felhom-controller/controller/internal/util/msgerr.go

package util

import (
	"errors"
	"fmt"
	"gitea.dooplex.hu/admin/felhom-controller/internal/i18n"
)

// ── An error that carries the KEY of the sentence it is, not only the sentence ────────────────────
//
// Localisation slice 2 release B (R-557). 179 Hungarian sentences in this product are built deep
// inside a package with `fmt.Errorf` and printed by whoever catches them — a page, a JSON answer, a
// flash. They cannot be translated where they are SHOWN, because by then they are a finished string;
// and they must not be translated where they are MADE, because that code has no request and no
// language. So the error carries its key and the display end renders it.
//
//	return util.MsgErrorf(stacks.ErrRequiredField, "deploy.field_required", label)
//	...
//	http.Error(w, s.errText(r, err), 400)     // the household's language
//	if errors.Is(err, stacks.ErrRequiredField) { … }   // still true — R-553's signal is intact
//
// THREE PROPERTIES, each earned:
//
//  1. `Error()` RETURNS THE HUNGARIAN TEXT, byte for byte what `fmt.Errorf` produced. Every printer
//     that has not been converted — a log line, a third-party wrapper, `%v` in a Printf — keeps
//     printing exactly what it printed before. That is what makes converting 179 producers safe
//     without converting all their printers in the same commit, and it is what the parity gate
//     measures.
//  2. `errors.Is` STILL WORKS, for the kind AND for a wrapped cause. `Unwrap() []error` returns the
//     kind first and then any error among the arguments, so `errors.Is(err, ErrRequiredField)` and
//     `errors.Is(err, os.ErrNotExist)` are both answerable. The predecessor `KindErrorf` returned the
//     kind alone and dropped the cause; this does not.
//  3. AN ERROR ARGUMENT IS RENDERED RECURSIVELY. `fmt.Errorf("formázás sikertelen: %w", err)` is a
//     sentence wrapping a sentence. Passing the inner error as an ARGUMENT (not as pre-rendered text)
//     lets `Text(lang, …)` translate the whole chain — the outer in the household's language and the
//     inner too, when the inner also carries a key. An inner error with no key prints itself, which
//     is the right answer for a restic or docker message that is not ours to translate.

// MsgErrorf builds an error whose message is the bundle's `key` filled with `args`.
//
// `kind` may be nil. When it is not, it is the sentinel `errors.Is` tests — the same contract
// KindErrorf established (R-553), and the reason a converted producer does not break a decision.
func MsgErrorf(kind error, key string, args ...interface{}) error {
	return &msgError{kind: kind, key: key, args: args}
}

// MsgError is MsgErrorf with no kind, for a producer nothing branches on.
func MsgError(key string, args ...interface{}) error {
	return &msgError{key: key, args: args}
}

type msgError struct {
	kind error
	key  string
	args []interface{}
}

// Error returns the Hungarian sentence — the default language, and the bytes the literal carried.
func (e *msgError) Error() string { return e.Text(i18n.Default) }

// Text renders the message in lang, rendering any error argument that also carries a key.
func (e *msgError) Text(lang string) string {
	b, err := i18n.Shared()
	if err != nil {
		return e.key
	}
	if len(e.args) == 0 {
		return b.Msg(lang, e.key)
	}
	out := make([]interface{}, len(e.args))
	for i, a := range e.args {
		if inner, ok := a.(error); ok {
			out[i] = ErrText(lang, inner)
			continue
		}
		out[i] = a
	}
	return b.Msgf(lang, e.key, out...)
}

// Key returns the bundle key, so a caller can name it without rendering it.
func (e *msgError) Key() string { return e.key }

// Args returns the message's parameters as strings, for a carrier that can only hold text — the
// `fa` parameters of a flash in a redirect URL.
//
// An ERROR argument is rendered in HUNGARIAN here, deliberately and with a known cost: the carrier is
// written by one request and read by another, so the writer cannot know the reader's language, and a
// nested foreign sentence (restic, docker) is not translatable anyway. The OUTER message still
// follows the reader's language, which is the sentence that carries the meaning.
func (e *msgError) Args() []string {
	out := make([]string, 0, len(e.args))
	for _, a := range e.args {
		if inner, ok := a.(error); ok {
			out = append(out, ErrText(i18n.Default, inner))
			continue
		}
		out = append(out, fmt.Sprintf("%v", a))
	}
	return out
}

// Unwrap returns the kind FIRST and then every error among the arguments, so `errors.Is` answers for
// the decision signal and for the cause alike.
func (e *msgError) Unwrap() []error {
	var out []error
	if e.kind != nil {
		out = append(out, e.kind)
	}
	for _, a := range e.args {
		if inner, ok := a.(error); ok && inner != nil {
			out = append(out, inner)
		}
	}
	return out
}

// Msg is the interface a message-carrying error satisfies. Declared so a caller can test for the
// capability rather than for this concrete type — the point is "does this error know its key", not
// "was it built by this constructor".
type Msg interface {
	error
	Text(lang string) string
	Key() string
	Args() []string
}

// AsMsg reports whether err (or anything it wraps) carries a message key, and returns it.
func AsMsg(err error) (Msg, bool) {
	var m Msg
	if errors.As(err, &m) {
		return m, true
	}
	return nil, false
}

// ErrText is the one function every display end calls: the error's sentence in lang.
//
// An error that carries a key is rendered in lang. **Anything else is returned verbatim** — a restic
// message, a docker message, an ssh message, a Go stdlib error. That is not a gap: those sentences
// are not written here and are not translated here (10-localisation.md §9, the rule R-553
// established). A customer seeing restic's own English is seeing what restic said.
func ErrText(lang string, err error) string {
	if err == nil {
		return ""
	}
	if m, ok := AsMsg(err); ok {
		return m.Text(lang)
	}
	return err.Error()
}