Re: [PATCH v2] Restore terminal state in mi_thread_exit (PR gdb/17627)

[Simon Marchi <simon dot marchi at ericsson dot com>]

On 2014-12-10 12:12 PM, Pedro Alves wrote:
> On 12/05/2014 04:12 PM, Simon Marchi wrote:
>>> That doesn't follow though.  We have a ton of places that call
>>> target_terminal_our that don't need to put back the terminal in
>>> whatever state is was before.  The reason is that usually we'll
>>> be printing after the inferior stopped for some event.  If we end
>>> up re-resuming it, we'll call target_terminal_inferior then.
>>
>> I am not sure I understand that part. The thread exit is noticed because
>> of a breakpoint being hit (a breakpoint in libthread-db).
> 
> Nope, it's not.  When that breakpoint is hit, the only thing we do
> is mark the thread as "dying", in linux-thread-db.c:detach_thread.
> 
> The thread exit is noticed when the kernel reports an exit status
> to the waitpid calls in linux-nat.c (WIFEXITED/WIFSIGNALED)
> (and few other corner cases in that same file).  Look for exit_lwp.
> E.g.:
> 
> (top-gdb) bt
> #0  delete_thread (ptid=...) at /home/pedro/gdb/mygit/src/gdb/thread.c:371
> #1  0x00000000004b76ee in exit_lwp (lp=0x1682000) at /home/pedro/gdb/mygit/src/gdb/linux-nat.c:895
> #2  0x00000000004bbb20 in linux_nat_filter_event (lwpid=19001, status=0, new_pending_p=0x7fffffffcf50) at /home/pedro/gdb/mygit/src/gdb/linux-nat.c:2847
> #3  0x00000000004bc6d8 in linux_nat_wait_1 (ops=0xddf5e0, ptid=..., ourstatus=0x7fffffffd300, target_options=1) at /home/pedro/gdb/mygit/src/gdb/linux-nat.c:3136
> #4  0x00000000004bd583 in linux_nat_wait (ops=0xddf5e0, ptid=..., ourstatus=0x7fffffffd300, target_options=1) at /home/pedro/gdb/mygit/src/gdb/linux-nat.c:3505
> #5  0x00000000004c597b in thread_db_wait (ops=0xd4f0c0 <thread_db_ops>, ptid=..., ourstatus=0x7fffffffd300, options=1)
>     at /home/pedro/gdb/mygit/src/gdb/linux-thread-db.c:1532
> #6  0x000000000065c037 in delegate_wait (self=0xd4f0c0 <thread_db_ops>, arg1=..., arg2=0x7fffffffd300, arg3=1) at /home/pedro/gdb/mygit/src/gdb/target-delegates.c:116
> #7  0x000000000066c465 in target_wait (ptid=..., status=0x7fffffffd300, options=1) at /home/pedro/gdb/mygit/src/gdb/target.c:2175
> #8  0x00000000006165ff in fetch_inferior_event (client_data=0x0) at /home/pedro/gdb/mygit/src/gdb/infrun.c:3241
> #9  0x0000000000639d57 in inferior_event_handler (event_type=INF_REG_EVENT, client_data=0x0) at /home/pedro/gdb/mygit/src/gdb/inf-loop.c:57
> 
>> In all-stop, I
>> would expect that when the thread-exit breakpoint is hit, we stop all
>> threads, then process the event (display the message). After that, we
>> would need to resume the remaining threads, so the application continues
>> executing. If so, we would call target_terminal_inferior there.
> 
> Nope, thread exits are actually handled inside linux-nat.c, without
> stopping all threads.
> 
> (I've got a patch to stop native gdb from using the create and death
> event breakpoints, like gdbserver, btw.  Not necessary when we have
> PTRACE_EVENT_CLONE.)
> 
>>
>> But, I also remember you mentioning that these thread-db breakpoints are
>> not handled the same way as regular breakpoints. If I inspect the
>> target-stack when debugging a threaded program:
>>
>>     The current target stack is:
>>       - multi-thread (multi-threaded child process.)
>>       - native (Native process)
>>       - exec (Local exec file)
>>       - None (None)
>>
>> I would guess that the layer responsible for the all-stop behavior (stop
>> all the threads when one is stopped) is the native process one.
> 
> Right.
> 
>> The
>> thread-db breakpoint handling, however, is done in the multi-thread layer,
>> so the native target is not reached for handling these. So when the
>> message is printed, the other threads are actually still running.
>>
>> Is this last explanation correct?
>>
>>> But in the thread exit case, there's nothing to be re-resumed,
>>> so we need to care about it "manually".  If you take a backtrace
>>> at the point the thread exit event is printed, we should be
>>> still deep within the target backend code.
>>>
>>> Please update the commit log to clarify this.
>>
>> I would write this:
>>
>> We need to manually restore the terminal setting in this particular observer.
>> In the case of the other observers that call target_terminal_ours, gdb will end
> 
> "other MI observers"?

Ok.

>> up resuming the inferior later in the execution and call
>> target_terminal_inferior. In the case of the thread exit event, we still need
>> to call target_terminal_ours to be able to print something, but there is nothing
>> that gdb will need to resume after that. We therefore need to call
>> target_terminal_inferior ourselves.
>>
>> is this correct?
> 
> Yes.

