Avoid stdio in SIGINT handler

* admin/merge-gnulib (GNULIB_MODULES): Add ignore-value.
* lib/gnulib.mk, m4/gnulib-comp.m4: Regenerate.
* lib/ignore-value.h: New file, from gnulib.
* src/keyboard.c: Include it.
(write_stdout, read_stdin): New functions.
(handle_interrupt): Use them instead of printf and getchar,
and avoid fflush when handling signals.

5 files changed, 108 insertions, 28 deletions

diff --git a/admin/merge-gnulib b/admin/merge-gnulib
index 363bb23d119..40b5b78ee8f 100755
--- a/admin/merge-gnulib
+++ b/admin/merge-gnulib
@@ -32,7 +32,7 @@ GNULIB_MODULES='
   dtoastr dtotimespec dup2 environ execinfo faccessat
   fcntl fcntl-h fdatasync fdopendir filemode fstatat fsync
   getloadavg getopt-gnu gettime gettimeofday gitlog-to-changelog
-  intprops largefile lstat
+  ignore-value intprops largefile lstat
   manywarnings memrchr mkostemp mktime
   pipe2 pselect pthread_sigmask putenv qcopy-acl readlink readlinkat
   sig2str socklen stat-time stdalign stddef stdio
diff --git a/lib/gnulib.mk b/lib/gnulib.mk
index 97ed5b1f40f..b920cbbc9bb 100644
--- a/lib/gnulib.mk
+++ b/lib/gnulib.mk
@@ -21,7 +21,7 @@
 # the same distribution terms as the rest of that program.
 #
 # Generated by gnulib-tool.
-# Reproduce by: gnulib-tool --import --lib=libgnu --source-base=lib --m4-base=m4 --doc-base=doc --tests-base=tests --aux-dir=build-aux --avoid=close --avoid=dup --avoid=fchdir --avoid=flexmember --avoid=fstat --avoid=malloc-posix --avoid=msvc-inval --avoid=msvc-nothrow --avoid=open --avoid=openat-die --avoid=opendir --avoid=raise --avoid=save-cwd --avoid=select --avoid=setenv --avoid=sigprocmask --avoid=stdarg --avoid=stdbool --avoid=threadlib --avoid=unsetenv --makefile-name=gnulib.mk --conditional-dependencies --no-libtool --macro-prefix=gl --no-vc-files alloca-opt binary-io byteswap c-ctype c-strcase careadlinkat close-stream count-one-bits count-trailing-zeros crypto/md5 crypto/sha1 crypto/sha256 crypto/sha512 dtoastr dtotimespec dup2 environ execinfo faccessat fcntl fcntl-h fdatasync fdopendir filemode fstatat fsync getloadavg getopt-gnu gettime gettimeofday gitlog-to-changelog intprops largefile lstat manywarnings memrchr mkostemp mktime pipe2 pselect pthread_sigmask putenv qcopy-acl readlink readlinkat sig2str socklen stat-time stdalign stddef stdio stpcpy strftime strtoimax strtoumax symlink sys_stat sys_time time time_r time_rz timegm timer-time timespec-add timespec-sub unsetenv update-copyright utimens vla warnings
+# Reproduce by: gnulib-tool --import --lib=libgnu --source-base=lib --m4-base=m4 --doc-base=doc --tests-base=tests --aux-dir=build-aux --avoid=close --avoid=dup --avoid=fchdir --avoid=flexmember --avoid=fstat --avoid=malloc-posix --avoid=msvc-inval --avoid=msvc-nothrow --avoid=open --avoid=openat-die --avoid=opendir --avoid=raise --avoid=save-cwd --avoid=select --avoid=setenv --avoid=sigprocmask --avoid=stdarg --avoid=stdbool --avoid=threadlib --avoid=unsetenv --makefile-name=gnulib.mk --conditional-dependencies --no-libtool --macro-prefix=gl --no-vc-files alloca-opt binary-io byteswap c-ctype c-strcase careadlinkat close-stream count-one-bits count-trailing-zeros crypto/md5 crypto/sha1 crypto/sha256 crypto/sha512 dtoastr dtotimespec dup2 environ execinfo faccessat fcntl fcntl-h fdatasync fdopendir filemode fstatat fsync getloadavg getopt-gnu gettime gettimeofday gitlog-to-changelog ignore-value intprops largefile lstat manywarnings memrchr mkostemp mktime pipe2 pselect pthread_sigmask putenv qcopy-acl readlink readlinkat sig2str socklen stat-time stdalign stddef stdio stpcpy strftime strtoimax strtoumax symlink sys_stat sys_time time time_r time_rz timegm timer-time timespec-add timespec-sub unsetenv update-copyright utimens vla warnings
 
