‘signal’ no longer returns

Although for decades ‘signal’ has been documented to not return,
a corner case in the Lisp debugger causes ‘signal’ to return.
Remove the corner case and adjust Emacs internals accordingly.
An alternative would be to document the corner case, but this
would complicate the Lisp API unnecessarily.  (Bug#24047)
* src/eval.c (signal_or_quit): New function, with most of the
old contents of Fsignal.
(quit): New function, which uses signal_or_quit and which
might return.  All keyboard-based callers of Fsignal (Qquit,
Qnil) changed to use this new function instead.
(Fsignal): Use signal_or_quit.  Now _Noreturn.  All callers
changed.
(xsignal): Move to lisp.h.
* src/lisp.h (xsignal): Now an inline function, as it's now
just an alias for Fsignal.

13 files changed, 54 insertions, 39 deletions

diff --git a/src/bytecode.c b/src/bytecode.c
index 05bc9fcdb08..ee1b79f1826 100644
--- a/src/bytecode.c
+++ b/src/bytecode.c
@@ -386,7 +386,7 @@ relocate_byte_stack (void)
         Vquit_flag = Qnil;                              \
         if (EQ (Vthrow_on_input, flag))                 \
           Fthrow (Vthrow_on_input, Qt);                 \
-        Fsignal (Qquit, Qnil);                          \
+        quit ();                                        \
       }                                                 \
     else if (pending_signals)                           \
       process_pending_signals ();                       \
diff --git a/src/charset.c b/src/charset.c
index 95a9c577d52..05469aa2650 100644
--- a/src/charset.c
+++ b/src/charset.c
@@ -843,9 +843,9 @@ usage: (define-charset-internal ...)  */)
   int nchars;
 
   if (nargs != charset_arg_max)
-    return Fsignal (Qwrong_number_of_arguments,
-                    Fcons (intern ("define-charset-internal"),
-                           make_number (nargs)));
+    Fsignal (Qwrong_number_of_arguments,
+             Fcons (intern ("define-charset-internal"),
+                    make_number (nargs)));
 
   attrs = Fmake_vector (make_number (charset_attr_max), Qnil);
 
diff --git a/src/coding.c b/src/coding.c
index 29c90f0a401..a8ddc817565 100644
--- a/src/coding.c
+++ b/src/coding.c
@@ -10546,9 +10546,9 @@ usage: (define-coding-system-internal ...)  */)
   return Qnil;
 
  short_args:
-  return Fsignal (Qwrong_number_of_arguments,
-                  Fcons (intern ("define-coding-system-internal"),
-                         make_number (nargs)));
+  Fsignal (Qwrong_number_of_arguments,
+           Fcons (intern ("define-coding-system-internal"),
+                  make_number (nargs)));
 }
 
 
diff --git a/src/eval.c b/src/eval.c
index 72facd5db64..33b82f74b64 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -1431,6 +1431,7 @@ push_handler_nosignal (Lisp_Object tag_ch_val, enum handlertype handlertype)
 }
 
 
+static Lisp_Object signal_or_quit (Lisp_Object, Lisp_Object, bool);
 static Lisp_Object find_handler_clause (Lisp_Object, Lisp_Object);
 static bool maybe_call_debugger (Lisp_Object conditions, Lisp_Object sig,
                                  Lisp_Object data);
@@ -1444,7 +1445,7 @@ process_quit_flag (void)
     Fkill_emacs (Qnil);
   if (EQ (Vthrow_on_input, flag))
     Fthrow (Vthrow_on_input, Qt);
-  Fsignal (Qquit, Qnil);
+  quit ();
 }
 
 DEFUN ("signal", Fsignal, Ssignal, 2, 2, 0,
@@ -1460,9 +1461,29 @@ DATA should be a list.  Its elements are printed as part of the error message.
 See Info anchor `(elisp)Definition of signal' for some details on how this
 error message is constructed.
 If the signal is handled, DATA is made available to the handler.
-See also the function `condition-case'.  */)
+See also the function `condition-case'.  */
+       attributes: noreturn)
   (Lisp_Object error_symbol, Lisp_Object data)
 {
+  signal_or_quit (error_symbol, data, false);
+  eassume (false);
+}
+
+/* Quit, in response to a keyboard quit request.  */
+Lisp_Object
+quit (void)
+{
+  return signal_or_quit (Qquit, Qnil, true);
+}
+
+/* Signal an error, or quit.  ERROR_SYMBOL and DATA are as with Fsignal.
+   If KEYBOARD_QUIT, this is a quit; ERROR_SYMBOL should be
+   Qquit and DATA should be Qnil, and this function may return.
+   Otherwise this function is like Fsignal and does not return.  */
+
+static Lisp_Object
+signal_or_quit (Lisp_Object error_symbol, Lisp_Object data, bool keyboard_quit)
+{
   /* When memory is full, ERROR-SYMBOL is nil,
      and DATA is (REAL-ERROR-SYMBOL . REAL-DATA).
      That is a special case--don't do this in other situations.  */
@@ -1542,7 +1563,7 @@ See also the function `condition-case'.  */)
         = maybe_call_debugger (conditions, error_symbol, data);
       /* We can't return values to code which signaled an error, but we
          can continue code which has signaled a quit.  */
-      if (debugger_called && EQ (real_error_symbol, Qquit))
+      if (keyboard_quit && debugger_called && EQ (real_error_symbol, Qquit))
         return Qnil;
     }
 
