linux-thread-db.c not only caller of add_thread, -> gdb segv

["Douglas Evans" <dje@google.com>]

Hi. I'm trying to decide what's the best way to fix a bug I've found.
linux-thread-db.c is not the only caller of add_thread, e.g. infrun.c
calls it too.
linux-thread-db.c assumes thread_info.private is non-NULL, e.g. here:

static int
clear_lwpid_callback (struct thread_info *thread, void *dummy)
{
  /* If we know that our thread implementation is 1-to-1, we could
save
     a certain amount of information; it's not clear how much, so we
     are always conservative.  */

  thread->private->th_valid = 0;
  thread->private->ti_valid = 0;

  return 0;
}

called from here:

#0  0x00000000004681a4 in clear_lwpid_callback (thread=0xb132d0,
dummy=0x0) at ../../src/gdb/linux-thread-db.c:765
#1  0x00000000004f8480 in iterate_over_threads (callback=0x46818d
<clear_lwpid_callback>, data=0x0) at ../../src/gdb/thread.c:204
#2  0x00000000004682a6 in thread_db_resume (ptid={pid = -1, lwp = 0,
tid = 0}, step=0, signo=TARGET_SIGNAL_0) at
../../src/gdb/linux-thread-db.c:782
#3  0x00000000004eb83d in resume (step=0, sig=TARGET_SIGNAL_0) at
../../src/gdb/infrun.c:614
#4  0x00000000004ebc14 in proceed (addr=18446744073709551615,
siggnal=TARGET_SIGNAL_DEFAULT, step=0) at ../../src/gdb/infrun.c:805

This is iterating over all threads, some of which were created via
add_thread calls outside of linux-thread-db.c, and thus
thread_info.private is NULL and boom, segv.

I can see two solutions
1) ensure all calls to add_thread properly initialize thread_info.private
   (via callback or whatever)
2) have linux-thread-db.c always check thread_info.private before
dereferencing it.

Comments?