[xfer] Add xfer_check_order() utility function

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

src/core/xfer.c

 
 	return xfer_deliver ( intf, iobuf, &meta );
 }
+
+/**
+ * Check that data is delivered strictly in order
+ *
+ * @v meta		Data transfer metadata
+ * @v pos		Current position
+ * @v len		Length of data
+ * @ret rc		Return status code
+ */
+int xfer_check_order ( struct xfer_metadata *meta, size_t *pos, size_t len ) {
+	size_t new_pos;
+
+	/* Allow out-of-order zero-length packets (as used by xfer_seek()) */
+	if ( len == 0 )
+		return 0;
+
+	/* Calculate position of this delivery */
+	new_pos = *pos;
+	if ( meta->flags & XFER_FL_ABS_OFFSET )
+		new_pos = 0;
+	new_pos += meta->offset;
+
+	/* Fail if delivery position is not equal to current position */
+	if ( new_pos != *pos )
+		return -EPROTO;
+
+	/* Update current position */
+	*pos += len;
+
+	return 0;
+}

src/include/ipxe/xfer.h

 extern int __attribute__ (( format ( printf, 2, 3 ) ))
 xfer_printf ( struct interface *intf, const char *format, ... );
 extern int xfer_seek ( struct interface *intf, off_t offset );
+extern int xfer_check_order ( struct xfer_metadata *meta, size_t *pos,
+			      size_t len );
 
 #endif /* _IPXE_XFER_H */