Don't delete thread_info if refcount isn't zero

Don't delete thread_info if refcount isn't zero

I build GDB with asan, and run test case hook-stop.exp, and threadapply.exp,
I got the following asan error,

=================================================================^M
^[[1m^[[31m==2291==ERROR: AddressSanitizer: heap-use-after-free on address 0x6160000999c4 at pc 0x000000826022 bp 0x7ffd28a8ff70 sp 0x7ffd28a8ff60^M
^[[1m^[[0m^[[1m^[[34mREAD of size 4 at 0x6160000999c4 thread T0^[[1m^[[0m^M
    #0 0x826021 in release_stop_context_cleanup ../../binutils-gdb/gdb/infrun.c:8203^M
    #1 0x72798a in do_my_cleanups ../../binutils-gdb/gdb/common/cleanups.c:154^M
    #2 0x727a32 in do_cleanups(cleanup*) ../../binutils-gdb/gdb/common/cleanups.c:176^M
    #3 0x826895 in normal_stop() ../../binutils-gdb/gdb/infrun.c:8381^M
    #4 0x815208 in fetch_inferior_event(void*) ../../binutils-gdb/gdb/infrun.c:4011^M
    #5 0x868aca in inferior_event_handler(inferior_event_type, void*) ../../binutils-gdb/gdb/inf-loop.c:44^M
....
^[[1m^[[32m0x6160000999c4 is located 68 bytes inside of 568-byte region [0x616000099980,0x616000099bb8)^M
^[[1m^[[0m^[[1m^[[35mfreed by thread T0 here:^[[1m^[[0m^M
    #0 0x7fb0bc1312ca in __interceptor_free (/usr/lib/x86_64-linux-gnu/libasan.so.2+0x982ca)^M
    #1 0xb8c62f in xfree(void*) ../../binutils-gdb/gdb/common/common-utils.c:100^M
    #2 0x83df67 in free_thread ../../binutils-gdb/gdb/thread.c:207^M
    #3 0x83dfd2 in init_thread_list() ../../binutils-gdb/gdb/thread.c:223^M
    #4 0x805494 in kill_command ../../binutils-gdb/gdb/infcmd.c:2595^M
....

Detaching from program: /home/yao.qi/SourceCode/gnu/build-with-asan/gdb/testsuite/outputs/gdb.threads/threadapply/threadapply, process 2399^M
=================================================================^M
^[[1m^[[31m==2387==ERROR: AddressSanitizer: heap-use-after-free on address 0x6160000a98c0 at pc 0x00000083fd28 bp 0x7ffd401c3110 sp 0x7ffd401c3100^M
^[[1m^[[0m^[[1m^[[34mREAD of size 4 at 0x6160000a98c0 thread T0^[[1m^[[0m^M
    #0 0x83fd27 in thread_alive ../../binutils-gdb/gdb/thread.c:741^M
    #1 0x844277 in thread_apply_all_command ../../binutils-gdb/gdb/thread.c:1804^M
....
^M
^[[1m^[[32m0x6160000a98c0 is located 64 bytes inside of 568-byte region [0x6160000a9880,0x6160000a9ab8)^M
^[[1m^[[0m^[[1m^[[35mfreed by thread T0 here:^[[1m^[[0m^M
    #0 0x7f59a7e322ca in __interceptor_free (/usr/lib/x86_64-linux-gnu/libasan.so.2+0x982ca)^M
    #1 0xb8c62f in xfree(void*) ../../binutils-gdb/gdb/common/common-utils.c:100^M
    #2 0x83df67 in free_thread ../../binutils-gdb/gdb/thread.c:207^M
    #3 0x83dfd2 in init_thread_list() ../../binutils-gdb/gdb/thread.c:223^M

This patch fixes the issue by deleting thread_info object if it is
deletable, otherwise, mark it as exited (by set_thread_exited).
Function set_thread_exited is shared from delete_thread_1.  This patch
also moves field "refcount" to private and methods incref and
decref.  Additionally, we stop using "ptid_t" in
"struct current_thread_cleanup" to reference threads, instead we use
"thread_info" directly.  Due to this change, we don't need
restore_current_thread_ptid_changed anymore.

gdb:

2017-04-10  Yao Qi  <yao.qi@linaro.org>

	PR gdb/19942
	* gdbthread.h (thread_info::deletable): New method.
	(thread_info::incref): New method.
	(thread_info::decref): New method.
	(thread_info::refcount): Move it to private.
	* infrun.c (save_stop_context): Call inc_refcount.
	(release_stop_context_cleanup): Likewise.
	* thread.c (set_thread_exited): New function.
	(init_thread_list): Delete "tp" only it is deletable, otherwise
	call set_thread_exited.
	(delete_thread_1): Call set_thread_exited.
	(current_thread_cleanup) <inferior_pid>: Remove.
	<thread>: New field.
	(restore_current_thread_ptid_changed): Removed.
	(do_restore_current_thread_cleanup): Adjust.
	(restore_current_thread_cleanup_dtor): Don't call
	find_thread_ptid.
	(set_thread_refcount): Use dec_refcount.
	(make_cleanup_restore_current_thread): Adjust.
	(thread_apply_all_command): Call inc_refcount.
	(_initialize_thread): Don't call
	observer_attach_thread_ptid_changed.