nbd/server: Optimize final chunk of sparse read

If we are careful to handle 0-length read requests correctly,
we can optimize our sparse read to send the NBD_REPLY_FLAG_DONE
bit on our last OFFSET_DATA or OFFSET_HOLE chunk rather than
needing a separate chunk.
Signed-off-by: NEric Blake <eblake@redhat.com>
Message-Id: <20171107030912.23930-3-eblake@redhat.com>
Reviewed-by: NVladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>

nbd/server.c

@@ -1339,12 +1339,14 @@ static int coroutine_fn nbd_co_send_sparse_read(NBDClient *client,
                                              offset + progress,
                                              size - progress, &pnum, NULL,
                                              NULL);
+        bool final;
 
         if (status < 0) {
             error_setg_errno(errp, -status, "unable to check for holes");
             return status;
         }
         assert(pnum && pnum <= size - progress);
+        final = progress + pnum == size;
         if (status & BDRV_BLOCK_ZERO) {
             NBDStructuredReadHole chunk;
             struct iovec iov[] = {
@@ -1353,7 +1355,8 @@ static int coroutine_fn nbd_co_send_sparse_read(NBDClient *client,
 
             trace_nbd_co_send_structured_read_hole(handle, offset + progress,
                                                    pnum);
-            set_be_chunk(&chunk.h, 0, NBD_REPLY_TYPE_OFFSET_HOLE,
+            set_be_chunk(&chunk.h, final ? NBD_REPLY_FLAG_DONE : 0,
+                         NBD_REPLY_TYPE_OFFSET_HOLE,
                          handle, sizeof(chunk) - sizeof(chunk.h));
             stq_be_p(&chunk.offset, offset + progress);
             stl_be_p(&chunk.length, pnum);
@@ -1366,7 +1369,7 @@ static int coroutine_fn nbd_co_send_sparse_read(NBDClient *client,
                 break;
             }
             ret = nbd_co_send_structured_read(client, handle, offset + progress,
-                                              data + progress, pnum, false,
+                                              data + progress, pnum, final,
                                               errp);
         }
 
@@ -1375,9 +1378,6 @@ static int coroutine_fn nbd_co_send_sparse_read(NBDClient *client,
         }
         progress += pnum;
     }
-    if (!ret) {
-        ret = nbd_co_send_structured_done(client, handle, errp);
-    }
     return ret;
 }
 
@@ -1542,7 +1542,8 @@ static coroutine_fn void nbd_trip(void *opaque)
             }
         }
 
-        if (client->structured_reply && !(request.flags & NBD_CMD_FLAG_DF)) {
+        if (client->structured_reply && !(request.flags & NBD_CMD_FLAG_DF) &&
+            request.len) {
             ret = nbd_co_send_sparse_read(req->client, request.handle,
                                           request.from, req->data, request.len,
                                           &local_err);