[SSSD] [sssd PR#5299][opened] dp: fix potential race condition in provider's sbus server

[pbrezina]

   URL: https://github.com/SSSD/sssd/pull/5299
Author: pbrezina
 Title: #5299: dp: fix potential race condition in provider's sbus server
Action: opened

PR body:
"""
We can hit a segfault if provider start is somehow delayed.

- dp_init_send
  - sbus_server_create_and_connect_send
    - sbus_server_create (*)
- dp_init_done (callback for sbus_server_create_and_connect_send)
  - sbus_server_create_and_connect_recv
  - sbus_server_set_on_connection (sets clients data and creates dp_cli)

At (*) sbus server is already created and accepts new connections once
we get into tevent loop. So it is possible that the client connects to
server before sbus_server_set_on_connection is called and thus the client
is not properly initialized. However it should not happen in normal start
because providers are started before responders and it can happen only if
data provider startup is somehow delay.

You can use this diff to reproduce the crash:
```diff
--- a/src/providers/data_provider_be.c
+++ b/src/providers/data_provider_be.c
@@ -702,6 +702,8 @@ int main(int argc, const char *argv[])
     uid_t uid;
     gid_t gid;

+    sleep(5);
+
     struct poptOption long_options[] = {
         POPT_AUTOHELP
         SSSD_MAIN_OPTS
```

Resolves:
https://github.com/SSSD/sssd/issues/5298
"""

To pull the PR as Git branch:
git remote add ghsssd https://github.com/SSSD/sssd
git fetch ghsssd pull/5299/head:pr5299
git checkout pr5299