Always pass signals to the right thread

Always pass signals to the right thread

Currently, GDB can pass a signal to the wrong thread in several
different but related scenarios.

E.g., if thread 1 stops for signal SIGFOO, the user switches to thread
2, and then issues "continue", SIGFOO is actually delivered to thread
2, not thread 1.  This obviously messes up programs that use
pthread_kill to send signals to specific threads.

This has been a known issue for a long while.  Back in 2008 when I
made stop_signal be per-thread (2020b7ab), I kept the behavior -- see
code in 'proceed' being removed -- wanting to come back to it later.
The time has finally come now.

The patch fixes this -- on resumption, intercepted signals are always
delivered to the thread that had intercepted them.

Another example: if thread 1 stops for a breakpoint, the user switches
to thread 2, and then issues "signal SIGFOO", SIGFOO is actually
delivered to thread 1, not thread 2, because 'proceed' first switches
to thread 1 to step over its breakpoint...  If the user deletes the
breakpoint before issuing "signal FOO", then the signal is delivered
to thread 2 (the current thread).

"signal SIGFOO" can be used for two things: inject a signal in the
program while the program/thread had stopped for none, bypassing
"handle nopass"; or changing/suppressing a signal the program had
stopped for.  These scenarios are really two faces of the same coin,
and GDB can't really guess what the user is trying to do.  GDB might
have intercepted signals in more than one thread even (see the new
signal-command-multiple-signals-pending.exp test).  At least in the
inject case, it's obviously clear to me that the user means to deliver
the signal to the currently selected thread, so best is to make the
command's behavior consistent and easy to explain.

Then, if the user is trying to suppress/change a signal the program
had stopped for instead of injecting a new signal, but, the user had
changed threads meanwhile, then she will be surprised that with:

  (gdb) continue
  Thread 1 stopped for signal SIGFOO.
  (gdb) thread 2
  (gdb) signal SIGBAR

... GDB actually delivers SIGFOO to thread 1, and SIGBAR to thread 2
(with scheduler-locking off, which is the default, because then
"signal" or any other resumption command resumes all threads).

So the patch makes GDB detect that, and ask for confirmation:

  (gdb) thread 1
  [Switching to thread 1 (Thread 10979)]
  (gdb) signal SIGUSR2
  Note:
    Thread 3 previously stopped with signal SIGUSR2, User defined signal 2.
    Thread 2 previously stopped with signal SIGUSR1, User defined signal 1.
  Continuing thread 1 (the current thread) with specified signal will
  still deliver the signals noted above to their respective threads.
  Continue anyway? (y or n)

All these scenarios are covered by the new tests.

Tested on x86_64 Fedora 20, native and gdbserver.

gdb/
2014-07-25  Pedro Alves  <palves@redhat.com>

	* NEWS: Mention signal passing and "signal" command changes.
	* gdbthread.h (struct thread_suspend_state) <stop_signal>: Extend
	comment.
	* breakpoint.c (until_break_command): Adjust clear_proceed_status
	call.
	* infcall.c (run_inferior_call): Adjust clear_proceed_status call.
	* infcmd.c (proceed_thread_callback, continue_1, step_once)
	(jump_command): Adjust clear_proceed_status call.
	(signal_command): Warn if other thread that are resumed have
	signals that will be delivered.  Adjust clear_proceed_status call.
	(until_next_command, finish_command)
	(proceed_after_attach_callback, attach_command_post_wait)
	(attach_command): Adjust clear_proceed_status call.
	* infrun.c (proceed_after_vfork_done): Likewise.
	(proceed_after_attach_callback): Adjust comment.
	(clear_proceed_status_thread): Clear stop_signal if not in pass
	state.
	(clear_proceed_status_callback): Delete.
	(clear_proceed_status): New 'step' parameter.  Only clear the
	proceed status of threads the command being prepared is about to
	resume.
	(proceed): If passed in an explicit signal, override stop_signal
	with it.  Don't pass the last stop signal to the thread we're
	resuming.
	(init_wait_for_inferior): Adjust clear_proceed_status call.
	(switch_back_to_stepped_thread): Clear the signal if it should not
	be passed.
	* infrun.h (clear_proceed_status): New 'step' parameter.
	(user_visible_resume_ptid): Add comment.
	* linux-nat.c (linux_nat_resume_callback): Don't check whether the
	signal is in pass state.
	* remote.c (append_pending_thread_resumptions): Likewise.
	* mi/mi-main.c (proceed_thread): Adjust clear_proceed_status call.

gdb/doc/
2014-07-25  Pedro Alves  <palves@redhat.com>
	    Eli Zaretskii  <eliz@gnu.org>

	* gdb.texinfo (Signaling) <signal command>: Explain what happens
	with multi-threaded programs.

gdb/testsuite/
2014-07-25  Pedro Alves  <palves@redhat.com>

	* gdb.threads/signal-command-handle-nopass.c: New file.
	* gdb.threads/signal-command-handle-nopass.exp: New file.
	* gdb.threads/signal-command-multiple-signals-pending.c: New file.
	* gdb.threads/signal-command-multiple-signals-pending.exp: New file.
	* gdb.threads/signal-delivered-right-thread.c: New file.
	* gdb.threads/signal-delivered-right-thread.exp: New file.