[PULL 16/37] nbd/server: Simplify export shutdown

[Kevin Wolf <kwolf@redhat.com>]

Closing export is somewhat convoluted because nbd_export_close() and
nbd_export_put() call each other and the ways they actually end up being
nested is not necessarily obvious.

However, it is not really necessary to call nbd_export_close() from
nbd_export_put() when putting the last reference because it only does
three things:

1. Close all clients. We're going to refcount 0 and all clients hold a
   reference, so we know there is no active client any more.

2. Close the user reference (represented by exp->name being non-NULL).
   The same argument applies: If the export were still named, we would
   still have a reference.

3. Freeing exp->description. This is really cleanup work to be done when
   the export is finally freed. There is no reason to already clear it
   while clients are still in the process of shutting down.

So after moving the cleanup of exp->description, the code can be
simplified so that only nbd_export_close() calls nbd_export_put(), but
never the other way around.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20200924152717.287415-13-kwolf@redhat.com>
Acked-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
---
 nbd/server.c | 17 ++++-------------
 1 file changed, 4 insertions(+), 13 deletions(-)

diff --git a/nbd/server.c b/nbd/server.c
index 1cc915f01d..fb70374df5 100644
--- a/nbd/server.c
+++ b/nbd/server.c
@@ -1678,8 +1678,6 @@ void nbd_export_close(NBDExport *exp)
         QTAILQ_REMOVE(&exports, exp, next);
         QTAILQ_INSERT_TAIL(&closed_exports, exp, next);
     }
-    g_free(exp->description);
-    exp->description = NULL;
     nbd_export_put(exp);
 }
 
@@ -1706,19 +1704,12 @@ void nbd_export_get(NBDExport *exp)
 void nbd_export_put(NBDExport *exp)
 {
     assert(exp->refcount > 0);
-    if (exp->refcount == 1) {
-        nbd_export_close(exp);
-    }
-
-    /* nbd_export_close() may theoretically reduce refcount to 0. It may happen
-     * if someone calls nbd_export_put() on named export not through
-     * nbd_export_set_name() when refcount is 1. So, let's assert that
-     * it is > 0.
-     */
-    assert(exp->refcount > 0);
     if (--exp->refcount == 0) {
         assert(exp->name == NULL);
-        assert(exp->description == NULL);
+        assert(QTAILQ_EMPTY(&exp->clients));
+
+        g_free(exp->description);
+        exp->description = NULL;
 
         if (exp->blk) {
             if (exp->eject_notifier_blk) {
-- 
2.25.4