smbd calls gencache_stabilize() from exit_server_common() upon process shutdown.
But the signal handlers for SIGTERM, SIGHUP have been unregistered by TALLOC_FREE(global_smbXsrv_client) already then.
So, gencache_stabilize() can be interrupted and killed at any time if smbd receives signals.
This leads to a silent gencache_notrans.tdb and gencache.tdb corruption.

Proposed Fixes:
1 Move gencache_stabilize() before TALLOC_FREEs.
2 Let gencache_stabilize() block signals.
3 Do we really need gencache_stabilize() on smbd? Smbd does not seem to use gencache so often.

Logs:

# /usr/local/samba/sbin/smbd --version
Version 4.9.0pre1-GIT-2610a3f

# cat /proc/10442/status
Name:   smbd

SigBlk: 0000000000001880
SigIgn: 0000000000000000
SigCgt: 0000000180014461

Let smbd exit.

(gdb) bt
#0  0x00007f4c75d9fab9 in gencache_stabilize ()
   from /usr/local/samba/lib/libsmbconf.so.0
#1  0x00007f4c780472ad in exit_server_common ()
   from /usr/local/samba/lib/private/libsmbd-base-samba4.so
#2  0x00007f4c780472f1 in smbd_exit_server_cleanly ()
   from /usr/local/samba/lib/private/libsmbd-base-samba4.so
#3  0x00007f4c749acc17 in exit_server_cleanly ()
   from /usr/local/samba/lib/private/libsmbd-shim-samba4.so
#4  0x00007f4c7800d4cd in smbd_server_connection_terminate_ex ()
   from /usr/local/samba/lib/private/libsmbd-base-samba4.so
#5  0x00007f4c7801e4d4 in smbd_smb2_request_create_done ()
   from /usr/local/samba/lib/private/libsmbd-base-samba4.so

SigBlk: 0000000000001880
SigIgn: 0000000000000000
SigCgt: 0000000180010460 XXX SIGTERM mask 0x4000 is gone.

# kill -TERM 10442

(gdb) c
Continuing.

Program received signal SIGTERM, Terminated.
0x00007f4c75d9fab9 in gencache_stabilize ()
   from /usr/local/samba/lib/libsmbconf.so.0
(gdb) c
Continuing.

Program terminated with signal SIGTERM, Terminated.
The program no longer exists.
(gdb)