[Qemu-devel] [PATCH] nbd: strict nbd_wr_syncv

[Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>]

nbd_wr_syncv is called either from coroutine or from client negotiation
code, when socket is in blocking mode. So, -EAGAIN is impossible.

Furthermore, EAGAIN is confusing, as, what to read/write again? With
EAGAIN as a return code we don't know how much data is already
read or written by the function, so in case of EAGAIN the whole
communication is broken.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
---

Hi all!

If I understand right, nbd_wr_syncv is called either from coroutine
or from client negotiation code, when socket is in blocking mode.
So, some thoughts

1. let's establish this with an assert, because returning EAGAIN is
   confusing (see above)
2. should we in case of non-coroutine context start this coroutine in 
   nbd_wr_syncv, like in bdrv_prwv_co, and use non-blocking mode?
3. is there any problems or disadvantages of moving  client negotiation
   to coroutine too?

 nbd/common.c | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/nbd/common.c b/nbd/common.c
index dccbb8e9de..4db45b3ede 100644
--- a/nbd/common.c
+++ b/nbd/common.c
@@ -20,6 +20,10 @@
 #include "qapi/error.h"
 #include "nbd-internal.h"
 
+/* nbd_wr_syncv
+ * The function may be called from coroutine or from non-coroutine context.
+ * When called from non-coroutine context @ioc must be in blocking mode.
+ */
 ssize_t nbd_wr_syncv(QIOChannel *ioc,
                      struct iovec *iov,
                      size_t niov,
@@ -42,11 +46,8 @@ ssize_t nbd_wr_syncv(QIOChannel *ioc,
             len = qio_channel_writev(ioc, local_iov, nlocal_iov, &local_err);
         }
         if (len == QIO_CHANNEL_ERR_BLOCK) {
-            if (qemu_in_coroutine()) {
-                qio_channel_yield(ioc, do_read ? G_IO_IN : G_IO_OUT);
-            } else {
-                return -EAGAIN;
-            }
+            assert(qemu_in_coroutine());
+            qio_channel_yield(ioc, do_read ? G_IO_IN : G_IO_OUT);
             continue;
         }
         if (len < 0) {
-- 
2.11.1