[xferbuf] Record maximum required size

Record the maximum size required when writing into a data transfer
buffer.  This allows the maximum size to be determined even if
allocation fails (e.g. due to a fixed-size buffer or an out-of-memory
condition).

In the case of a fixed-size buffer (which may already be larger than
required), this allows the caller to determine the actual size used
for written data.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/core/xferbuf.c

@@ -63,6 +63,7 @@ void xferbuf_detach ( struct xfer_buffer *xferbuf ) {
 
 	xferbuf->data = NULL;
 	xferbuf->len = 0;
+	xferbuf->max = 0;
 	xferbuf->pos = 0;
 }
 
@@ -87,6 +88,10 @@ void xferbuf_free ( struct xfer_buffer *xferbuf ) {
 static int xferbuf_ensure_size ( struct xfer_buffer *xferbuf, size_t len ) {
 	int rc;
 
+	/* Record maximum required size */
+	if ( len > xferbuf->max )
+		xferbuf->max = len;
+
 	/* If buffer is already large enough, do nothing */
 	if ( len <= xferbuf->len )
 		return 0;

src/include/ipxe/xferbuf.h

@@ -21,6 +21,8 @@ struct xfer_buffer {
 	void *data;
 	/** Size of data */
 	size_t len;
+	/** Maximum required size of data */
+	size_t max;
 	/** Current offset within data */
 	size_t pos;
 	/** Data transfer buffer operations */