bug#15561: Timer can miss its SIGALRM

bug#15561: Timer can miss its SIGALRM

[Barry OReilly]

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Looking over the timer code, the do_pending_atimers function does:

      block_atimers ();
      run_timers ();
      unblock_atimers ();

The last line of run_timers is:

  set_alarm ();

But what happens if the next timer happens to be soon, and Emacs
receives SIGALRM inbetween set_alarm and unblock_timers?

What is the purpose of sigmasking SIGALRM anyway? If it means to block
SIGALRM during timers, that doesn't always happen because timers run
within timers. The end of an inner timer would unblock SIGALRM and
the outer timer would finish with the sigmask unblocked.

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bug#15561: periodic timer stops running

[Barry OReilly]

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> Are you talking about a SIGALRM received during the execution of
> do_pending_atimers, between run_timer's call to set_alarm and
> do_pending_atimers's call to unblock_atimers? If so, the SIGALRM
> should be held by the operating system during that period, and Emacs
> won't be informed of the SIGALRM until unblock_atimers does its
> thing. Sorry, I don't see how this would cause a timer to stop.

I see you're right, because SIGALRM isn't specified with SIG_IGN at
any point.

While verifying that, I found in sys_subshell:

  struct save_signal saved_handlers[5];
  [...]
#ifdef USABLE_SIGIO
  saved_handlers[3].code = SIGIO;
  saved_handlers[4].code = 0;
#else
  saved_handlers[3].code = 0;
#endif

Shouldn't the else case initialize saved_handlers[4]? On the off
chance it is garbage valued coincidentally as SIGALRM, the subsequent
SIG_IGN could drop a pending SIGALRM.

Peter, which OS do you run? What is USABLE_SIGIO in your src/config.h?

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bug#15561: periodic timer stops running

[Barry OReilly]

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save_signal_handlers (struct save_signal *saved_handlers)
{
  while (saved_handlers->code)

Nevermind, it short circuits at the first 0.

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bug#15561: periodic timer stops running

[Peter Münster]

On Fri, Feb 28 2014, Barry OReilly wrote:

> Peter, which OS do you run?

Hi,

GNU/Linux (openSUSE-12.3).


> What is USABLE_SIGIO in your src/config.h?

It's 1.


The patch did not help.


Very probably a timer stops running just after system wake-up (after
suspend to ram), because at that moment several timers are triggered at
the same time.

Thanks for your efforts,
-- 
           Peter

bug#15561: periodic timer stops running

[Barry OReilly]

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> Very probably a timer stops running just after system wake-up (after
> suspend to ram), because at that moment several timers are triggered
> at the same time.

I tried testing several timers going off at once, but reproduced no
problem.

;;; -*- lexical-binding:t -*-
(require 'cl-lib)
(let ((noninteractive nil))
  (cl-dotimes (timer-id 8)
    (run-at-time '(0 0 0 0)
                 0.1
                 (lambda ()
                   (message "%s DEBUG: Timer ID=%s" (current-time)
timer-id)
                   (sit-for 0 t)))))

All 8 fire without stopping.

Is it always the same timer that stops running, or have you seen
different timers in Emacs stop?

Could you come up with a recipe that allows us to reproduce the bug?

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bug#15561: periodic timer stops running

[Peter Münster]

On Tue, Mar 04 2014, Barry OReilly wrote:

> Is it always the same timer that stops running, or have you seen
> different timers in Emacs stop?

Hi Barry,

Different timers.


> Could you come up with a recipe that allows us to reproduce the bug?

I don't know. It would be difficult:
- in happens only once or twice per day
- the timers depend on my personal environment: checking email, news,
  Bitcoin value, the todo-list of my org-file and so on
- some timers depend on idle-time

Would it help, if I a add some code to the start and to the end of each
timer function, that prints interesting information to the *Messages*
buffer? And when it happens again, we would get some trace?

-- 
           Peter

bug#15561: periodic timer stops running

[Peter Münster]

Perhaps related:

The following function make some beeps for some seconds. It is called
from a timer. Sometimes, the `cancel-timer' does not work, and emacs
continues to beep.

--8<---------------cut here---------------start------------->8---
(defun org-notify-action-ding (plist)
  "Make noise."
  (let ((timer (run-with-timer 0 1 'ding)))
    (run-with-timer (or (plist-get plist :duration) 3) nil
                    'cancel-timer timer)))
--8<---------------cut here---------------end--------------->8---

Or is this another problem/bug?

