netcmd: Workaround issue backing up offline domain with lmdb >= 0.9.26

netcmd: Workaround issue backing up offline domain with lmdb >= 0.9.26

The LMDB change "ITS#9278 fix robust mutex cleanup for FreeBSD" released
in version 0.9.26 makes samba-tool domain backup offline to fail with
the following error:

Failed to connect to 'mdb:///tmp/foo/private/sam.ldb.d/CN=CONFIGURATION,DC=FOO,DC=EXAMPLE,DC=COM.ldb' with backend 'mdb': Unable to load ltdb cache records for backend 'ldb_mdb backend'
module samba_dsdb initialization failed : Operations error
Unable to load modules for /tmp/foo/private/sam.ldb.bak-offline: Unable to load ltdb cache records for backend 'ldb_mdb backend'
ERROR(ldb): uncaught exception - Unable to load ltdb cache records for backend 'ldb_mdb backend'
  File "/usr/local/samba/lib64/python3.6/site-packages/samba/netcmd/__init__.py", line 186, in _run
    return self.run(*args, **kwargs)
  File "/usr/local/samba/lib64/python3.6/site-packages/samba/netcmd/domain_backup.py", line 1147, in run
    session_info=system_session(), lp=lp)
  File "/usr/local/samba/lib64/python3.6/site-packages/samba/samdb.py", line 72, in __init__
    options=options)
  File "/usr/local/samba/lib64/python3.6/site-packages/samba/__init__.py", line 114, in __init__
    self.connect(url, flags, options)
  File "/usr/local/samba/lib64/python3.6/site-packages/samba/samdb.py", line 87, in connect
    options=options)

The error occurs opening the backed ldb to write the backup date and the
next SID, a call to pthread_mutex_lock in mdb_txn_renew0 (frame 8) returns
EINVAL:

  #0  0x00007ff63c2f1bea in wait4 () from /lib64/libc.so.6
  #1  0x00007ff63c26f3a3 in do_system () from /lib64/libc.so.6
  #2  0x00007ff63bc71e94 in smb_panic_default (why=0x7ffed481b7d0 "Signal 6: Aborted") at ../../lib/util/fault.c:153
  #3  0x00007ff63bc72168 in smb_panic (why=0x7ffed481b7d0 "Signal 6: Aborted") at ../../lib/util/fault.c:200
  #4  0x00007ff63bc71c82 in fault_report (sig=6) at ../../lib/util/fault.c:81
  #5  0x00007ff63bc71c97 in sig_fault (sig=6) at ../../lib/util/fault.c:92
  #6  <signal handler called>
  #7  0x00007ff63c2178b5 in raise () from /lib64/libpthread.so.0
  #8  0x00007ff637602e65 in mdb_txn_renew0 (txn=txn@entry=0x55d6f97fb800) at mdb.c:2710
  #9  0x00007ff637603ae8 in mdb_txn_begin (env=0x55d6f85dfa80, parent=0x0, flags=131072, ret=0x55d6f89c0928)
      at mdb.c:2912
  #10 0x00007ff6376236cc in lmdb_lock_read (module=0x55d6f8c5f4b0) at ../../lib/ldb/ldb_mdb/ldb_mdb.c:585
  #11 0x00007ff637641de6 in ldb_kv_cache_load (module=0x55d6f8c5f4b0) at ../../lib/ldb/ldb_key_value/ldb_kv_cache.c:450
  #12 0x00007ff637638792 in ldb_kv_init_store (ldb_kv=0x55d6f8af2a80, name=0x7ff637625675 "ldb_mdb backend",
      ldb=0x55d6f8cd22b0, options=0x0, _module=0x7ffed481c248) at ../../lib/ldb/ldb_key_value/ldb_kv.c:2166
  #13 0x00007ff6376247ba in lmdb_connect (ldb=0x55d6f8cd22b0,
      url=0x55d6f85d41f0 "mdb:///tmp/foo/private/sam.ldb.d/CN=CONFIGURATION,DC=FOO,DC=EXAMPLE,DC=COM.ldb", flags=64,
      options=0x0, _module=0x7ffed481c248) at ../../lib/ldb/ldb_mdb/ldb_mdb.c:1143
  #14 0x00007ff63bd94d2f in ldb_module_connect_backend (ldb=0x55d6f8cd22b0,
      url=0x55d6f85d41f0 "mdb:///tmp/foo/private/sam.ldb.d/CN=CONFIGURATION,DC=FOO,DC=EXAMPLE,DC=COM.ldb",
      options=0x0, backend_module=0x7ffed481c248) at ../../lib/ldb/common/ldb_modules.c:221
  #15 0x00007ff6375a4baf in new_partition_from_dn (ldb=0x55d6f8cd22b0, data=0x55d6f858bed0, mem_ctx=0x55d6f8a03cd0,
      dn=0x55d6f9865450, filename=0x55d6f860b6da "sam.ldb.d/CN=CONFIGURATION,DC=FOO,DC=EXAMPLE,DC=COM.ldb",
      backend_db_store=0x55d6f9d378e0 "mdb", partition=0x7ffed481c308)
      at ../../source4/dsdb/samdb/ldb_modules/partition_init.c:257
  #16 0x00007ff6375a57b9 in partition_reload_if_required (module=0x55d6f8972d10, data=0x55d6f858bed0, parent=0x0)
      at ../../source4/dsdb/samdb/ldb_modules/partition_init.c:513
  #17 0x00007ff6375a3b04 in partition_read_lock (module=0x55d6f8972d10)
      at ../../source4/dsdb/samdb/ldb_modules/partition.c:1492
  #18 0x00007ff63bd9631e in ldb_next_read_lock (module=0x55d6f8972d10) at ../../lib/ldb/common/ldb_modules.c:662
  #19 0x00007ff637484857 in schema_read_lock (module=0x55d6f9377e40)
      at ../../source4/dsdb/samdb/ldb_modules/schema_load.c:614
  #20 0x00007ff63bd9631e in ldb_next_read_lock (module=0x55d6f9377e40) at ../../lib/ldb/common/ldb_modules.c:662
  #21 0x00007ff6374b5402 in samba_dsdb_init (module=0x55d6f91c3cd0)
      at ../../source4/dsdb/samdb/ldb_modules/samba_dsdb.c:483
  #22 0x00007ff63bd95283 in ldb_module_init_chain (ldb=0x55d6f8cd22b0, module=0x55d6f91c3cd0)
      at ../../lib/ldb/common/ldb_modules.c:363
  #23 0x00007ff63bd95645 in ldb_load_modules (ldb=0x55d6f8cd22b0, options=0x0)
      at ../../lib/ldb/common/ldb_modules.c:445
  #24 0x00007ff63bd90663 in ldb_connect (ldb=0x55d6f8cd22b0,
      url=0x7ff6377d98f8 "/tmp/foo/private/sam.ldb.bak-offline", flags=64, options=0x0)
      at ../../lib/ldb/common/ldb.c:274
  #25 0x00007ff63bddb32f in py_ldb_connect (self=0x7ff63778afc0, args=(), Python Exception <class 'gdb.error'> There is no member named ma_keys.:
  kwargs=) at ../../lib/ldb/pyldb.c:1235

Deleting the previous samdb instance by setting it to None before opening the
backed ldb workaround the problem until we find the real problem here.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=14676

Signed-off-by: Samuel Cabrero <scabrero@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>