 
 MOSTLYCLEANFILES += core *.stackdump
@@ -567,6 +567,13 @@ EXTRA_libgnu_a_SOURCES += group-member.c
 
 ## end   gnulib module group-member
 
+## begin gnulib module ignore-value
+
+
+EXTRA_DIST += ignore-value.h
+
+## end   gnulib module ignore-value
+
 ## begin gnulib module intprops
 
 
diff --git a/lib/ignore-value.h b/lib/ignore-value.h
new file mode 100644
index 00000000000..6713d96f84e
--- /dev/null
+++ b/lib/ignore-value.h
@@ -0,0 +1,50 @@
+/* ignore a function return without a compiler warning.  -*- coding: utf-8 -*-
+
+   Copyright (C) 2008-2016 Free Software Foundation, Inc.
+
+   This program is free software: you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+/* Written by Jim Meyering, Eric Blake and Pádraig Brady.  */
+
+/* Use "ignore_value" to avoid a warning when using a function declared with
+   gcc's warn_unused_result attribute, but for which you really do want to
+   ignore the result.  Traditionally, people have used a "(void)" cast to
+   indicate that a function's return value is deliberately unused.  However,
+   if the function is declared with __attribute__((warn_unused_result)),
+   gcc issues a warning even with the cast.
+
+   Caution: most of the time, you really should heed gcc's warning, and
+   check the return value.  However, in those exceptional cases in which
+   you're sure you know what you're doing, use this function.
+
+   For the record, here's one of the ignorable warnings:
+   "copy.c:233: warning: ignoring return value of 'fchown',
+   declared with attribute warn_unused_result".  */
+
+#ifndef _GL_IGNORE_VALUE_H
+#define _GL_IGNORE_VALUE_H
+
+/* Normally casting an expression to void discards its value, but GCC
+   versions 3.4 and newer have __attribute__ ((__warn_unused_result__))
+   which may cause unwanted diagnostics in that case.  Use __typeof__
+   and __extension__ to work around the problem, if the workaround is
+   known to be needed.  */
+#if 3 < __GNUC__ + (4 <= __GNUC_MINOR__)
+# define ignore_value(x) \
+    (__extension__ ({ __typeof__ (x) __x = (x); (void) __x; }))
+#else
+# define ignore_value(x) ((void) (x))
+#endif
+
+#endif
diff --git a/m4/gnulib-comp.m4 b/m4/gnulib-comp.m4
index 69920a84afd..27ca70a0d09 100644
--- a/m4/gnulib-comp.m4
+++ b/m4/gnulib-comp.m4
@@ -91,6 +91,7 @@ AC_DEFUN([gl_EARLY],
   # Code from module gettimeofday:
   # Code from module gitlog-to-changelog:
   # Code from module group-member:
+  # Code from module ignore-value:
   # Code from module include_next:
   # Code from module intprops:
   # Code from module inttypes-incomplete:
@@ -905,6 +906,7 @@ AC_DEFUN([gl_FILE_LIST], [
   lib/gettimeofday.c
   lib/gl_openssl.h
   lib/group-member.c
+  lib/ignore-value.h
   lib/intprops.h
   lib/inttypes.in.h
   lib/lstat.c
diff --git a/src/keyboard.c b/src/keyboard.c
index fcafd0bc9a1..6bdfc1aa084 100644
--- a/src/keyboard.c
+++ b/src/keyboard.c
@@ -64,6 +64,8 @@ along with GNU Emacs.  If not, see <http://www.gnu.org/licenses/>.  */
 #include <unistd.h>
 #include <fcntl.h>
 
+#include <ignore-value.h>
+
 #ifdef HAVE_WINDOW_SYSTEM
 #include TERM_HEADER
 #endif /* HAVE_WINDOW_SYSTEM */
@@ -10206,6 +10208,21 @@ deliver_interrupt_signal (int sig)
   deliver_process_signal (sig, handle_interrupt_signal);
 }
 
