[Nbd] [PATCH] doc: Restore a lost sentence about NBD_REP_ERR_BLOCK_SIZE_REQD

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

The previous patch accidentally dropped the suggestion that a
server replying with NBD_REP_ERR_BLOCK_SIZE_REQD should first
send NBD_REP_INFO with NBD_INFO_BLOCK_SIZE, so that the client
doesn't have to do yet another round trip NBD_OPT_INFO to learn
what sizes the server plans to enforce.

Signed-off-by: Eric Blake <eblake@...696...>
---

extension-info branch

 doc/proto.md | 13 +++++++------
 1 file changed, 7 insertions(+), 6 deletions(-)

diff --git a/doc/proto.md b/doc/proto.md
index 1a0f1cf..da65df0 100644
--- a/doc/proto.md
+++ b/doc/proto.md
@@ -949,12 +949,13 @@ of the newstyle negotiation.
     - `NBD_REP_ERR_BLOCK_SIZE_REQD`: The server requires the client to
       request block size constraints using `NBD_INFO_BLOCK_SIZE` prior
       to entering transmission phase, because the server will be using
-      non-default block sizes constraints. The server MUST NOT send this
-      error if block size constraints were requested with
-      `NBD_INFO_BLOCK_SIZE` with the `NBD_OPT_INFO` or `NBD_OPT_GO`
-      request. The server SHOULD NOT send this error if it is using
-      default block size constraints or block size constraints
-      negotiated out of band.
+      non-default block sizes constraints. The server SHOULD first
+      send at least an `NBD_INFO_BLOCK_SIZE` information reply before
+      giving this error.  The server MUST NOT send this error if block
+      size constraints were requested with `NBD_INFO_BLOCK_SIZE` with
+      the `NBD_OPT_INFO` or `NBD_OPT_GO` request. The server SHOULD
+      NOT send this error if it is using default block size
+      constraints or block size constraints negotiated out of band.

     Additionally, if TLS has not been initiated, the server MAY reply
     with `NBD_REP_ERR_TLS_REQD` (instead of `NBD_REP_ERR_UNKNOWN`) to
-- 
2.5.5