bug#48839: 28.0.50; Emacs freezes and takes 100% CPU with C-h v l

[Tassilo Horn <thorn@fastmail.fm>]

Eli Zaretskii <eliz@gnu.org> writes:

Hi Eli and Daniel,

>> Looking at the marginalia code, I can see no evil, and indeed I can
>> un-marginaliarize the sample to
>> 
>>     (let ((print-escape-newlines t)
>>           (print-escape-control-characters t)
>>           (print-escape-multibyte t))
>>       (string-width (prin1-to-string load-history)))
>> 
>> which freezes my emacs in the same way.  Concretely, `prin1-to-string'
>> finishes and returns a 1.3 MB string and then `string-width' will run
>> indefinitely on that.
>
> string-width on the current master has a design bug, which causes it
> to be VERY slow on very long strings, if those strings don't include
> newlines or similar characters.  I'm working on fixing that design
> bug, but meanwhile: why does marginalia need to compute the width of
> such very long strings? that sounds like a design bug in marginalia.
> The simplest fix is not to compute string-width of any string whose
> length is greater than, say, 300 characters.  Does that help in this
> case?

Yes, with this patch I can make the issue go away with no observable
difference in the things marginalia displays.

--8<---------------cut here---------------start------------->8---
diff -u --label /home/horn/.emacs.d/elpa/marginalia-20210530.158/marginalia.el --label \#\<buffer\ marginalia.el\> /home/horn/.emacs.d/elpa/marginalia-20210530.158/marginalia.el /tmp/buffer-content-yqLXyF
--- /home/horn/.emacs.d/elpa/marginalia-20210530.158/marginalia.el
+++ #<buffer marginalia.el>
@@ -450,8 +450,12 @@
      ((marginalia--symbol-class sym) :face 'marginalia-type)
      ((let ((print-escape-newlines t)
             (print-escape-control-characters t)
-            (print-escape-multibyte t))
-        (prin1-to-string (if (boundp sym) (symbol-value sym) 'unbound)))
+            (print-escape-multibyte t)
+            (str-val (prin1-to-string
+                      (if (boundp sym)
+                          (symbol-value sym)
+                        'unbound))))
+        (substring str-val 0 (min (length str-val) 300)))
       :truncate (/ marginalia-truncate-width 3) :face 'marginalia-variable)
      ((documentation-property sym 'variable-documentation)
       :truncate marginalia-truncate-width :face 'marginalia-documentation))))

Diff finished.  Sat Jun  5 10:09:24 2021
--8<---------------cut here---------------end--------------->8---

Bye,
Tassilo