@@ -1569,16 +1590,6 @@ See also the function `condition-case'.  */)
   fatal ("%s", SDATA (string));
 }
 
-/* Internal version of Fsignal that never returns.
-   Used for anything but Qquit (which can return from Fsignal).  */
-
-void
-xsignal (Lisp_Object error_symbol, Lisp_Object data)
-{
-  Fsignal (error_symbol, data);
-  emacs_abort ();
-}
-
 /* Like xsignal, but takes 0, 1, 2, or 3 args instead of a list.  */
 
 void
diff --git a/src/fileio.c b/src/fileio.c
index b1f9d3cf73a..66ea761ca56 100644
--- a/src/fileio.c
+++ b/src/fileio.c
@@ -4513,7 +4513,7 @@ by calling `format-decode', which see.  */)
                              PT - BEG, Z - PT - inserted);
 
   if (read_quit)
-    Fsignal (Qquit, Qnil);
+    quit ();
 
   /* Retval needs to be dealt with in all cases consistently.  */
   if (NILP (val))
diff --git a/src/keyboard.c b/src/keyboard.c
index 8901ff055e7..ed4968486c3 100644
--- a/src/keyboard.c
+++ b/src/keyboard.c
@@ -696,7 +696,7 @@ recursive_edit_1 (void)
 
   val = command_loop ();
   if (EQ (val, Qt))
-    Fsignal (Qquit, Qnil);
+    quit ();
   /* Handle throw from read_minibuf when using minibuffer
      while it's active but we're in another window.  */
   if (STRINGP (val))
@@ -7581,7 +7581,7 @@ menu_item_eval_property_1 (Lisp_Object arg)
   /* If we got a quit from within the menu computation,
      quit all the way out of it.  This takes care of C-] in the debugger.  */
   if (CONSP (arg) && EQ (XCAR (arg), Qquit))
-    Fsignal (Qquit, Qnil);
+    quit ();
 
   return Qnil;
 }
@@ -10407,7 +10407,7 @@ handle_interrupt (bool in_signal_handler)
           immediate_quit = false;
           pthread_sigmask (SIG_SETMASK, &empty_mask, 0);
           saved = gl_state;
-          Fsignal (Qquit, Qnil);
+          quit ();
           gl_state = saved;
         }
       else
diff --git a/src/lisp.h b/src/lisp.h
index 39877d7bb4d..089f3977cd2 100644
--- a/src/lisp.h
+++ b/src/lisp.h
@@ -3879,7 +3879,12 @@ extern void run_hook_with_args_2 (Lisp_Object, Lisp_Object, Lisp_Object);
 extern Lisp_Object run_hook_with_args (ptrdiff_t nargs, Lisp_Object *args,
                                        Lisp_Object (*funcall)
                                        (ptrdiff_t nargs, Lisp_Object *args));
-extern _Noreturn void xsignal (Lisp_Object, Lisp_Object);
+extern Lisp_Object quit (void);
+INLINE _Noreturn void
+xsignal (Lisp_Object error_symbol, Lisp_Object data)
+{
+  Fsignal (error_symbol, data);
+}
 extern _Noreturn void xsignal0 (Lisp_Object);
 extern _Noreturn void xsignal1 (Lisp_Object, Lisp_Object);
 extern _Noreturn void xsignal2 (Lisp_Object, Lisp_Object, Lisp_Object);
diff --git a/src/nsmenu.m b/src/nsmenu.m
index 83ded6daca3..d1f4b020bb0 100644
--- a/src/nsmenu.m
+++ b/src/nsmenu.m
@@ -1843,7 +1843,7 @@ ns_popup_dialog (struct frame *f, Lisp_Object header, Lisp_Object contents)
 
   if (EQ (ret, Qundefined) && window_closed)
     /* Make close button pressed equivalent to C-g.  */
-    Fsignal (Qquit, Qnil);
+    quit ();
 