TIA for any help,
-- 
           Peter

bug#15561: periodic timer stops running

[Barry OReilly]

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> I don't know. It would be difficult:
> - in happens only once or twice per day
> - the timers depend on my personal environment: checking email,
>   news, Bitcoin value, the todo-list of my org-file and so on
> - some timers depend on idle-time

> Would it help, if I a add some code to the start and to the end of
> each timer function, that prints interesting information to the
> *Messages* buffer? And when it happens again, we would get some
> trace?

After the timers stop, do they start up again when you 'kill -s ALRM'
the Emacs process?

Maybe you could put a print statement in the handle_alarm_signal
function to confirm you get the signal. If you do, then perhaps start
debugging at the do_pending_atimers function.

If the problem is that you're not getting the signal, write a small
program that implements a simple timer using SIGALRM, run it
continuously, and see if it stops too under the same conditions.

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bug#15561: periodic timer stops running

[Peter Münster]

On Wed, Mar 12 2014, Barry OReilly wrote:

> After the timers stop, do they start up again when you 'kill -s ALRM'
> the Emacs process?

The timers don't stop anymore since my distribution upgrade from
opensuse-12.3 to 13.1. Very odd...

I don't understand why, but the problem seems to be solved.

-- 
           Peter

bug#15561: periodic timer stops running

[Barry OReilly]

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I think applying Paul Eggert's patch is all that's left to this bug
report then. Since it solves a theoretical flaw rather than one
witnessed, maybe it's appropriate for trunk?

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bug#15561: periodic timer stops running

[Paul Eggert]

Barry OReilly wrote:
> I think applying Paul Eggert's patch is all that's left to this bug
> report then. Since it solves a theoretical flaw rather than one
> witnessed, maybe it's appropriate for trunk?

Sounds good; done as trunk bzr 116018.

bug#15561: periodic timer stops running

[Peter Münster]

Hi,

It happened again: 2 periodic timers have stopped running.


On Wed, Mar 12 2014, Barry OReilly wrote:

> After the timers stop, do they start up again when you 'kill -s ALRM'
> the Emacs process?

No.


> Maybe you could put a print statement in the handle_alarm_signal
> function to confirm you get the signal.

Emacs goes into the handle_alarm_signal function when sending the
signal.


> If you do, then perhaps start debugging at the do_pending_atimers
> function.

I've tried, but I don't really know, where to look...

Here the value of my timer-list:

--8<---------------cut here---------------start------------->8---
Value: ([t 21335 18354 590042 20 org-notify-process nil nil 953248]
 [t 21335 20714 647433 9000 gnus-demon-run-callback
    (pm/rescan-news 900 9000)
    nil 138000]
 [nil 21336 7511 917609 300 savehist-autosave nil nil 848940]
 [nil 21336 7547 595349 300 gnus-demon-run-callback
      (gnus-group-save-newsrc 120 300)
      nil 925000]
 [nil 21336 7547 880984 60 pm/newmail-update nil nil 696000]
 [nil 21336 7641 823147 900 gnus-demon-run-callback
      (pm/btc-checker 60 900)
      nil 609000]
 [nil 21336 7727 644172 420 gnus-demon-run-callback
      (gnus-delay-send-queue 180 420)
      nil 412000]
 [nil 21336 7947 507382 3600 url-cookie-write-file nil nil 982000]
 [nil 21336 10875 981456 nil password-cache-remove
      ("auth-source-magic (:max 1 :host (\"news.gmane.org\" \"news.gmane.org\") :port (\"119\" \"nntp\" \"nntp\" \"563\" \"nntps\" \"snews\"))")
      nil 731000]
 [nil 21336 13307 647529 6000 gnus-demon-run-callback
      (pm/check-mail 180 6000)
      nil 832000])
--8<---------------cut here---------------end--------------->8---

What does the first entry in the vector mean (t or nil)? The 2 first
entries have stopped.

What could I do to debug this?

TIA for any hints,
-- 
           Peter

periodic timer stops running

[Peter Münster]

Hi,

I use several timers, for example "(run-with-timer 20 20 'org-notify-process)"

But sometimes, a timer just stops running, and I don't know, what event
triggers this problem.

Is it perhaps related to:
http://debbugs.gnu.org/cgi/bugreport.cgi?bug=15561 ?

