feat: unify generated configuration loading and extensible decoders

config/loader.go

@@ -0,0 +1,175 @@
+package config
+
+import (
+	"errors"
+	"io"
+	"os"
+	"path/filepath"
+	"reflect"
+	"strings"
+)
+
+const maxConfigBytes = 1 << 20
+const maxConfigDepth = 64
+const maxConfigNodes = 100000
+
+// Decoder must not retain or mutate input and must support concurrent calls
+// when its Loader is shared. It is trusted application code, not a sandbox.
+type Decoder interface {
+	Decode([]byte) (Document, error)
+}
+type DecoderFunc func([]byte) (Document, error)
+
+func (f DecoderFunc) Decode(data []byte) (Document, error) { return f(data) }
+
+// Loader owns an immutable per-instance decoder registry. No global registration.
+type Loader struct {
+	decoders map[string]Decoder
+	sealed   bool
+}
+type LoaderOption func(*Loader) error
+
+// WithDecoder adds a new extension; builtin/duplicate extensions cannot be replaced.
+func WithDecoder(extension string, decoder Decoder) LoaderOption {
+	return func(l *Loader) error {
+		if l == nil || l.decoders == nil || l.sealed {
+			return errors.New("config: decoder registry cannot be changed after construction")
+		}
+		ext := strings.ToLower(strings.TrimPrefix(extension, "."))
+		if ext == "" || strings.ContainsAny(ext, "./\\ \t\r\n") {
+			return errors.New("config: invalid decoder extension")
+		}
+		if decoder == nil {
+			return errors.New("config: nil decoder")
+		}
+		v := reflect.ValueOf(decoder)
+		if (v.Kind() == reflect.Pointer || v.Kind() == reflect.Func || v.Kind() == reflect.Map || v.Kind() == reflect.Slice || v.Kind() == reflect.Interface) && v.IsNil() {
+			return errors.New("config: nil decoder")
+		}
+		if _, exists := l.decoders[ext]; exists {
+			return errors.New("config: decoder extension already registered")
+		}
+		l.decoders[ext] = decoder
+		return nil
+	}
+}
+
+func NewLoader(options ...LoaderOption) (*Loader, error) {
+	l := &Loader{decoders: map[string]Decoder{"json": JSONDecoder{}, "yaml": YAMLDecoder{}, "yml": YAMLDecoder{}, "xml": XMLDecoder{}}}
+	for _, o := range options {
+		if o == nil {
+			return nil, errors.New("config: nil loader option")
+		}
+		if err := o(l); err != nil {
+			return nil, err
+		}
+	}
+	l.sealed = true
+	return l, nil
+}
+
+var defaultLoader, _ = NewLoader()
+
+func selectLoader(loaders []*Loader) (*Loader, error) {
+	if len(loaders) == 0 {
+		return defaultLoader, nil
+	}
+	if len(loaders) != 1 || loaders[0] == nil {
+		return nil, errors.New("config: expected one non-nil loader")
+	}
+	return loaders[0], nil
+}
+func (l *Loader) read(path string) (doc Document, err error) {
+	defer func() {
+		if err != nil {
+			var d *Error
+			if errors.As(err, &d) {
+				c := *d
+				c.File = path
+				err = &c
+			} else {
+				reason := "cannot read configuration file"
+				if errors.Is(err, os.ErrNotExist) {
+					reason = "configuration file does not exist"
+				} else if errors.Is(err, os.ErrPermission) {
+					reason = "permission denied reading configuration file"
+				}
+				err = &Error{File: path, Reason: reason, cause: err}
+			}
+		}
+	}()
+	decoder := l.decoders[strings.TrimPrefix(strings.ToLower(filepath.Ext(path)), ".")]
+	if decoder == nil {
+		return doc, &Error{Reason: "unsupported configuration format"}
+	}
+	info, e := os.Stat(path)
+	if e != nil {
+		return doc, e
+	}
+	if !info.Mode().IsRegular() {
+		return doc, &Error{Reason: "configuration must be a regular file"}
+	}
+	f, e := os.Open(path)
+	if e != nil {
+		return doc, e
+	}
+	defer f.Close()
+	info, e = f.Stat()
+	if e != nil {
+		return doc, e
+	}
+	if !info.Mode().IsRegular() {
+		return doc, &Error{Reason: "configuration must be a regular file"}
+	}
+	data, e := io.ReadAll(io.LimitReader(f, maxConfigBytes+1))
+	if e != nil {
+		return doc, e
+	}
+	if len(data) > maxConfigBytes {
+		return doc, &Error{Reason: "file exceeds 1 MiB"}
+	}
+	doc, err = decodeSafely(decoder, data)
+	if err != nil {
+		return doc, err
+	}
+	if _, ok := doc.Value.(map[string]any); !ok {
+		return doc, doc.diagnostic("", "root must be an object", nil)
+	}
+	return doc, nil
+}
+func decodeSafely(decoder Decoder, data []byte) (doc Document, err error) {
+	defer func() {
+		if recover() != nil {
+			err = &Error{Reason: "decoder panicked"}
+		}
+	}()
+	doc, err = decoder.Decode(data)
+	if err != nil {
+		var d *Error
+		if !errors.As(err, &d) {
+			err = &Error{Reason: "invalid configuration syntax", cause: err}
+		}
+	}
+	return
+}
+
+// DecodeFile overlays a bounded document onto independent defaults. With no
+// loader argument it accepts JSON/YAML/XML. It never opens dependencies.
+func DecodeFile[T any](path string, defaults T, loaders ...*Loader) (T, error) {
+	var zero T
+	l, err := selectLoader(loaders)
+	if err != nil {
+		return zero, err
+	}
+	doc, err := l.read(path)
+	if err != nil {
+		return zero, err
+	}
+	result, err := bindDocument(doc, defaults)
+	if err != nil {
+		d := validationError(err)
+		d.File = path
+		return zero, d
+	}
+	return result, nil
+}