc.) are not recovered —
// they still crash the program, as they should.
func Async[T any](fn func() T) <-chan Expect[T] {
	ch := make(chan Expect[T], 1)
	go func() {
		var (
			val      T
			finished bool
		)
		defer func() {
			if v := recover(); v != nil {
				if err, ok := v.(*stackError); ok {
					// Must() panic — treat as a collected failure.
					ch <- Fail[T](err)
					return
				}
				panic(v) // genuine runtime panic — crash the program
			}
			if finished {
				ch <- Ok(val)
				return
			}
			// goroutineID is looked up here, on the error path only.
			if stored, ok := gErrors.LoadAndDelete(goroutineID()); ok {
				ch <- Fail[T](stored.(error))
			} else {
				ch <- Fail[T](errors.New("goroutine exited unexpectedly"))
			}
		}()
		val = fn()
		finished = true
	}()
	return ch
}

// Go runs fn in a new goroutine and blocks until it completes, returning the
// result as Expect[T].  It is a convenience wrapper around [Async] for the
// common single-goroutine case.
//
//	msg := result.Go(func() string {
//	    cfg := loadConfig().Expect("load config")
//	    return greet(cfg).Expect("greet")
//	})
func Go[T any](fn func() T) Expect[T] {
	return <-Async(fn)
}

// Run runs fn in a new goroutine and returns any error produced by
// [Expect.Expect] or [Expect.Expectf] calls within fn. Blocks until fn
// completes or the goroutine exits.
//
//	func run() error {
//	    return result.Run(func() {
//	        port := parsePort(cfg.Port).Expect("load config port")
//	        _ = port // happy path continues …
//	    })
//	}
func Run(fn func()) error {
	return Go(func() struct{} {
		fn()
		return struct{}{}
	}).Err()
}

// Catch recovers a panic produced by [Expect.Must] and stores it in *errp.
// For [Expect.Expect] and [Expect.Expectf] unwinding, use [Go] or [Run]
// instead — those exit via runtime.Goexit and are not recoverable by Catch.
// Panics that are not errors (e.g. nil-pointer dereferences) are re-panicked.
func Catch(errp *error) {
	v := recover()
	if v == nil {
		return
	}
	if err, ok := v.(error); ok {
		*errp = err
		return
	}
	panic(v) // not an error — let it propagate
}

// StackTrace returns the call stack captured when [Expect.Expect],
// [Expect.Expectf], or [Expect.Must] stored or panicked with an error.
// Frames are formatted as "function\n\tfile:line\n" starting at the call
// site of the failing Expect/Expectf/Must call.
// Returns an empty string if err was not produced by this package or if
// [CaptureStack] was false when the error was captured.
func StackTrace(err error) string {
	var s *stackError
	if !errors.As(err, &s) || len(s.stack) == 0 {
		return ""
	}
	frames := runtime.CallersFrames(s.stack)
	var b strings.Builder
	for {
		f, more := frames.Next()
		if f.Function != "runtime.goexit" {
			fmt.Fprintf(&b, "%s\n\t%s:%d\n", f.Function, f.File, f.Line)
		}
		if !more {
			break
		}
	}
	return b.String()
}

// stackError wraps an error with program counters captured at the failure site.
type stackError struct {
	err   error
	stack []uintptr // nil when CaptureStack is false
}

func (s *stackError) Error() string { return s.err.Error() }
func (s *stackError) Unwrap() error { return s.err }