It happens about once a day. How could I solve this issue please?

TIA for any hints,
-- 
           Peter

bug#15561: periodic timer stops running

[Paul Eggert]

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Following up to:

http://lists.gnu.org/archive/html/emacs-devel/2014-02/msg00593.html

Does the attached patch fix things for you?  It's relative to Emacs 
trunk bzr 116586.

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=== modified file 'src/atimer.c'
--- src/atimer.c	2014-01-01 07:43:34 +0000
+++ src/atimer.c	2014-02-27 20:15:21 +0000
@@ -50,22 +50,17 @@
 /* Block/unblock SIGALRM.  */
 
 static void
-sigmask_atimers (int how)
+block_atimers (sigset_t *oldset)
 {
   sigset_t blocked;
   sigemptyset (&blocked);
   sigaddset (&blocked, SIGALRM);
-  pthread_sigmask (how, &blocked, 0);
-}
-static void
-block_atimers (void)
-{
-  sigmask_atimers (SIG_BLOCK);
-}
-static void
-unblock_atimers (void)
-{
-  sigmask_atimers (SIG_UNBLOCK);
+  pthread_sigmask (SIG_BLOCK, &blocked, oldset);
+}
+static void
+unblock_atimers (sigset_t const *oldset)
+{
+  pthread_sigmask (SIG_SETMASK, oldset, 0);
 }
 
 /* Function prototypes.  */
@@ -98,6 +93,7 @@
 	      atimer_callback fn, void *client_data)
 {
   struct atimer *t;
+  sigset_t oldset;
 
   /* Round TIME up to the next full second if we don't have
      itimers.  */
@@ -122,7 +118,7 @@
   t->fn = fn;
   t->client_data = client_data;
 
-  block_atimers ();
+  block_atimers (&oldset);
 
   /* Compute the timer's expiration time.  */
   switch (type)
@@ -143,7 +139,7 @@
 
   /* Insert the timer in the list of active atimers.  */
   schedule_atimer (t);
-  unblock_atimers ();
+  unblock_atimers (&oldset);
 
   /* Arrange for a SIGALRM at the time the next atimer is ripe.  */
   set_alarm ();
@@ -158,8 +154,9 @@
 cancel_atimer (struct atimer *timer)
 {
   int i;
+  sigset_t oldset;
 
-  block_atimers ();
+  block_atimers (&oldset);
 
   for (i = 0; i < 2; ++i)
     {
@@ -186,7 +183,7 @@
 	}
     }
 
-  unblock_atimers ();
+  unblock_atimers (&oldset);
 }
 
 
@@ -217,7 +214,8 @@
 void
 stop_other_atimers (struct atimer *t)
 {
-  block_atimers ();
+  sigset_t oldset;
+  block_atimers (&oldset);
 
   if (t)
     {
@@ -242,7 +240,7 @@
 
   stopped_atimers = append_atimer_lists (atimers, stopped_atimers);
   atimers = t;
-  unblock_atimers ();
+  unblock_atimers (&oldset);
 }
 
 
@@ -256,8 +254,9 @@
     {
       struct atimer *t = atimers;
       struct atimer *next;
+      sigset_t oldset;
 
-      block_atimers ();
+      block_atimers (&oldset);
       atimers = stopped_atimers;
       stopped_atimers = NULL;
 
@@ -268,7 +267,7 @@
 	  t = next;
 	}
 
-      unblock_atimers ();
+      unblock_atimers (&oldset);
     }
 }
 
@@ -381,9 +380,10 @@
 {
   if (atimers)
     {
-      block_atimers ();
+      sigset_t oldset;
+      block_atimers (&oldset);
       run_timers ();
-      unblock_atimers ();
+      unblock_atimers (&oldset);
     }
 }
 

=== modified file 'src/callproc.c'
--- src/callproc.c	2014-01-30 17:22:30 +0000
+++ src/callproc.c	2014-02-27 20:18:18 +0000
@@ -108,20 +108,20 @@
 /* Block SIGCHLD.  */
 
 void
-block_child_signal (void)
+block_child_signal (sigset_t *oldset)
 {
   sigset_t blocked;
   sigemptyset (&blocked);
   sigaddset (&blocked, SIGCHLD);
-  pthread_sigmask (SIG_BLOCK, &blocked, 0);
+  pthread_sigmask (SIG_BLOCK, &blocked, oldset);
 }
 
