Fixes after review

After discussion, we decided to simplify attribute translation for now. Full translation requires support for UTF-8 values encoded according to RFC 8187/7230, this can be clarified and implemented in further PRs.

1. Remove translation tables
2. Use simple translation rules only, for now ignoring UTF-8 keys and values

- `X-Attribute-NEOFS-` prefixed headers considered well-known system attributes
- System Attribute key is uppercased
- System Attribute key's `-` translates to `_`
- `X-Attribute-` prefixed headers considered regular object attributes
- Normal attribute key is a result of http header prefix trimming
- Value string should be left as is, for now

```
HTTP:
X-Attribute-NEOFS-Expiration-Epoch: 123
X-Attribute-FileName: cat.jpg

NeoFS:
__NEOFS__EXPIRATION_EPOCH: "123"
FileName: "cat.jpg"
```

Signed-off-by: Evgeniy Kulikov <kim@nspcc.ru>

filter.go

@@ -2,63 +2,33 @@ package main
 
 import (
 	"bytes"
-	"strconv"
-	"strings"
 
-	"github.com/nspcc-dev/neofs-api-go/pkg/object"
-	"github.com/spf13/viper"
 	"github.com/valyala/fasthttp"
 	"go.uber.org/zap"
 )
 
-type (
-	HeaderFilter interface {
-		Filter(header *fasthttp.RequestHeader) map[string]string
-	}
+const (
+	userAttributeHeaderPrefix  = "X-Attribute-"
+	neofsAttributeHeaderPrefix = "NEOFS-"
 
-	headerFilter struct {
-		logger  *zap.Logger
-		mapping map[string]string
-	}
+	systemAttributePrefix = "__NEOFS__"
 )
 
-const userAttributeHeaderPrefix = "X-Attribute-"
+func systemTranslator(key []byte) []byte {
+	// replace `NEOFS-` with `__NEOFS__`
+	key = bytes.Replace(key, []byte(neofsAttributeHeaderPrefix), []byte(systemAttributePrefix), 1)
 
-func newHeaderFilter(l *zap.Logger, v *viper.Viper) HeaderFilter {
-	filter := &headerFilter{
-		logger:  l,
-		mapping: make(map[string]string),
-	}
+	// replace `-` with `_`
+	key = bytes.ReplaceAll(key, []byte("-"), []byte("_"))
 
-	for i := 0; ; i++ {
-		index := strconv.Itoa(i)
-		key := strings.Join([]string{cfgUploaderHeader, index, cfgUploaderHeaderKey}, ".")
-		rep := strings.Join([]string{cfgUploaderHeader, index, cfgUploaderHeaderVal}, ".")
-
-		keyValue := v.GetString(key)
-		repValue := v.GetString(rep)
-
-		if keyValue == "" || repValue == "" {
-			break
-		}
-
-		filter.mapping[keyValue] = repValue
-
-		l.Debug("load upload header table value",
-			zap.String("key", keyValue),
-			zap.String("val", repValue))
-	}
-
-	// Default values
-	filter.mapping[object.AttributeFileName] = object.AttributeFileName
-	filter.mapping[object.AttributeTimestamp] = object.AttributeTimestamp
-
-	return filter
+	// replace with uppercase
+	return bytes.ToUpper(key)
 }
 
-func (h *headerFilter) Filter(header *fasthttp.RequestHeader) map[string]string {
+func filterHeaders(l *zap.Logger, header *fasthttp.RequestHeader) map[string]string {
 	result := make(map[string]string)
 	prefix := []byte(userAttributeHeaderPrefix)
+	system := []byte(neofsAttributeHeaderPrefix)
 
 	header.VisitAll(func(key, val []byte) {
 		// checks that key and val not empty
@@ -71,27 +41,27 @@ func (h *headerFilter) Filter(header *fasthttp.RequestHeader) map[string]string
 			return
 		}
 
-		// checks that after removing attribute prefix we had not empty key
-		if key = bytes.TrimPrefix(key, prefix); len(key) == 0 {
+		// removing attribute prefix
+		key = bytes.TrimPrefix(key, prefix)
+
+		// checks that it's a system NeoFS header
+		if bytes.HasPrefix(key, system) {
+			key = systemTranslator(key)
+		}
+
+		// checks that attribute key not empty
+		if len(key) == 0 {
 			return
 		}
 
-		// checks mapping table and if we found record store it
-		// at resulting hashmap
-		if name, ok := h.mapping[string(key)]; ok {
-			result[name] = string(val)
+		// make string representation of key / val
+		k, v := string(key), string(val)
 
-			h.logger.Debug("add attribute to result object",
-				zap.String("key", name),
-				zap.String("val", string(val)))
+		result[k] = v
 
-			return
-		}
-
-		// otherwise inform that attribute will be ignored
-		h.logger.Debug("ignore attribute",
-			zap.String("key", string(key)),
-			zap.String("val", string(val)))
+		l.Debug("add attribute to result object",
+			zap.String("key", k),
+			zap.String("val", v))
 	})
 
 	return result