In order to support running an NBD export on inactive nodes, we must
make sure to return errors for any operations that aren't allowed on
inactive nodes. Reads are the only operation we know we need for
inactive images, so to err on the side of caution, return errors for
everything else, even if some operations could possibly be okay.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
---
 nbd/server.c | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/nbd/server.c b/nbd/server.c
index f64e47270c..2076fb2666 100644
--- a/nbd/server.c
+++ b/nbd/server.c
@@ -2026,6 +2026,7 @@ static void nbd_export_delete(BlockExport *blk_exp)
 const BlockExportDriver blk_exp_nbd = {
     .type               = BLOCK_EXPORT_TYPE_NBD,
     .instance_size      = sizeof(NBDExport),
+    .supports_inactive  = true,
     .create             = nbd_export_create,
     .delete             = nbd_export_delete,
     .request_shutdown   = nbd_export_request_shutdown,
@@ -2920,6 +2921,22 @@ static coroutine_fn int nbd_handle_request(NBDClient *client,
     NBDExport *exp = client->exp;
     char *msg;
     size_t i;
+    bool inactive;
+
+    WITH_GRAPH_RDLOCK_GUARD() {
+        inactive = bdrv_is_inactive(blk_bs(exp->common.blk));
+        if (inactive) {
+            switch (request->type) {
+            case NBD_CMD_READ:
+                /* These commands are allowed on inactive nodes */
+                break;
+            default:
+                /* Return an error for the rest */
+                return nbd_send_generic_reply(client, request, -EPERM,
+                                              "export is inactive", errp);
+            }
+        }
+    }
 
     switch (request->type) {
     case NBD_CMD_CACHE:
-- 
2.48.1