 /* Unblock SIGCHLD.  */
 
 void
-unblock_child_signal (void)
+unblock_child_signal (sigset_t const *oldset)
 {
-  pthread_sigmask (SIG_SETMASK, &empty_mask, 0);
+  pthread_sigmask (SIG_SETMASK, oldset, 0);
 }
 
 /* Return the current buffer's working directory, or the home
@@ -162,7 +162,8 @@
 void
 record_kill_process (struct Lisp_Process *p, Lisp_Object tempfile)
 {
-  block_child_signal ();
+  sigset_t oldset;
+  block_child_signal (&oldset);
 
   if (p->alive)
     {
@@ -171,7 +172,7 @@
       kill (- p->pid, SIGKILL);
     }
 
-  unblock_child_signal ();
+  unblock_child_signal (&oldset);
 }
 
 /* Clean up files, file descriptors and processes created by Fcall_process.  */
@@ -313,6 +314,7 @@
   char *tempfile = NULL;
   int pid;
 #else
+  sigset_t oldset;
   pid_t pid;
 #endif
   int child_errno;
@@ -629,7 +631,7 @@
 #ifndef MSDOS
 
   block_input ();
-  block_child_signal ();
+  block_child_signal (&oldset);
 
 #ifdef WINDOWSNT
   pid = child_setup (filefd, fd_output, fd_error, new_argv, 0, current_dir);
@@ -671,7 +673,7 @@
 
   if (pid == 0)
     {
-      unblock_child_signal ();
+      unblock_child_signal (&oldset);
 
       setsid ();
 
@@ -707,7 +709,7 @@
 	}
     }
 
-  unblock_child_signal ();
+  unblock_child_signal (&oldset);
   unblock_input ();
 
 #endif /* not MSDOS */

=== modified file 'src/keyboard.c'
--- src/keyboard.c	2014-02-24 14:26:49 +0000
+++ src/keyboard.c	2014-02-27 20:23:38 +0000
@@ -10295,6 +10295,9 @@
 handle_interrupt (bool in_signal_handler)
 {
   char c;
+  sigset_t blocked;
+  sigemptyset (&blocked);
+  sigaddset (&blocked, SIGINT);
 
   cancel_echoing ();
 
@@ -10306,9 +10309,6 @@
 	  /* If SIGINT isn't blocked, don't let us be interrupted by
 	     a SIGINT.  It might be harmful due to non-reentrancy
 	     in I/O functions.  */
-	  sigset_t blocked;
-	  sigemptyset (&blocked);
-	  sigaddset (&blocked, SIGINT);
 	  pthread_sigmask (SIG_BLOCK, &blocked, 0);
 	}
 
@@ -10393,7 +10393,7 @@
 	  struct gcpro gcpro1, gcpro2, gcpro3, gcpro4;
 
 	  immediate_quit = 0;
-	  pthread_sigmask (SIG_SETMASK, &empty_mask, 0);
+	  pthread_sigmask (SIG_UNBLOCK, &blocked, 0);
 	  saved = gl_state;
 	  GCPRO4 (saved.object, saved.global_code,
 		  saved.current_syntax_table, saved.old_prop);
@@ -10414,7 +10414,7 @@
         }
     }
 
-  pthread_sigmask (SIG_SETMASK, &empty_mask, 0);
+  pthread_sigmask (SIG_UNBLOCK, &blocked, 0);
 
 /* TODO: The longjmp in this call throws the NS event loop integration off,
          and it seems to do fine without this.  Probably some attention

=== modified file 'src/lisp.h'
--- src/lisp.h	2014-01-03 06:47:27 +0000
+++ src/lisp.h	2014-02-27 20:32:01 +0000
@@ -4237,8 +4237,8 @@
 extern void sys_subshell (void);
 extern void sys_suspend (void);
 extern void discard_tty_input (void);
-extern void block_tty_out_signal (void);
-extern void unblock_tty_out_signal (void);
+extern void block_tty_out_signal (sigset_t *);
+extern void unblock_tty_out_signal (sigset_t const *);
 extern void init_sys_modes (struct tty_display_info *);
 extern void reset_sys_modes (struct tty_display_info *);
 extern void init_all_sys_modes (void);

