bug#24047: [PROPOSED PATCH] ‘signal’ no longer returns

[Paul Eggert]

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.
* src/eval.c (maybe_call_debugger): Now returns void, not bool.
Caller changed.
(Fsignal): Now _Noreturn.  Callers adjusted accordingly.
(xsignal): Move to lisp.h.
* src/lisp.h (process_quit_flag): Now _Noreturn.
(xsignal): Now an inline function, as it's now simply an alias
for Fsignal.
---
 src/charset.c  |    6 +++---
 src/coding.c   |    6 +++---
 src/eval.c     |   33 ++++++---------------------------
 src/keyboard.c |    4 ----
 src/lisp.h     |    8 ++++++--
 5 files changed, 18 insertions(+), 39 deletions(-)

diff --git a/src/charset.c b/src/charset.c
index 95a9c57..05469aa 100644
--- a/src/charset.c
+++ b/src/charset.c
@@ -843,9 +843,9 @@ range of code points (in CHARSET) of target characters.  */)
   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 29c90f0..a8ddc81 100644
--- a/src/coding.c
+++ b/src/coding.c
@@ -10546,9 +10546,9 @@ assigned to each coding category (see 
`coding-category-list').
   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 72facd5..a850446 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -1432,7 +1432,7 @@ struct handler *
 
 
 static Lisp_Object find_handler_clause (Lisp_Object, Lisp_Object);
-static bool maybe_call_debugger (Lisp_Object conditions, Lisp_Object sig,
+static void maybe_call_debugger (Lisp_Object conditions, Lisp_Object sig,
                                 Lisp_Object data);
 
 void
@@ -1460,7 +1460,8 @@ static bool maybe_call_debugger (Lisp_Object conditions, 
Lisp_Object sig,
 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)
 {
   /* When memory is full, ERROR-SYMBOL is nil,
@@ -1537,14 +1538,7 @@ static bool maybe_call_debugger (Lisp_Object conditions, 
Lisp_Object sig,
          /* Special handler that means "print a message and run debugger
             if requested".  */
          || EQ (h->tag_or_ch, Qerror)))
-    {
-      bool debugger_called
-       = 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))
-       return Qnil;
-    }
+    maybe_call_debugger (conditions, error_symbol, data);
 
   if (!NILP (clause))
     {
@@ -1569,16 +1563,6 @@ static bool maybe_call_debugger (Lisp_Object conditions, 
Lisp_Object sig,
   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
@@ -1700,7 +1684,7 @@ static bool maybe_call_debugger (Lisp_Object conditions, 
Lisp_Object sig,
     = SIG is the error symbol, and DATA is the rest of the data.
     = SIG is nil, and DATA is (SYMBOL . REST-OF-DATA).
       This is for memory-full errors only.  */
-static bool
+static void
 maybe_call_debugger (Lisp_Object conditions, Lisp_Object sig, Lisp_Object data)
 {
   Lisp_Object combined_data;
@@ -1719,12 +1703,7 @@ static bool maybe_call_debugger (Lisp_Object conditions, 
Lisp_Object sig,
       && ! skip_debugger (conditions, combined_data)
       /* RMS: What's this for?  */
       && when_entered_debugger < num_nonmacro_input_events)
-    {
-      call_debugger (list2 (Qerror, combined_data));
-      return 1;
-    }
-
-  return 0;
+    call_debugger (list2 (Qerror, combined_data));
 }
 
 static Lisp_Object
diff --git a/src/keyboard.c b/src/keyboard.c
index 8901ff0..6b5528d 100644
--- a/src/keyboard.c
+++ b/src/keyboard.c
@@ -10402,13 +10402,9 @@ shows the events before all translations (except for 
input methods).
         then quit right away.  */
       if (immediate_quit && NILP (Vinhibit_quit))
        {
-         struct gl_state_s saved;
-
          immediate_quit = false;
          pthread_sigmask (SIG_SETMASK, &empty_mask, 0);
-         saved = gl_state;
          Fsignal (Qquit, Qnil);
-         gl_state = saved;
        }
       else
         { /* Else request quit when it's safe.  */
diff --git a/src/lisp.h b/src/lisp.h
index 39877d7..50ee8ea 100644
--- a/src/lisp.h
+++ b/src/lisp.h
@@ -3230,7 +3230,7 @@ struct handler
 extern void process_pending_signals (void);
 extern bool volatile pending_signals;
 
-extern void process_quit_flag (void);
+extern _Noreturn void process_quit_flag (void);
 #define QUIT                                           \
   do {                                                 \
     if (!NILP (Vquit_flag) && NILP (Vinhibit_quit))    \
@@ -3879,7 +3879,11 @@ extern void map_obarray (Lisp_Object, void (*) 
(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);
+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);
-- 
1.7.9.5