Linux gdbserver confused when event randomization picks process exit event

The tail end of linux_wait_1 isn't expecting that the select_event_lwp
machinery can pick a whole-process exit event to report to GDB.  When
that happens, both gdb and gdbserver end up quite confused:

 ...
 (gdb)
 [Thread 24971.24971] #1 stopped.
 0x0000003615a011f0 in ?? ()
 c&
 Continuing.
 (gdb) [New Thread 24971.24981]
 [New Thread 24983.24983]
 [New Thread 24971.24982]

 [Thread 24983.24983] #3 stopped.
 0x0000003615ebc7cc in __libc_fork () at ../nptl/sysdeps/unix/sysv/linux/fork.c:130
 130       pid = ARCH_FORK ();
 [New Thread 24984.24984]
 Error in re-setting breakpoint -16: PC register is not available
 Error in re-setting breakpoint -17: PC register is not available
 Error in re-setting breakpoint -18: PC register is not available
 Error in re-setting breakpoint -19: PC register is not available
 Error in re-setting breakpoint -24: PC register is not available
 Error in re-setting breakpoint -25: PC register is not available
 Error in re-setting breakpoint -26: PC register is not available
 Error in re-setting breakpoint -27: PC register is not available
 Error in re-setting breakpoint -28: PC register is not available
 Error in re-setting breakpoint -29: PC register is not available
 Error in re-setting breakpoint -30: PC register is not available
 PC register is not available
 (gdb)

gdb/gdbserver/ChangeLog:
2015-08-06  Pedro Alves  <palves@redhat.com>

	* linux-low.c (add_lwp): Set waitstatus to TARGET_WAITKIND_IGNORE.
	(linux_thread_alive): Use lwp_is_marked_dead.
	(extended_event_reported): Delete.
	(linux_wait_1): Check if waitstatus is TARGET_WAITKIND_IGNORE
	instead of extended_event_reported.
	(mark_lwp_dead): Don't set the 'dead' flag.  Store the waitstatus
	as well.
	(lwp_is_marked_dead): New function.
	(lwp_running): Use lwp_is_marked_dead.
	* linux-low.h: Delete 'dead' field, and update 'waitstatus's
	comment.

1 files changed, 4 insertions, 7 deletions

diff --git a/gdb/gdbserver/linux-low.h b/gdb/gdbserver/linux-low.h
index 24fb015..f8f6e78 100644
--- a/gdb/gdbserver/linux-low.h
+++ b/gdb/gdbserver/linux-low.h
@@ -261,16 +261,13 @@ struct lwp_info
      event already received in a wait()).  */
   int stopped;
 
-  /* If this flag is set, the lwp is known to be dead already (exit
-     event already received in a wait(), and is cached in
-     status_pending).  */
-  int dead;
-
   /* When stopped is set, the last wait status recorded for this lwp.  */
   int last_status;
 
-  /* This is used to store extended ptrace event information until
-     it is reported to GDB.  */
+  /* If WAITSTATUS->KIND != TARGET_WAITKIND_IGNORE, the waitstatus for
+     this LWP's last event, to pass to GDB without any further
+     processing.  This is used to store extended ptrace event
+     information or exit status until it can be reported to GDB.  */
   struct target_waitstatus waitstatus;
 
   /* When stopped is set, this is where the lwp last stopped, with