fix(model): correct bufferpool handling; simplify (#10113)

The copier routine refactor resulted in bad buffer pool handling,
putting a buffer back into the pool twice. This simplifies and removes
the danger prone Upgrade() method.

lib/protocol/protocol.go

@@ -512,8 +512,9 @@ func (c *rawConnection) readMessageAfterHeader(hdr *bep.Header, fourByteBuf []by
 	// Then comes the message
 
 	buf := BufferPool.Get(int(msgLen))
+	defer BufferPool.Put(buf)
+
 	if _, err := io.ReadFull(c.cr, buf); err != nil {
-		BufferPool.Put(buf)
 		return nil, fmt.Errorf("reading message: %w", err)
 	}
 
@@ -525,7 +526,6 @@ func (c *rawConnection) readMessageAfterHeader(hdr *bep.Header, fourByteBuf []by
 
 	case bep.MessageCompression_MESSAGE_COMPRESSION_LZ4:
 		decomp, err := lz4Decompress(buf)
-		BufferPool.Put(buf)
 		if err != nil {
 			return nil, fmt.Errorf("decompressing message: %w", err)
 		}
@@ -541,14 +541,11 @@ func (c *rawConnection) readMessageAfterHeader(hdr *bep.Header, fourByteBuf []by
 
 	msg, err := newMessage(hdr.Type)
 	if err != nil {
-		BufferPool.Put(buf)
 		return nil, err
 	}
 	if err := proto.Unmarshal(buf, msg); err != nil {
-		BufferPool.Put(buf)
 		return nil, fmt.Errorf("unmarshalling message: %w", err)
 	}
-	BufferPool.Put(buf)
 
 	return msg, nil
 }
@@ -567,16 +564,16 @@ func (c *rawConnection) readHeader(fourByteBuf []byte) (*bep.Header, error) {
 	// Then comes the header
 
 	buf := BufferPool.Get(int(hdrLen))
+	defer BufferPool.Put(buf)
+
 	if _, err := io.ReadFull(c.cr, buf); err != nil {
-		BufferPool.Put(buf)
 		return nil, fmt.Errorf("reading header: %w", err)
 	}
 
 	var hdr bep.Header
 	err := proto.Unmarshal(buf, &hdr)
-	BufferPool.Put(buf)
 	if err != nil {
-		return nil, fmt.Errorf("unmarshalling header: %w %x", err, buf)
+		return nil, fmt.Errorf("unmarshalling header: %w", err)
 	}
 
 	metricDeviceRecvDecompressedBytes.WithLabelValues(c.idString).Add(float64(2 + len(buf)))