[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.

1 files changed, 3 insertions, 5 deletions

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 );