nbd/server: Fix structured read of length 0

The NBD spec was recently clarified to state that a read of length 0
should not be attempted by a compliant client; but that a server must
still handle it correctly in an unspecified manner (that is, either
a successful no-op or an error reply, but not a crash) [1].  However,
it also implies that NBD_REPLY_TYPE_OFFSET_DATA must have a non-zero
payload length, but our existing code was replying with a chunk
that a picky client could reject as invalid because it was missing
a payload (our own client implementation was recently patched to be
that picky, after first fixing it to not send 0-length requests).

We are already doing successful no-ops for 0-length writes and for
non-structured reads; so for consistency, we want structured reply
reads to also be a no-op.  The easiest way to do this is to return
a NBD_REPLY_TYPE_NONE chunk; this is best done via a new helper
function (especially since future patches for other structured
replies may benefit from using the same helper).

[1] https://github.com/NetworkBlockDevice/nbd/commit/ee926037

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <20171108215703.9295-8-eblake@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>

nbd/server.c

@@ -1273,6 +1273,21 @@ static inline void set_be_chunk(NBDStructuredReplyChunk *chunk, uint16_t flags,
     stl_be_p(&chunk->length, length);
 }
 
+static int coroutine_fn nbd_co_send_structured_done(NBDClient *client,
+                                                    uint64_t handle,
+                                                    Error **errp)
+{
+    NBDStructuredReplyChunk chunk;
+    struct iovec iov[] = {
+        {.iov_base = &chunk, .iov_len = sizeof(chunk)},
+    };
+
+    trace_nbd_co_send_structured_done(handle);
+    set_be_chunk(&chunk, NBD_REPLY_FLAG_DONE, NBD_REPLY_TYPE_NONE, handle, 0);
+
+    return nbd_co_send_iov(client, iov, 1, errp);
+}
+
 static int coroutine_fn nbd_co_send_structured_read(NBDClient *client,
                                                     uint64_t handle,
                                                     uint64_t offset,
@@ -1286,6 +1301,7 @@ static int coroutine_fn nbd_co_send_structured_read(NBDClient *client,
         {.iov_base = data, .iov_len = size}
     };
 
+    assert(size);
     trace_nbd_co_send_structured_read(handle, offset, data, size);
     set_be_chunk(&chunk.h, NBD_REPLY_FLAG_DONE, NBD_REPLY_TYPE_OFFSET_DATA,
                  handle, sizeof(chunk) - sizeof(chunk.h) + size);
@@ -1544,10 +1560,13 @@ reply:
         if (ret < 0) {
             ret = nbd_co_send_structured_error(req->client, request.handle,
                                                -ret, msg, &local_err);
-        } else {
+        } else if (reply_data_len) {
             ret = nbd_co_send_structured_read(req->client, request.handle,
                                               request.from, req->data,
                                               reply_data_len, &local_err);
+        } else {
+            ret = nbd_co_send_structured_done(req->client, request.handle,
+                                              &local_err);
         }
     } else {
         ret = nbd_co_send_simple_reply(req->client, request.handle,

nbd/trace-events

@@ -55,6 +55,7 @@ nbd_receive_request(uint32_t magic, uint16_t flags, uint16_t type, uint64_t from
 nbd_blk_aio_attached(const char *name, void *ctx) "Export %s: Attaching clients to AIO context %p\n"
 nbd_blk_aio_detach(const char *name, void *ctx) "Export %s: Detaching clients from AIO context %p\n"
 nbd_co_send_simple_reply(uint64_t handle, uint32_t error, const char *errname, int len) "Send simple reply: handle = %" PRIu64 ", error = %" PRIu32 " (%s), len = %d"
+nbd_co_send_structured_done(uint64_t handle) "Send structured reply done: handle = %" PRIu64
 nbd_co_send_structured_read(uint64_t handle, uint64_t offset, void *data, size_t size) "Send structured read data reply: handle = %" PRIu64 ", offset = %" PRIu64 ", data = %p, len = %zu"
 nbd_co_send_structured_error(uint64_t handle, int err, const char *errname, const char *msg) "Send structured error reply: handle = %" PRIu64 ", error = %d (%s), msg = '%s'"
 nbd_co_receive_request_decode_type(uint64_t handle, uint16_t type, const char *name) "Decoding type: handle = %" PRIu64 ", type = %" PRIu16 " (%s)"