=== modified file 'src/process.c'
--- src/process.c	2014-02-22 21:08:22 +0000
+++ src/process.c	2014-02-27 20:19:36 +0000
@@ -1660,6 +1660,7 @@
   bool pty_flag = 0;
   char pty_name[PTY_NAME_SIZE];
   Lisp_Object lisp_pty_name = Qnil;
+  sigset_t oldset;
 
   inchannel = outchannel = -1;
 
@@ -1725,7 +1726,7 @@
   setup_process_coding_systems (process);
 
   block_input ();
-  block_child_signal ();
+  block_child_signal (&oldset);
 
 #ifndef WINDOWSNT
   /* vfork, and prevent local vars from being clobbered by the vfork.  */
@@ -1849,7 +1850,7 @@
       signal (SIGPIPE, SIG_DFL);
 
       /* Stop blocking SIGCHLD in the child.  */
-      unblock_child_signal ();
+      unblock_child_signal (&oldset);
 
       if (pty_flag)
 	child_setup_tty (xforkout);
@@ -1868,7 +1869,7 @@
     p->alive = 1;
 
   /* Stop blocking in the parent.  */
-  unblock_child_signal ();
+  unblock_child_signal (&oldset);
   unblock_input ();
 
   if (pid < 0)
@@ -7055,8 +7056,9 @@
 catch_child_signal (void)
 {
   struct sigaction action, old_action;
+  sigset_t oldset;
   emacs_sigaction_init (&action, deliver_child_signal);
-  block_child_signal ();
+  block_child_signal (&oldset);
   sigaction (SIGCHLD, &action, &old_action);
   eassert (! (old_action.sa_flags & SA_SIGINFO));
 
@@ -7065,7 +7067,7 @@
       = (old_action.sa_handler == SIG_DFL || old_action.sa_handler == SIG_IGN
 	 ? dummy_handler
 	 : old_action.sa_handler);
-  unblock_child_signal ();
+  unblock_child_signal (&oldset);
 }
 
 \f

=== modified file 'src/process.h'
--- src/process.h	2014-01-01 07:43:34 +0000
+++ src/process.h	2014-02-27 20:19:31 +0000
@@ -213,8 +213,8 @@
 
 /* Defined in callproc.c.  */
 
-extern void block_child_signal (void);
-extern void unblock_child_signal (void);
+extern void block_child_signal (sigset_t *);
+extern void unblock_child_signal (sigset_t const *);
 extern Lisp_Object encode_current_directory (void);
 extern void record_kill_process (struct Lisp_Process *, Lisp_Object);
 

=== modified file 'src/sound.c'
--- src/sound.c	2014-01-01 07:43:34 +0000
+++ src/sound.c	2014-02-27 20:31:39 +0000
@@ -702,7 +702,7 @@
 {
   int val;
 #ifdef USABLE_SIGIO
-  sigset_t blocked;
+  sigset_t oldset, blocked;
 #endif
 
   eassert (sd->fd >= 0);
@@ -714,7 +714,7 @@
 #ifdef USABLE_SIGIO
   sigemptyset (&blocked);
   sigaddset (&blocked, SIGIO);
-  pthread_sigmask (SIG_BLOCK, &blocked, 0);
+  pthread_sigmask (SIG_BLOCK, &blocked, &oldset);
 #endif
 
   val = sd->format;
@@ -748,7 +748,7 @@
 
   turn_on_atimers (1);
 #ifdef USABLE_SIGIO
-  pthread_sigmask (SIG_UNBLOCK, &blocked, 0);
+  pthread_sigmask (SIG_SETMASK, &oldset, 0);
 #endif
 }
 
@@ -764,10 +764,10 @@
 	 be interrupted by a signal.  Block the ones we know to cause
 	 troubles.  */
 #ifdef USABLE_SIGIO
-      sigset_t blocked;
+      sigset_t blocked, oldset;
       sigemptyset (&blocked);
       sigaddset (&blocked, SIGIO);
-      pthread_sigmask (SIG_BLOCK, &blocked, 0);
+      pthread_sigmask (SIG_BLOCK, &blocked, &oldset);
 #endif
       turn_on_atimers (0);
 
@@ -776,7 +776,7 @@
 
       turn_on_atimers (1);
 #ifdef USABLE_SIGIO
-      pthread_sigmask (SIG_UNBLOCK, &blocked, 0);
+      pthread_sigmask (SIG_SETMASK, &oldset, 0);
 #endif
 