+/* Output MSG directly to standard output, without buffering.  Ignore
+   failures.  This is safe in a signal handler.  */
+static void
+write_stdout (char const *msg)
+{
+  ignore_value (write (STDOUT_FILENO, msg, strlen (msg)));
+}
+
+/* Read a byte from stdin, without buffering.  Safe in signal handlers.  */
+static int
+read_stdin (void)
+{
+  char c;
+  return read (STDIN_FILENO, &c, 1) == 1 ? c : EOF;
+}
 
 /* If Emacs is stuck because `inhibit-quit' is true, then keep track
    of the number of times C-g has been requested.  If C-g is pressed
@@ -10242,9 +10259,9 @@ handle_interrupt (bool in_signal_handler)
           sigemptyset (&blocked);
           sigaddset (&blocked, SIGINT);
           pthread_sigmask (SIG_BLOCK, &blocked, 0);
+          fflush (stdout);
         }
 
-      fflush (stdout);
       reset_all_sys_modes ();
 
 #ifdef SIGTSTP
@@ -10260,8 +10277,9 @@ handle_interrupt (bool in_signal_handler)
       /* Perhaps should really fork an inferior shell?
          But that would not provide any way to get back
          to the original shell, ever.  */
-      printf ("No support for stopping a process on this operating system;\n");
-      printf ("you can continue or abort.\n");
+      write_stdout ("No support for stopping a process"
+                    " on this operating system;\n"
+                    "you can continue or abort.\n");
 #endif /* not SIGTSTP */
 #ifdef MSDOS
       /* We must remain inside the screen area when the internal terminal
@@ -10272,46 +10290,49 @@ handle_interrupt (bool in_signal_handler)
          the code used for auto-saving doesn't cope with the mark bit.  */
       if (!gc_in_progress)
         {
-          printf ("Auto-save? (y or n) ");
-          fflush (stdout);
-          if (((c = getchar ()) & ~040) == 'Y')
+          write_stdout ("Auto-save? (y or n) ");
+          c = read_stdin ();
+          if ((c & 040) == 'Y')
             {
               Fdo_auto_save (Qt, Qnil);
 #ifdef MSDOS
-              printf ("\r\nAuto-save done");
-#else /* not MSDOS */
-              printf ("Auto-save done\n");
-#endif /* not MSDOS */
+              write_stdout ("\r\nAuto-save done");
+#else
+              write_stdout ("Auto-save done\n");
+#endif
             }
-          while (c != '\n') c = getchar ();
+          while (c != '\n')
+            c = read_stdin ();
         }
       else
         {
           /* During GC, it must be safe to reenable quitting again.  */
           Vinhibit_quit = Qnil;
+          write_stdout
+            (
 #ifdef MSDOS
-          printf ("\r\n");
-#endif /* not MSDOS */
-          printf ("Garbage collection in progress; cannot auto-save now\r\n");
-          printf ("but will instead do a real quit after garbage collection ends\r\n");
-          fflush (stdout);
+             "\r\n"
+#endif
+             "Garbage collection in progress; cannot auto-save now\r\n"
+             "but will instead do a real quit"
+             " after garbage collection ends\r\n");
         }
 
 #ifdef MSDOS
-      printf ("\r\nAbort?  (y or n) ");
-#else /* not MSDOS */
-      printf ("Abort (and dump core)? (y or n) ");
-#endif /* not MSDOS */
-      fflush (stdout);
-      if (((c = getchar ()) & ~040) == 'Y')
+      write_stdout ("\r\nAbort?  (y or n) ");
+#else
+      write_stdout ("Abort (and dump core)? (y or n) ");
+#endif
+      c = read_stdin ();
+      if ((c & ~040) == 'Y')
         emacs_abort ();
-      while (c != '\n') c = getchar ();
+      while (c != '\n')
+        c = read_stdin ();
 #ifdef MSDOS
-      printf ("\r\nContinuing...\r\n");
+      write_stdout ("\r\nContinuing...\r\n");
 #else /* not MSDOS */
-      printf ("Continuing...\n");
+      write_stdout ("Continuing...\n");
 #endif /* not MSDOS */
-      fflush (stdout);
       init_all_sys_modes ();
     }
   else