[iobuf] Add iob_disown() and use it where it simplifies code

There are many functions that take ownership of the I/O buffer they
are passed as a parameter.  The caller should not retain a pointer to
the I/O buffer.  Use iob_disown() to automatically nullify the
caller's pointer, e.g.:

    xfer_deliver_iob ( xfer, iob_disown ( iobuf ) );

This will ensure that iobuf is set to NULL for any code after the call
to xfer_deliver_iob().

iob_disown() is currently used only in places where it simplifies the
code, by avoiding an extra line explicitly setting the I/O buffer
pointer to NULL.  It should ideally be used with each call to any
function that takes ownership of an I/O buffer.  (The SSA
optimisations will ensure that use of iob_disown() gets optimised away
in cases where the caller makes no further use of the I/O buffer
pointer anyway.)

If gcc ever introduces an __attribute__((free)), indicating that use
of a function argument after a function call should generate a
warning, then we should use this to identify all applicable function
call sites, and add iob_disown() as necessary.

5 files changed, 9 insertions, 14 deletions

diff --git a/src/net/arp.c b/src/net/arp.c
index 011d4fef..ba9ebf48 100644
--- a/src/net/arp.c
+++ b/src/net/arp.c
@@ -265,8 +265,8 @@ static int arp_rx ( struct io_buffer *iobuf, struct net_device *netdev,
 	memcpy ( arp_sender_ha ( arphdr ), netdev->ll_addr, arphdr->ar_hln );
 
 	/* Send reply */
-	net_tx ( iobuf, netdev, &arp_protocol, arp_target_ha (arphdr ) );
-	iobuf = NULL;
+	net_tx ( iob_disown ( iobuf ), netdev, &arp_protocol,
+		 arp_target_ha ( arphdr ) );
 
  done:
 	free_iob ( iobuf );
diff --git a/src/net/tcp/http.c b/src/net/tcp/http.c
index 4dc1ab73..46b5077a 100644
--- a/src/net/tcp/http.c
+++ b/src/net/tcp/http.c
@@ -340,8 +340,7 @@ static int http_socket_deliver_iob ( struct xfer_interface *socket,
 			/* Once we're into the data phase, just fill
 			 * the data buffer
 			 */
-			rc = http_rx_data ( http, iobuf );
-			iobuf = NULL;
+			rc = http_rx_data ( http, iob_disown ( iobuf ) );
 			goto done;
 		case HTTP_RX_RESPONSE:
 		case HTTP_RX_HEADER:
diff --git a/src/net/udp.c b/src/net/udp.c
index 63c2d9ee..57edb9c6 100644
--- a/src/net/udp.c
+++ b/src/net/udp.c
@@ -328,8 +328,7 @@ static int udp_rx ( struct io_buffer *iobuf, struct sockaddr_tcpip *st_src,
 	memset ( &meta, 0, sizeof ( meta ) );
 	meta.src = ( struct sockaddr * ) st_src;
 	meta.dest = ( struct sockaddr * ) st_dest;
-	rc = xfer_deliver_iob_meta ( &udp->xfer, iobuf, &meta );
-	iobuf = NULL;
+	rc = xfer_deliver_iob_meta ( &udp->xfer, iob_disown ( iobuf ), &meta );
 
  done:
 	free_iob ( iobuf );
diff --git a/src/net/udp/dhcp.c b/src/net/udp/dhcp.c
index 26c50172..6ebf6021 100644
--- a/src/net/udp/dhcp.c
+++ b/src/net/udp/dhcp.c
@@ -976,9 +976,8 @@ static int dhcp_tx ( struct dhcp_session *dhcp ) {
 
 	/* Transmit the packet */
 	iob_put ( iobuf, dhcppkt.len );
-	rc = xfer_deliver_iob_meta ( &dhcp->xfer, iobuf, &meta );
-	iobuf = NULL;
-	if ( rc != 0 ) {
+	if ( ( rc = xfer_deliver_iob_meta ( &dhcp->xfer, iob_disown ( iobuf ),
+					    &meta ) ) != 0 ) {
 		DBGC ( dhcp, "DHCP %p could not transmit UDP packet: %s\n",
 		       dhcp, strerror ( rc ) );
 		goto done;
diff --git a/src/net/udp/tftp.c b/src/net/udp/tftp.c
index 13734b0f..ec6b1b40 100644
--- a/src/net/udp/tftp.c
+++ b/src/net/udp/tftp.c
@@ -763,9 +763,8 @@ static int tftp_rx_data ( struct tftp_request *tftp,
 	memset ( &meta, 0, sizeof ( meta ) );
 	meta.whence = SEEK_SET;
 	meta.offset = offset;
-	rc = xfer_deliver_iob_meta ( &tftp->xfer, iobuf, &meta );
-	iobuf = NULL;
-	if ( rc != 0 ) {
+	if ( ( rc = xfer_deliver_iob_meta ( &tftp->xfer, iob_disown ( iobuf ),
+					    &meta ) ) != 0 ) {
 		DBGC ( tftp, "TFTP %p could not deliver data: %s\n",
 		       tftp, strerror ( rc ) );
 		goto done;
@@ -887,8 +886,7 @@ static int tftp_rx ( struct tftp_request *tftp,
 		rc = tftp_rx_oack ( tftp, iobuf->data, len );
 		break;
 	case htons ( TFTP_DATA ):
-		rc = tftp_rx_data ( tftp, iobuf );
-		iobuf = NULL;
+		rc = tftp_rx_data ( tftp, iob_disown ( iobuf ) );
 		break;
 	case htons ( TFTP_ERROR ):
 		rc = tftp_rx_error ( tftp, iobuf->data, len );