scratch/handler-bind d67bfb6e80d 2/2: eval.c: Add new var `lisp-eval-depth-reserve`

[Stefan Monnier]

branch: scratch/handler-bind
commit d67bfb6e80d9a725860ca1d0236b85a591ee8013
Author: Stefan Monnier <monnier@iro.umontreal.ca>
Commit: Stefan Monnier <monnier@iro.umontreal.ca>

    eval.c: Add new var `lisp-eval-depth-reserve`
    
    Rather than blindly increase `max-lisp-eval-depth` when entering the
    debugger or running `signal-hook-function`, use this new "reserve"
    to keep track of how much we have grown the stack for "debugger"
    purposes so that for example recursive calls to `signal-hook-function`
    can't eat up the whole C stack.
    
    * src/eval.c (max_ensure_room): Rewrite.
    (restore_stack_limits): Move before `max_ensure_room`.  Rewrite.
    (call_debugger, signal_or_quit): Adjust calls accordingly.
    Also grow `max-lisp-eval-depth` for `hander-bind` handlers.
    (init_eval_once): Don't initialize `max_lisp_eval_depth` here.
    (syms_of_eval): Initialize it here instead.
    Add new var `lisp-eval-depth-reserve`.
---
 src/eval.c | 55 ++++++++++++++++++++++++++++++++-----------------------
 1 file changed, 32 insertions(+), 23 deletions(-)

diff --git a/src/eval.c b/src/eval.c
index 2fa987b6072..31948eb5c9c 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -212,7 +212,6 @@ void
 init_eval_once (void)
 {
   /* Don't forget to update docs (lispref node "Eval").  */
-  max_lisp_eval_depth = 1600;
   Vrun_hooks = Qnil;
   pdumper_do_now_and_after_load (init_eval_once_for_pdumper);
 }
@@ -248,22 +247,29 @@ init_eval (void)
   redisplay_deep_handler = NULL;
 }
 
-/* Ensure that *M is at least A + B if possible, or is its maximum
-   value otherwise.  */
-
 static void
-max_ensure_room (intmax_t *m, intmax_t a, intmax_t b)
+restore_stack_limits (Lisp_Object data)
 {
-  intmax_t sum = ckd_add (&sum, a, b) ? INTMAX_MAX : sum;
-  *m = max (*m, sum);
+  intmax_t old_depth;
+  integer_to_intmax (data, &old_depth);
+  lisp_eval_depth_reserve += max_lisp_eval_depth - old_depth;
+  max_lisp_eval_depth = old_depth;
 }
 
-/* Unwind-protect function used by call_debugger.  */
+/* Try and ensure that we have at least B dpeth available.  */
 
 static void
-restore_stack_limits (Lisp_Object data)
+max_ensure_room (intmax_t b)
 {
-  integer_to_intmax (data, &max_lisp_eval_depth);
+  intmax_t sum = ckd_add (&sum, lisp_eval_depth, b) ? INTMAX_MAX : sum;
+  intmax_t diff = min (sum - max_lisp_eval_depth, lisp_eval_depth_reserve);
+  if (diff <= 0)
+    return;
+  intmax_t old_depth = max_lisp_eval_depth;
+  max_lisp_eval_depth += diff;
+  lisp_eval_depth_reserve -= diff;
+  /* Restore limits after leaving the debugger.  */
+  record_unwind_protect (restore_stack_limits, make_int (old_depth));
 }
 
 /* Call the Lisp debugger, giving it argument ARG.  */
@@ -274,16 +280,12 @@ call_debugger (Lisp_Object arg)
   bool debug_while_redisplaying;
   specpdl_ref count = SPECPDL_INDEX ();
   Lisp_Object val;
-  intmax_t old_depth = max_lisp_eval_depth;
 
   /* The previous value of 40 is too small now that the debugger
      prints using cl-prin1 instead of prin1.  Printing lists nested 8
      deep (which is the value of print-level used in the debugger)
      currently requires 77 additional frames.  See bug#31919.  */
-  max_ensure_room (&max_lisp_eval_depth, lisp_eval_depth, 100);
-
-  /* Restore limits after leaving the debugger.  */
-  record_unwind_protect (restore_stack_limits, make_int (old_depth));
+  max_ensure_room (100);
 
 #ifdef HAVE_WINDOW_SYSTEM
   if (display_hourglass_p)
@@ -1789,16 +1791,13 @@ signal_or_quit (Lisp_Object error_symbol, Lisp_Object 
data, bool keyboard_quit)
 
   /* This hook is used by edebug.  */
   if (! NILP (Vsignal_hook_function)
-      && ! NILP (error_symbol)
-      /* Don't try to call a lisp function if we've already overflowed
-         the specpdl stack.  */
-      && specpdl_ptr < specpdl_end)
+      && ! NILP (error_symbol))
     {
-      /* Edebug takes care of restoring these variables when it exits.  */
-      max_ensure_room (&max_lisp_eval_depth, lisp_eval_depth, 20);
-
+      specpdl_ref count = SPECPDL_INDEX ();
+      max_ensure_room (20);
       /* FIXME: 'handler-bind' makes `signal-hook-function' obsolete?  */
       call2 (Vsignal_hook_function, error_symbol, data);
+      unbind_to (count, Qnil);
     }
 
   conditions = Fget (real_error_symbol, Qerror_conditions);
@@ -1836,9 +1835,12 @@ signal_or_quit (Lisp_Object error_symbol, Lisp_Object 
data, bool keyboard_quit)
                Lisp_Object error_data
                  = (NILP (error_symbol)
                     ? data : Fcons (error_symbol, data));
+               specpdl_ref count = SPECPDL_INDEX ();
+               max_ensure_room (20);
                push_handler (make_fixnum (skip + h->bytecode_dest),
                              SKIP_CONDITIONS);
                call1 (h->val, error_data);
+               unbind_to (count, Qnil);
                pop_handler ();
              }
            continue;
@@ -1888,8 +1890,8 @@ signal_or_quit (Lisp_Object error_symbol, Lisp_Object 
data, bool keyboard_quit)
       && NILP (Vinhibit_debugger)
       && !NILP (Ffboundp (Qdebug_early)))
     {
-      max_ensure_room (&max_lisp_eval_depth, lisp_eval_depth, 100);
       specpdl_ref count = SPECPDL_INDEX ();
+      max_ensure_room (100);
       AUTO_STRING (redisplay_trace, "*Redisplay-trace*");
       Lisp_Object redisplay_trace_buffer;
       AUTO_STRING (gap, "\n\n\n\n"); /* Separates things in *Redisplay-trace* 
*/
@@ -4332,6 +4334,13 @@ actual stack overflow in C, which would be fatal for 
Emacs.
 You can safely make it considerably larger than its default value,
 if that proves inconveniently small.  However, if you increase it too far,
 Emacs could overflow the real C stack, and crash.  */);
+  max_lisp_eval_depth = 1600;
+
+  DEFVAR_INT ("lisp-eval-depth-reserve", lisp_eval_depth_reserve,
+             doc: /* Extra depth that can be allocated to handle errors.
+This is the max depth that the system will add to `max-lisp-eval-depth'
+when calling debuggers or `handler-bind' handlers.  */);
+  lisp_eval_depth_reserve = 200;
 
   DEFVAR_LISP ("quit-flag", Vquit_flag,
               doc: /* Non-nil causes `eval' to abort, unless `inhibit-quit' is 
non-nil.