netstack: return err from Writes on invalid fd

Author: ignoramous

intra/netstack/fdbased.go

@@ -53,9 +53,6 @@ var _ FdSwapper = (*linkFdSwap)(nil)
 // placeholder FD for whenever existing FD wrapped in struct fds is closed.
 const invalidfd int = -1
 
-// Should WritePackets return NoSuchFile error if the fd is invalid?
-const errorOnInvalidFD = false
-
 // wrapttl is the time to wait for the dispatcher to wrap up (close a previous FD).
 const waitttl = wrapttl
 
@@ -505,10 +502,7 @@ func (e *endpoint) WritePackets(pkts stack.PacketBufferList) (int, tcpip.Error)
 
 	if fd == invalidfd {
 		log.E("ns: tun(-1): WritePackets (to tun): fd invalid (pkts: %d)", pkts.Len())
-		if errorOnInvalidFD {
-			return 0, &tcpip.ErrNoSuchFile{}
-		}
-		return 0, nil
+		return 0, &tcpip.ErrNoSuchFile{}
 	}
 
 	batch := make([]unix.Iovec, 0, batchSz)
@@ -637,10 +631,7 @@ func (e *endpoint) InjectOutbound(dest tcpip.Address, packet *buffer.View) tcpip
 
 	if !f.ok() {
 		log.E("ns: tun(%d): inject-outbound (to tun) to dst(%v): endpoint not attached", fd, dest)
-		if errorOnInvalidFD {
-			return &tcpip.ErrUnknownDevice{}
-		}
-		return nil // nothing to do
+		return &tcpip.ErrUnknownDevice{}
 	}
 
 	b := packet.AsSlice()