discover_class_cache is not threadsafe

[purushottamc]

As per analysis,

In our project one thread is trying to use variable discover_class_cache while another one trying to flush it out. so, it seems the node->name vaiable was getting flushed before the discover_class_cache->name variable found NULL during backtrace analysis as mentioned below:

Core was generated by `/usr/bin/dbus-daemon --system --address=systemd: --nofork --nopidfile --systemd'.
Program terminated with signal SIGSEGV, Segmentation fault.
#0 0x0000007f8a640194 in ?? ()
[Current thread is 1 (LWP 9)]

Thread 1 (LWP 9):
#0 strcmp () at ../sysdeps/aarch64/strcmp.S:156
#1 0x0000007f8a779414 in get_class_cache_entry_name (s=s@entry=0x556fcee092 "dbus") at stringrep.c:36
#2 string_to_security_class (s=s@entry=0x556fcee092 "dbus") at stringrep.c:195
#3 0x000000556fce0f8c in bus_selinux_check (sender_sid=sender_sid@entry=0x5572740ce0, override_sid=0x5572747730, requested=requested@entry=0x556fcee1a1 "send_msg", auxdata=auxdata@entry=0x7fe1982c80, target_class=0x556fcee092 "dbus") at ../../dbus-1.12.20/bus/selinux.c:410
#4 0x000000556fce15b4 in bus_selinux_check (target_class=0x556fcee092 "dbus", auxdata=0x7fe1982c80, requested=0x556fcee1a1 "send_msg", override_sid=, sender_sid=0x5572740ce0) at ../../dbus-1.12.20/bus/selinux.c:639
#5 bus_selinux_allows_send (sender=sender@entry=0x5572740760, proposed_recipient=proposed_recipient@entry=0x0, msgtype=0x7f8a8b40a1 "method_call", interface=0x5572741960 "org.freedesktop.DBus", member=0x5572741948 "AddMatch", error_name=0x0, destination=destination@entry=0x5572741980 "org.freedesktop.DBus", activation_entry=activation_entry@entry=0x0, error=error@entry=0x7fe1982e48) at ../../dbus-1.12.20/bus/selinux.c:639
#6 0x000000556fcd3f1c in bus_context_check_security_policy (context=context@entry=0x557272b6b0, transaction=transaction@entry=0x55727453e0, sender=sender@entry=0x5572740760, addressed_recipient=addressed_recipient@entry=0x0, proposed_recipient=proposed_recipient@entry=0x0, message=message@entry=0x55727454a0, activation_entry=activation_entry@entry=0x0, error=error@entry=0x7fe1982e48) at ../../dbus-1.12.20/bus/bus.c:1611
#7 0x000000556fcdbc04 in bus_dispatch (message=0x55727454a0, connection=0x5572740760) at ../../dbus-1.12.20/bus/dispatch.c:383
#8 bus_dispatch_message_filter (connection=0x5572740760, message=0x55727454a0, user_data=) at ../../dbus-1.12.20/bus/dispatch.c:548
#9 0x0000007f8a890408 in dbus_connection_dispatch (connection=0x5572740760) at ../../dbus-1.12.20/dbus/dbus-connection.c:4854
#10 dbus_connection_dispatch (connection=connection@entry=0x5572740760) at ../../dbus-1.12.20/dbus/dbus-connection.c:4726
#11 0x000000556fce6578 in _dbus_loop_dispatch (loop=) at ../../dbus-1.12.20/dbus/dbus-mainloop.c:532
#12 _dbus_loop_dispatch (loop=0x557272b7e0) at ../../dbus-1.12.20/dbus/dbus-mainloop.c:513
#13 0x000000556fce6904 in _dbus_loop_iterate (loop=loop@entry=0x557272b7e0, block=block@entry=1) at ../../dbus-1.12.20/dbus/dbus-mainloop.c:862
#14 0x000000556fce6a70 in _dbus_loop_run (loop=0x557272b7e0) at ../../dbus-1.12.20/dbus/dbus-mainloop.c:888
#15 0x000000556fccff88 in main (argc=7, argv=0x7fe19834c8) at ../../dbus-1.12.20/bus/main.c:724

further debugging:
(gdb) f 1
#1 0x0000007f8a779414 in get_class_cache_entry_name (s=s@entry=0x556fcee092 "dbus") at stringrep.c:36
36 stringrep.c: No such file or directory.
(gdb) print discover_class_cache
$1 = (struct discover_class_node *) 0x0
(gdb) f 2
#2 string_to_security_class (s=s@entry=0x556fcee092 "dbus") at stringrep.c:195
195 in stringrep.c
(gdb) print discover_class_cache
$2 = (struct discover_class_node *) 0x0

[purushottamc]

Did changes as per above analysis,
#336
Could you please review this change?