feat: add strict TOML configuration validation

internal/config/decode.go

@@ -0,0 +1,91 @@
+package config
+
+import (
+	"bytes"
+	"fmt"
+	"os"
+	"time"
+	"unicode/utf8"
+
+	"github.com/pelletier/go-toml/v2"
+)
+
+// ClientOptions controls platform-specific static validation. Production uses
+// CheckFilesystem; documentation tests may perform syntax-only validation for
+// a non-native target.
+type ClientOptions struct {
+	Platform        Platform
+	CheckFilesystem bool
+}
+
+func LoadServer(path string) (*Server, error) {
+	data, err := os.ReadFile(path)
+	if err != nil {
+		return nil, fmt.Errorf("read server config: %w", err)
+	}
+	return DecodeServer(data)
+}
+
+func DecodeServer(data []byte) (*Server, error) {
+	return DecodeServerWithOverrides(data, nil)
+}
+
+// DecodeServerWithOverrides applies only explicitly present flag values after
+// TOML decoding. Keys come from the single registry in overrides.go.
+func DecodeServerWithOverrides(data []byte, overrides map[string]string) (*Server, error) {
+	var raw = defaultServerFile()
+	if err := strictDecode(data, &raw); err != nil {
+		return nil, fmt.Errorf("decode server config: %w", err)
+	}
+	if err := applyServerOverrides(&raw, overrides); err != nil {
+		return nil, err
+	}
+	return validateServer(raw)
+}
+
+func LoadClient(path string, platform Platform) (*Client, error) {
+	data, err := os.ReadFile(path)
+	if err != nil {
+		return nil, fmt.Errorf("read client config: %w", err)
+	}
+	return DecodeClient(data, ClientOptions{Platform: platform, CheckFilesystem: true})
+}
+
+func DecodeClient(data []byte, options ClientOptions) (*Client, error) {
+	return DecodeClientWithOverrides(data, options, nil)
+}
+
+// DecodeClientWithOverrides applies explicitly present flags after the file.
+func DecodeClientWithOverrides(data []byte, options ClientOptions, overrides map[string]string) (*Client, error) {
+	var raw = defaultClientFile()
+	if err := strictDecode(data, &raw); err != nil {
+		return nil, fmt.Errorf("decode client config: %w", err)
+	}
+	if err := applyClientOverrides(&raw, overrides); err != nil {
+		return nil, err
+	}
+	return validateClient(raw, options)
+}
+
+func strictDecode(data []byte, target any) error {
+	if !utf8.Valid(data) {
+		return fmt.Errorf("configuration is not valid UTF-8")
+	}
+	decoder := toml.NewDecoder(bytes.NewReader(data))
+	decoder.DisallowUnknownFields()
+	if err := decoder.Decode(target); err != nil {
+		return err
+	}
+	return nil
+}
+
+func parseDuration(name, value string, allowZero bool) (time.Duration, error) {
+	duration, err := time.ParseDuration(value)
+	if err != nil {
+		return 0, fmt.Errorf("%s: %w", name, err)
+	}
+	if duration < 0 || (!allowZero && duration == 0) {
+		return 0, fmt.Errorf("%s must be %s", name, map[bool]string{true: "non-negative", false: "positive"}[allowZero])
+	}
+	return duration, nil
+}