nbd: add empty .bdrv_reopen_prepare

Fixes commit job / qemu-img commit, when
commiting qcow2 file which is based on nbd export.

Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=1718727
Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
Message-id: 20190930213820.29777-2-mlevitsk@redhat.com
Signed-off-by: Max Reitz <mreitz@redhat.com>

block/nbd.c

@@ -1158,6 +1158,18 @@ static int coroutine_fn nbd_client_co_block_status(
         BDRV_BLOCK_OFFSET_VALID;
 }
 
+static int nbd_client_reopen_prepare(BDRVReopenState *state,
+                                     BlockReopenQueue *queue, Error **errp)
+{
+    BDRVNBDState *s = (BDRVNBDState *)state->bs->opaque;
+
+    if ((state->flags & BDRV_O_RDWR) && (s->info.flags & NBD_FLAG_READ_ONLY)) {
+        error_setg(errp, "Can't reopen read-only NBD mount as read/write");
+        return -EACCES;
+    }
+    return 0;
+}
+
 static void nbd_client_close(BlockDriverState *bs)
 {
     BDRVNBDState *s = (BDRVNBDState *)bs->opaque;
@@ -1798,6 +1810,7 @@ static BlockDriver bdrv_nbd = {
     .instance_size              = sizeof(BDRVNBDState),
     .bdrv_parse_filename        = nbd_parse_filename,
     .bdrv_file_open             = nbd_open,
+    .bdrv_reopen_prepare        = nbd_client_reopen_prepare,
     .bdrv_co_preadv             = nbd_client_co_preadv,
     .bdrv_co_pwritev            = nbd_client_co_pwritev,
     .bdrv_co_pwrite_zeroes      = nbd_client_co_pwrite_zeroes,
@@ -1820,6 +1833,7 @@ static BlockDriver bdrv_nbd_tcp = {
     .instance_size              = sizeof(BDRVNBDState),
     .bdrv_parse_filename        = nbd_parse_filename,
     .bdrv_file_open             = nbd_open,
+    .bdrv_reopen_prepare        = nbd_client_reopen_prepare,
     .bdrv_co_preadv             = nbd_client_co_preadv,
     .bdrv_co_pwritev            = nbd_client_co_pwritev,
     .bdrv_co_pwrite_zeroes      = nbd_client_co_pwrite_zeroes,
@@ -1842,6 +1856,7 @@ static BlockDriver bdrv_nbd_unix = {
     .instance_size              = sizeof(BDRVNBDState),
     .bdrv_parse_filename        = nbd_parse_filename,
     .bdrv_file_open             = nbd_open,
+    .bdrv_reopen_prepare        = nbd_client_reopen_prepare,
     .bdrv_co_preadv             = nbd_client_co_preadv,
     .bdrv_co_pwritev            = nbd_client_co_pwritev,
     .bdrv_co_pwrite_zeroes      = nbd_client_co_pwrite_zeroes,