Re: [Libguestfs] [libnbd PATCH v4 4/4] internal: Refactor layout of replies in sbuf
On Thu, Jun 08, 2023 at 09:17:38PM -0500, Eric Blake wrote:
In order to more easily add a third reply type with an even larger
header, but where the payload will look the same for both structured
and extended replies, it is nicer if simple and structured replies are
nested inside the same layer of sbuf.reply.hdr. Doing this also lets
us add an alias for accessing the cookie directly without picking
which header type we have on hand.
[...]
Although we do not use union payload with simple replies, a later
patch does use payload for two out of the three possible headers, and
it does not hurt for sbuf to call out storage for more than we use on
a particular state machine sequence. The commit is largely
mechanical, and there should be no semantic change.
Signed-off-by: Eric Blake <eblake(a)redhat.com>
---
lib/internal.h | 16 +++++--
generator/states-reply.c | 35 +++++++-------
generator/states-reply-chunk.c | 84 ++++++++++++++++-----------------
generator/states-reply-simple.c | 4 +-
4 files changed, 76 insertions(+), 63 deletions(-)
Here's what I'm leaning towards squashing in, now that patch 2/4
landed as 2e34ceb8.
diff --git a/generator/states-reply.c b/generator/states-reply.c
index af5f6135..0d200513 100644
--- a/generator/states-reply.c
+++ b/generator/states-reply.c
@@ -19,6 +19,11 @@
#include <assert.h>
#include <stddef.h>
+#define ASSERT_MEMBER_ALIAS(type, member_a, member_b) \
+ STATIC_ASSERT (offsetof (type, member_a) == offsetof (type, member_b) && \
+ sizeof ((type *)NULL)->member_a == \
+ sizeof ((type *)NULL)->member_b, member_alias)
+
/* State machine for receiving reply messages from the server.
*
* Note that we never block while in this sub-group. If there is
@@ -70,12 +75,10 @@ REPLY.START:
* structs (an intentional design decision in the NBD spec when
* structured replies were added).
*/
- STATIC_ASSERT (offsetof (union reply_header, simple.cookie) ==
- offsetof (union reply_header, cookie),
- _cookie_aliasing_simple);
- STATIC_ASSERT (offsetof (union reply_header, structured.cookie) ==
- offsetof (union reply_header, cookie),
- _cookie_aliasing_structured);
+ ASSERT_MEMBER_ALIAS (union reply_header, simple.magic, magic);
+ ASSERT_MEMBER_ALIAS (union reply_header, simple.cookie, cookie);
+ ASSERT_MEMBER_ALIAS (union reply_header, structured.magic, magic);
+ ASSERT_MEMBER_ALIAS (union reply_header, structured.cookie, cookie);
assert (h->reply_cmd == NULL);
assert (h->rlen == 0);
--
Eric Blake, Principal Software Engineer
Red Hat, Inc. +1-919-301-3266
Virtualization: qemu.org | libvirt.org