[Nbd] [PATCH] Don't error on NBD_FLAG_CMD_FUA on non-writes

[Eric Blake <eblake@...696...>]

Commit ab22e082 changed the server to reject all unknown
flags with EINVAL, but in the process broke qemu as a client
(which in qemu 2.5 unconditionally FUA on NBD_CMD_FLUSH).
Commit e82baa5f relaxed the protocol to explicitly document
that servers must not reject FUA on any command (but MAY
ignore it where the command does not actually cause a write).
Fix the reference implementation to match.

Signed-off-by: Eric Blake <eblake@...696...>
---
 nbd-server.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/nbd-server.c b/nbd-server.c
index b222a11..4edb883 100644
--- a/nbd-server.c
+++ b/nbd-server.c
@@ -1523,7 +1523,7 @@ static void handle_request(gpointer data, gpointer user_data) {
 	uint32_t flags = package->req->type & ~NBD_CMD_MASK_COMMAND;
 	struct nbd_reply rep;

-	if(flags != 0 && (type != NBD_CMD_WRITE || flags != NBD_CMD_FLAG_FUA)) {
+	if(flags & ~NBD_CMD_FLAG_FUA) {
 		msg(LOG_ERR, "E: received invalid flag %d on command %d, ignoring", flags, type);
 		goto error;
 	}
-- 
2.5.5