Subject: bug#60237: 30.0.50; tree sitter core dumps when I edebug view a node

From: Stefan Monnier <[email-masked]>
Date: Fri, 10 Mar 2023 13:28:20 -0500

>> I tried cargo-culting the cpu_gc_count stuff for the memory profiler,
>> see the patch below.  However, something is amiss: this assertion in
>> profiler.el sometimes triggers:
>>
>>     (maphash
>>      (lambda (backtrace _count)
>>        (let* ((max (1- (length backtrace)))
>>               (head (aref backtrace max))
>>               (best-parent nil)
>>               (best-match (1+ max))
>>               (parents (gethash head fun-map)))
>>          (pcase-dolist (`(,i . ,parent) parents)
>>            (when t ;; (<= (- max i) best-match) ;Else, it can't be better.
>>              (let ((match max)
>>                    (imatch i))
>>                (cl-assert (>= match imatch))  <<<<<<<<<<<<<<<<<<<<<<<<<<<<
>>                (cl-assert (function-equal (aref backtrace max)
>>                                           (aref parent i)))
>>
>> I cannot reliably reproduce this, and don't understand what causes the
>> assertion.  Any hints?
>
> Hmm... I just took a look but can't see neither why your change would
> be more likely to trigger this error than the existing code for the
> `cpu` case, nor why this assertion should always be true.

I can imagine corner cases where this could trigger, but they all
involve funny business where we change `profiler-max-stack-depth` during
a single profiling run (I think you'd need to write ad-hoc ELisp code
for that).  The only other explanation I can see is that we
somehow end up with a backtrace that includes `Automatic_GC` somewhere
not at the top (maybe this can happen with a `post-gc-hook`?).

If you manage to reproduce it, I'd be interested to know the value of
`backtrace` and `parent` when the assertion fails (and maybe just save
the `log` hash-table so we can look at it).  It might be a symptom of
another bug.

And I still can't see how/why this would happen only for the `memory`
profiler and not for the `cpu` profiler, so I assume it can also happen
for the `cpu` profiler and we've just been lucky not to bump into it yet.

This said, I think the patch below should fix it for the `cpu` profiler
and a similar change should fix it for your patch (and the patch is
arguably right in the sense that without this `nil` entry, the backtrace
entry created for `Automatic_GC` is not really complete).

        Stefan

diff --git a/src/profiler.c b/src/profiler.c
index 8247b2e90c6..295c47a2acd 100644
--- a/src/profiler.c
+++ b/src/profiler.c
@@ -423,7 +423,7 @@ DEFUN ("profiler-cpu-log", Fprofiler_cpu_log, Sprofiler_cpu_log,
      more for our use afterwards since we can't rely on its special
      pre-allocated keys anymore.  So we have to allocate a new one.  */
   cpu_log = profiler_cpu_running ? make_log () : Qnil;
-  Fputhash (make_vector (1, QAutomatic_GC),
+  Fputhash (CALLN (Fvector, QAutomatic_GC, Qnil),
 	    make_fixnum (cpu_gc_count),
 	    result);
   cpu_gc_count = 0;