Don’t stuff keyboard input uselessly

Also, document stuffing better.
* src/keyboard.c (stuff_buffered_input):
Give up on stuffing if it fails.
* src/sysdep.c (stuff_char): Return failure indication.

admin/notes/multi-tty

@@ -504,6 +504,8 @@ THINGS TO DO
 ** flow-ctrl.el must be updated.
 
 ** Fix stuff_char for multi-tty.  Doesn't seem to be of high priority.
+   For security reasons stuff_char does not work by default on many
+   platforms, which makes such a fix even lower priority.
 
 DIARY OF CHANGES
 ----------------

doc/lispref/os.texi

@@ -711,12 +711,11 @@ If @var{exit-data} is an integer, that is used as the exit status of
 the Emacs process.  (This is useful primarily in batch operation; see
 @ref{Batch Mode}.)
 
-If @var{exit-data} is a string, its contents are stuffed into the
-terminal input buffer so that the shell (or whatever program next reads
-input) can read them.  This is only possible on some systems.  Note that
-recent versions of GNU/Linux disable the system API required for
-stuffing the string into the terminal input, so this might not work on
-your system without special privileges.
+If @var{exit-data} is a string, its contents (followed by a newline) are
+stuffed into the terminal input buffer so that the shell (or whatever
+program next reads input) can read them.  However, stuffing is silently
+skipped if Emacs is not running interactively on a terminal, or if
+stuffing requires special privileges or is not supported on this platform.
 
 If @var{exit-data} is neither an integer nor a string, or is omitted,
 that means to use the (system-specific) exit status which indicates
@@ -816,6 +815,9 @@ before suspending Emacs, or this function signals an error.
 
 If @var{string} is non-@code{nil}, its characters are sent to Emacs's
 superior shell, to be read as terminal input.
+However, this action is skipped if Emacs is not running interactively on
+a terminal, or if stuffing characters into the terminal input requires
+special privileges or is not supported on this platform.
 
 Before suspending, @code{suspend-emacs} runs the normal hook
 @code{suspend-hook}.  After the user resumes Emacs,
@@ -840,7 +842,7 @@ Here is an example of how you could use these hooks:
 @c The sit-for prevents the @code{nil} that suspend-emacs returns
 @c hiding the message.
 
-Here is what you would see upon evaluating @code{(suspend-emacs "pwd")}:
+Here is what you would see upon evaluating @code{(suspend-emacs)}:
 
 @smallexample
 @group
@@ -851,8 +853,6 @@ Really suspend? @kbd{y}
 
 @group
 ---------- Parent Shell ----------
-bash$ pwd
-/home/username
 bash$ fg
 @end group
 

src/emacs.c

@@ -2947,9 +2947,9 @@ sort_args (int argc, char **argv)
 DEFUN ("kill-emacs", Fkill_emacs, Skill_emacs, 0, 2, "P",
        doc: /* Exit the Emacs job and kill it.
 If ARG is an integer, return ARG as the exit program code.
-If ARG is a string, stuff it as keyboard input.  (This might
-not work on modern systems due to security considerations, or
-not at all.)
+If ARG is a string, stuff it and then a newline as keyboard input,
+if Emacs is running interactively on a terminal and the platform
+supports and allows stuffing; this may need special privileges.
 Any other value of ARG, or ARG omitted, means return an
 exit code that indicates successful program termination.
 

src/keyboard.c

@@ -12421,19 +12421,15 @@ DEFUN ("suspend-emacs", Fsuspend_emacs, Ssuspend_emacs, 0, 1, "",
 If `cannot-suspend' is non-nil, or if the system doesn't support job
 control, run a subshell instead.
 
-If optional arg STUFFSTRING is non-nil, its characters are stuffed
-to be read as terminal input by Emacs's parent, after suspension.
+If optional arg STUFFSTRING is non-nil, stuff it and then a newline as
+keyboard input, if Emacs is running interactively on a terminal and the
+platform supports and allows stuffing; this may need special privileges.
 
 Before suspending, run the normal hook `suspend-hook'.
 After resumption run the normal hook `suspend-resume-hook'.
 