   return ret;
 }
diff --git a/src/term.c b/src/term.c
index 81908b370a5..d54ff115f9d 100644
--- a/src/term.c
+++ b/src/term.c
@@ -3759,7 +3759,7 @@ tty_menu_show (struct frame *f, int x, int y, int menuflags,
       /* Make "Cancel" equivalent to C-g unless FOR_CLICK (which means
          the menu was invoked with a mouse event as POSITION).  */
       if (!(menuflags & MENU_FOR_CLICK))
-        Fsignal (Qquit, Qnil);
+        quit ();
       break;
     }
 
diff --git a/src/w32fns.c b/src/w32fns.c
index d6b54d19a19..584e311230e 100644
--- a/src/w32fns.c
+++ b/src/w32fns.c
@@ -7584,7 +7584,7 @@ value of DIR as in previous invocations; this is standard Windows behavior.  */)
 
   /* Make "Cancel" equivalent to C-g.  */
   if (NILP (filename))
-    Fsignal (Qquit, Qnil);
+    quit ();
 
   return filename;
 }
diff --git a/src/w32menu.c b/src/w32menu.c
index 13296d9d855..7c66360becd 100644
--- a/src/w32menu.c
+++ b/src/w32menu.c
@@ -827,7 +827,7 @@ w32_menu_show (struct frame *f, int x, int y, int menuflags,
     {
       unblock_input ();
       /* Make "Cancel" equivalent to C-g.  */
-      Fsignal (Qquit, Qnil);
+      quit ();
     }
 
   unblock_input ();
@@ -1019,7 +1019,7 @@ w32_dialog_show (struct frame *f, Lisp_Object title,
     }
   else
     /* Make "Cancel" equivalent to C-g.  */
-    Fsignal (Qquit, Qnil);
+    quit ();
 
   return Qnil;
 }
@@ -1155,7 +1155,7 @@ simple_dialog_show (struct frame *f, Lisp_Object contents, Lisp_Object header)
   else if (answer == IDNO)
     lispy_answer = build_string ("No");
   else
-    Fsignal (Qquit, Qnil);
+    quit ();
 
   for (temp = XCDR (contents); CONSP (temp); temp = XCDR (temp))
     {
@@ -1177,8 +1177,7 @@ simple_dialog_show (struct frame *f, Lisp_Object contents, Lisp_Object header)
           return value;
         }
     }
-  Fsignal (Qquit, Qnil);
-  return Qnil;
+  return quit ();
 }
 #endif  /* !HAVE_DIALOGS  */
 
diff --git a/src/xfns.c b/src/xfns.c
index 798dc49bef5..c44997b3d6f 100644
--- a/src/xfns.c
+++ b/src/xfns.c
@@ -6346,7 +6346,7 @@ value of DIR as in previous invocations; this is standard Windows behavior.  */)
 
   /* Make "Cancel" equivalent to C-g.  */
   if (NILP (file))
-    Fsignal (Qquit, Qnil);
+    quit ();
 
   decoded_file = DECODE_FILE (file);
 
@@ -6418,7 +6418,7 @@ value of DIR as in previous invocations; this is standard Windows behavior.  */)
 
   /* Make "Cancel" equivalent to C-g.  */
   if (NILP (file))
-    Fsignal (Qquit, Qnil);
+    quit ();
 
   decoded_file = DECODE_FILE (file);
 
@@ -6469,7 +6469,7 @@ nil, it defaults to the selected frame. */)
   unblock_input ();
 
   if (NILP (font))
-    Fsignal (Qquit, Qnil);
+    quit ();
 
   return unbind_to (count, font);
 }
diff --git a/src/xmenu.c b/src/xmenu.c
index 9e1a817946a..9ab7bdf971f 100644
--- a/src/xmenu.c
+++ b/src/xmenu.c
@@ -1649,7 +1649,7 @@ x_menu_show (struct frame *f, int x, int y, int menuflags,
     {
       unblock_input ();
       /* Make "Cancel" equivalent to C-g.  */
-      Fsignal (Qquit, Qnil);
+      quit ();
     }
 
   unblock_input ();
@@ -1913,7 +1913,7 @@ x_dialog_show (struct frame *f, Lisp_Object title,
     }
   else
     /* Make "Cancel" equivalent to C-g.  */
-    Fsignal (Qquit, Qnil);
+    quit ();
 
   return Qnil;
 }
@@ -2304,7 +2304,7 @@ x_menu_show (struct frame *f, int x, int y, int menuflags,
       if (!(menuflags & MENU_FOR_CLICK))
         {
           unblock_input ();
-          Fsignal (Qquit, Qnil);
+          quit ();
         }
       break;
     }