       /* Close the device.  */

=== modified file 'src/sysdep.c'
--- src/sysdep.c	2014-01-01 07:43:34 +0000
+++ src/sysdep.c	2014-02-27 20:33:23 +0000
@@ -692,21 +692,21 @@
 /* Block and unblock SIGTTOU.  */
 
 void
-block_tty_out_signal (void)
+block_tty_out_signal (sigset_t *oldset)
 {
 #ifdef SIGTTOU
   sigset_t blocked;
   sigemptyset (&blocked);
   sigaddset (&blocked, SIGTTOU);
-  pthread_sigmask (SIG_BLOCK, &blocked, 0);
+  pthread_sigmask (SIG_BLOCK, &blocked, oldset);
 #endif
 }
 
 void
-unblock_tty_out_signal (void)
+unblock_tty_out_signal (sigset_t const *oldset)
 {
 #ifdef SIGTTOU
-  pthread_sigmask (SIG_SETMASK, &empty_mask, 0);
+  pthread_sigmask (SIG_SETMASK, oldset, 0);
 #endif
 }
 
@@ -721,10 +721,11 @@
 tcsetpgrp_without_stopping (int fd, pid_t pgid)
 {
 #ifdef SIGTTOU
+  sigset_t oldset;
   block_input ();
-  block_tty_out_signal ();
+  block_tty_out_signal (&oldset);
   tcsetpgrp (fd, pgid);
-  unblock_tty_out_signal ();
+  unblock_tty_out_signal (&oldset);
   unblock_input ();
 #endif
 }
@@ -1525,9 +1526,6 @@
 #endif
     }
 
-  if (! IEEE_FLOATING_POINT)
-    sigaddset (&action->sa_mask, SIGFPE);
-
   action->sa_handler = handler;
   action->sa_flags = emacs_sigaction_flags ();
 }
@@ -1643,7 +1641,10 @@
 static _Noreturn void
 handle_arith_signal (int sig)
 {
-  pthread_sigmask (SIG_SETMASK, &empty_mask, 0);
+  sigset_t blocked;
+  sigemptyset (&blocked);
+  sigaddset (&blocked, sig);
+  pthread_sigmask (SIG_UNBLOCK, &blocked, 0);
   xsignal0 (Qarith_error);
 }
 

=== modified file 'src/term.c'
--- src/term.c	2014-01-25 08:25:51 +0000
+++ src/term.c	2014-02-27 20:33:17 +0000
@@ -3952,9 +3952,10 @@
       /* setsid failed, presumably because Emacs is already a process
 	 group leader.  Fall back on the obsolescent way to dissociate
 	 a controlling tty.  */
-      block_tty_out_signal ();
+      sigset_t oldset;
+      block_tty_out_signal (&oldset);
       ioctl (fd, TIOCNOTTY, 0);
-      unblock_tty_out_signal ();
+      unblock_tty_out_signal (&oldset);
 #endif
     }
 }
@@ -3978,6 +3979,7 @@
   int status;
   struct tty_display_info *tty = NULL;
   struct terminal *terminal = NULL;
+  sigset_t oldset;
   bool ctty = false;  /* True if asked to open controlling tty.  */
 
   if (!terminal_type)
@@ -4064,11 +4066,11 @@
 
   /* On some systems, tgetent tries to access the controlling
      terminal.  */
-  block_tty_out_signal ();
+  block_tty_out_signal (&oldset);
   status = tgetent (tty->termcap_term_buffer, terminal_type);
   if (tty->termcap_term_buffer[TERMCAP_BUFFER_SIZE - 1])
     emacs_abort ();
-  unblock_tty_out_signal ();
+  unblock_tty_out_signal (&oldset);
 
   if (status < 0)
     {

bug#15561: periodic timer stops running

[Paul Eggert]

[Resending with corrected bug-report address; sorry about that.]

Barry OReilly wrote:
> But what happens if the next timer happens to be soon, and Emacs receives SIGALRM inbetween set_alarm and unblock_timers?

Are you talking about a SIGALRM received during the execution of 
do_pending_atimers, between run_timer's call to set_alarm and 
do_pending_atimers's call to unblock_atimers?  If so, the SIGALRM should 
be held by the operating system during that period, and Emacs won't be 
informed of the SIGALRM until unblock_atimers does its thing.  Sorry, I 
don't see how this would cause a timer to stop.