tdb: Make tdb robust against improper CLEAR_IF_FIRST restart

tdb: Make tdb robust against improper CLEAR_IF_FIRST restart

When winbind is restarted, there is a potential crash in tdb. Following
situation: We are in a cluster with ctdb. A winbind child hangs
in a request to the DC. Cluster monitoring decides the node has a
problem. Cluster monitoring decides to kill ctdbd. winbind child
still hangs in a RPC request. winbind parent figures that ctdb is
dead and immediately commits suicide. winbind parent is restarted by
cluster management, overwriting gencache.tdb with CLEAR_IF_FIRST. The
CLEAR_IF_FIRST logic as implemented now will not see that a child still
has the tdb open, only the parent holds the ACTIVE_LOCK due to performance
reasons. During the CLEAR_IF_FIRST logic is done, there is a very small
window where we ftruncate(tfd, 0) the file and re-write a proper header
without a lock. When during this small window the winbind child comes
back, wanting to store something into gencache.tdb, that winbind child
will crash with a SIGBUS.

Sounds unlikely? See:

[2012/09/29 07:02:31.871607,  0] lib/util.c:1183(smb_panic)
  PANIC (pid 1814517): internal error
[2012/09/29 07:02:31.877596,  0] lib/util.c:1287(log_stack_trace)
  BACKTRACE: 35 stack frames:
   #0 winbindd(log_stack_trace+0x1a) [0x7feb7d4ca18a]
   #1 winbindd(smb_panic+0x2b) [0x7feb7d4ca25b]
   #2 winbindd(+0x1a3cc4) [0x7feb7d4bacc4]
   #3 /lib64/libc.so.6(+0x32900) [0x7feb7a929900]
   #4 /lib64/libc.so.6(memcpy+0x35) [0x7feb7a97f355]
   #5 /usr/lib64/libtdb.so.1(+0x6e76) [0x7feb7b0b0e76]
   #6 /usr/lib64/libtdb.so.1(+0x3d37) [0x7feb7b0add37]
   #7 /usr/lib64/libtdb.so.1(+0x863d) [0x7feb7b0b263d]
   #8 /usr/lib64/libtdb.so.1(+0x8700) [0x7feb7b0b2700]
   #9 /usr/lib64/libtdb.so.1(+0x2505) [0x7feb7b0ac505]
   #10 /usr/lib64/libtdb.so.1(+0x25b7) [0x7feb7b0ac5b7]
   #11 /usr/lib64/libtdb.so.1(tdb_fetch+0x13) [0x7feb7b0ac633]
   #12 winbindd(gencache_set_data_blob+0x259) [0x7feb7d4d8449]
   #13 winbindd(gencache_set+0x53) [0x7feb7d4d85b3]
   #14 winbindd(gencache_del+0x5e) [0x7feb7d4d879e]
   #15 winbindd(saf_delete+0x93) [0x7feb7d54b693]
   #16 winbindd(+0xe507e) [0x7feb7d3fc07e]
   #17 winbindd(+0xe85e5) [0x7feb7d3ff5e5]
   #18 winbindd(+0xe65be) [0x7feb7d3fd5be]
   #19 winbindd(+0xe7562) [0x7feb7d3fe562]
   #20 winbindd(init_dc_connection+0x2e) [0x7feb7d3fe5be]
   #21 winbindd(+0xe75d9) [0x7feb7d3fe5d9]
   #22 winbindd(cm_connect_netlogon+0x58) [0x7feb7d3fe658]
   #23 winbindd(_wbint_PingDc+0x61) [0x7feb7d410991]
   #24 winbindd(+0x103175) [0x7feb7d41a175]
   #25 winbindd(winbindd_dual_ndrcmd+0xb7) [0x7feb7d4107d7]
   #26 winbindd(+0xf8609) [0x7feb7d40f609]
   #27 winbindd(+0xf9075) [0x7feb7d410075]
   #28 winbindd(tevent_common_loop_immediate+0xe8) [0x7feb7d4db198]
   #29 winbindd(run_events_poll+0x3c) [0x7feb7d4d93fc]
   #30 winbindd(+0x1c2b52) [0x7feb7d4d9b52]
   #31 winbindd(_tevent_loop_once+0x90) [0x7feb7d4d9f60]
   #32 winbindd(main+0x7b3) [0x7feb7d3e7aa3]
   #33 /lib64/libc.so.6(__libc_start_main+0xfd) [0x7feb7a915cdd]
   #34 winbindd(+0xce2a9) [0x7feb7d3e52a9]

This is in a winbind child, logfiles surrounding indicate the parent
was restarted.

This patch takes all chain locks around the CLEAR_IF_FIRST introduced
tdb_new_database.