Thanks for the clarifications, it's much clearer now. Thanks also for the help finding the issue.

I pushed the patch as such (only change is "other MI observers"):

>From 1abf3a143773819e195fceaa485060dcac9e6089 Mon Sep 17 00:00:00 2001
From: Simon Marchi <simon.marchi@ericsson.com>
Date: Wed, 10 Dec 2014 13:03:47 -0500
Subject: [PATCH] Restore terminal state in mi_thread_exit (PR gdb/17627)

When a thread exits, the terminal is left in mode "terminal_is_ours"
while the target executes.  This patch fixes that.

We need to manually restore the terminal setting in this particular
observer.  In the case of the other MI observers that call
target_terminal_ours, gdb will end up resuming the inferior later in the
execution and call target_terminal_inferior.  In the case of the thread
exit event, we still need to call target_terminal_ours to be able to
print something, but there is nothing that gdb will need to resume after
that. We therefore need to call target_terminal_inferior ourselves.

gdb/ChangeLog:

	PR gdb/17627
	* target.c (cleanup_restore_target_terminal): New function.
	(make_cleanup_restore_target_terminal): New function.
	* target.h (make_cleanup_restore_target_terminal): New
	declaration.
	* mi/mi-interp.c (mi_thread_exit): Use the new cleanup.

Signed-off-by: Simon Marchi <simon.marchi@ericsson.com>
---
 gdb/ChangeLog      |  8 ++++++++
 gdb/mi/mi-interp.c |  4 ++++
 gdb/target.c       | 34 ++++++++++++++++++++++++++++++++++
 gdb/target.h       |  4 ++++
 4 files changed, 50 insertions(+)

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index ad845a7..4aeb0cc 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,3 +1,11 @@
+2014-12-10  Simon Marchi  <simon.marchi@ericsson.com>
+
+	PR gdb/17627
+	* target.c (cleanup_restore_target_terminal): New function.
+	(make_cleanup_restore_target_terminal): New function.
+	* target.h (make_cleanup_restore_target_terminal): New declaration.
+	* mi/mi-interp.c (mi_thread_exit): Use the new cleanup.
+
 2014-12-08  Doug Evans  <dje@google.com>

 	* python/py-objfile.c (objfpy_get_owner): Increment refcount of result.
diff --git a/gdb/mi/mi-interp.c b/gdb/mi/mi-interp.c
index df2b558..60f0666 100644
--- a/gdb/mi/mi-interp.c
+++ b/gdb/mi/mi-interp.c
@@ -386,6 +386,7 @@ mi_thread_exit (struct thread_info *t, int silent)
 {
   struct mi_interp *mi;
   struct inferior *inf;
+  struct cleanup *old_chain;

   if (silent)
     return;
@@ -393,11 +394,14 @@ mi_thread_exit (struct thread_info *t, int silent)
   inf = find_inferior_pid (ptid_get_pid (t->ptid));

   mi = top_level_interpreter_data ();
+  old_chain = make_cleanup_restore_target_terminal ();
   target_terminal_ours ();
   fprintf_unfiltered (mi->event_channel,
 		      "thread-exited,id=\"%d\",group-id=\"i%d\"",
 		      t->num, inf->num);
   gdb_flush (mi->event_channel);
+
+  do_cleanups (old_chain);
 }

 /* Emit notification on changing the state of record.  */
diff --git a/gdb/target.c b/gdb/target.c
index ab5f2b9..7161e62 100644
--- a/gdb/target.c
+++ b/gdb/target.c
@@ -528,6 +528,40 @@ target_supports_terminal_ours (void)
   return 0;
 }

+/* Restore the terminal to its previous state (helper for
+   make_cleanup_restore_target_terminal). */
+
+static void
+cleanup_restore_target_terminal (void *arg)
+{
+  enum terminal_state *previous_state = arg;
+
+  switch (*previous_state)
+    {
+    case terminal_is_ours:
+      target_terminal_ours ();
+      break;
+    case terminal_is_ours_for_output:
+      target_terminal_ours_for_output ();
+      break;
+    case terminal_is_inferior:
+      target_terminal_inferior ();
+      break;
+    }
+}
+
+/* See target.h. */
+
+struct cleanup *
+make_cleanup_restore_target_terminal (void)
+{
+  enum terminal_state *ts = xmalloc (sizeof (*ts));
+
+  *ts = terminal_state;
+
+  return make_cleanup_dtor (cleanup_restore_target_terminal, ts, xfree);
+}
+
 static void
 tcomplain (void)
 {
diff --git a/gdb/target.h b/gdb/target.h
index d363b61..cff146a 100644
--- a/gdb/target.h
+++ b/gdb/target.h
@@ -1413,6 +1413,10 @@ extern void target_terminal_ours (void);

 extern int target_supports_terminal_ours (void);

+/* Make a cleanup that restores the state of the terminal to the current
+   state.  */
+extern struct cleanup *make_cleanup_restore_target_terminal (void);
+
 /* Print useful information about our terminal status, if such a thing
    exists.  */

-- 
2.1.3