 Some operating systems cannot stop the Emacs process and resume it later.
-On such systems, Emacs starts a subshell instead of suspending.
-
-On some operating systems, stuffing characters into terminal input
-buffer requires special privileges or is not supported at all.
-On such systems, calling this function with non-nil STUFFSTRING might
-either signal an error or silently fail to stuff the characters.  */)
+On such systems, Emacs starts a subshell instead of suspending.  */)
   (Lisp_Object stuffstring)
 {
   specpdl_ref count = SPECPDL_INDEX ();
@@ -12472,38 +12468,34 @@ either signal an error or silently fail to stuff the characters.  */)
   return Qnil;
 }
 
-/* If STUFFSTRING is a string, stuff its contents as pending terminal input.
-   Then in any case stuff anything Emacs has read ahead and not used.  */
+/* If STUFFSTRING is a string, stuff its contents and then a newline as
+   pending terminal input.  Then stuff anything Emacs has read ahead and
+   not used.  However, do nothing if stuffing does not work.  */
 
 void
 stuff_buffered_input (Lisp_Object stuffstring)
 {
 #ifdef SIGTSTP  /* stuff_char is defined if SIGTSTP.  */
-  register unsigned char *p;
+  int bad_stuff = 0;
 
   if (STRINGP (stuffstring))
     {
-      register ptrdiff_t count;
-
-      p = SDATA (stuffstring);
-      count = SBYTES (stuffstring);
-      while (count-- > 0)
-	stuff_char (*p++);
-      stuff_char ('\n');
+      char *p = SSDATA (stuffstring);
+      for (ptrdiff_t i = SBYTES (stuffstring); !bad_stuff && 0 < i; i--)
+	bad_stuff = stuff_char (*p++);
+      if (!bad_stuff)
+	bad_stuff = stuff_char ('\n');
     }
 
-  /* Anything we have read ahead, put back for the shell to read.  */
-  /* ?? What should this do when we have multiple keyboards??
-     Should we ignore anything that was typed in at the "wrong" kboard?
-
-     rms: we should stuff everything back into the kboard
-     it came from.  */
+  /* Anything we have read ahead, put back for the shell to read.
+     When we have multiple keyboards, we should stuff everything back
+     into the keyboard it came from, but fixing this is low priority as
+     many systems prohibit stuffing for security reasons.  */
   for (; kbd_fetch_ptr != kbd_store_ptr;
        kbd_fetch_ptr = next_kbd_event (kbd_fetch_ptr))
     {
-
-      if (kbd_fetch_ptr->kind == ASCII_KEYSTROKE_EVENT)
-	stuff_char (kbd_fetch_ptr->ie.code);
+      if (kbd_fetch_ptr->kind == ASCII_KEYSTROKE_EVENT && !bad_stuff)
+	bad_stuff = stuff_char (kbd_fetch_ptr->ie.code);
 
       clear_event (&kbd_fetch_ptr->ie);
     }

src/lisp.h

@@ -5280,7 +5280,7 @@ maybe_disable_address_randomization (int argc, char **argv)
 extern int emacs_exec_file (char const *, char *const *, char *const *);
 extern void init_standard_fds (void);
 extern char *emacs_get_current_dir_name (void);
-extern void stuff_char (char c);
+extern int stuff_char (char c);
 extern void init_foreground_group (void);
 extern void sys_subshell (void);
 extern void sys_suspend (void);

src/sysdep.c

@@ -390,23 +390,22 @@ discard_tty_input (void)
 
 /* Arrange for character C to be read as the next input from
    the terminal.
+   Return 0 on success, -1 otherwise.
    XXX What if we have multiple ttys?
 */
 
-void
+int
 stuff_char (char c)
 {
-  if (! (FRAMEP (selected_frame)
-	 && FRAME_LIVE_P (XFRAME (selected_frame))
-	 && FRAME_TERMCAP_P (XFRAME (selected_frame))))
-    return;
-
 /* Should perhaps error if in batch mode */
 #ifdef TIOCSTI
-  ioctl (fileno (CURTTY()->input), TIOCSTI, &c);
-#else /* no TIOCSTI */
-  error ("Cannot stuff terminal input characters in this version of Unix");
+  if (FRAMEP (selected_frame)
+      && FRAME_LIVE_P (XFRAME (selected_frame))
+      && FRAME_TERMCAP_P (XFRAME (selected_frame)))
+    return ioctl (fileno (CURTTY()->input), TIOCSTI, &c);
 #endif /* no TIOCSTI */
+
+  return -1;
 }
 
 #endif /* SIGTSTP */