Question about composite.c

[Gerry Agbobada <gagbobada@gmail.com>]

#+TITLE: empty_string_issue

The rest of this file has been edited with org-mode, to allow simple tangling to
create an elisp file to try to setup the bug I had ; and have
highlighting for the
diff I tried.

* Issue
While trying to use elisp code to get ligatures working on a build of emacs-27
branch, I noticed that a few ligatures from the table was freezing emacs.

I wasn't able to reproduce properly using =emacs -q=, but I was able to
reproduce it in 2 different major modes using a heavily customized emacs (Doom
Emacs). I was able to patch emacs C code to make my issue stop though; so I
mainly want to know if that is a logic error I was able to find out or just
something I should keep investigating on the elisp side.

* Recipe
** Emacs version
I'm playing with emacs-27 builds =--with-harfbuzz=.

The version is 27.0.60 with harfbuzz feature.

I think I was on commit =5841240= but I'm not 100% positive since I don't have
the machine easily available. I know I pulled emacs-27 between
Fri, 10 Jan 2020 08:00:00 GMT and Fri, 10 Jan 2020 11:00:00 GMT.

The =src/composite.c= file I was able to single out didn't change since then it
seems.

** Set up a buffer
This is extracted from microsoft/cascadia-code#153 on Github
[[https://github.com/microsoft/cascadia-code/issues/153][Link to the issue]]
#+BEGIN_SRC emacs-lisp :tangle yes
(defvar composition-ligature-table (make-char-table nil))
(require 'composite)

(let ((alist
       '(
         (?* . ".\\(?:\\(\\*\\*\\|[*>]\\)[*>]?\\)")
         )))
  (dolist (char-regexp alist)
    (set-char-table-range composition-ligature-table (car char-regexp)
                          `([,(cdr char-regexp) 0 font-shape-gstring]))))

(set-char-table-parent composition-ligature-table composition-function-table)

(setq-local composition-function-table composition-ligature-table)
#+END_SRC

** Test fonts
- Write a lot of * : *****
- Delete asterisks one by one (=DEL=)
- EXPECTED : asterisks get deleted one by one
- ACTUAL : emacs freezes and after modifying the =src/composite.c= file I found
  out the error is =Attempt to shape unibyte text= from this
[[https://github.com/emacs-mirror/emacs/blob/b651939aaf06f4f1ddcd1f750bb8825546027b96/src/composite.c#L1749][if
branch]] with an
  *empty string*.

I think my error may come from having a composition-table where a replacement
triggered by =prettify-symbols= occurs before the regex for
=composition-ligature-table= happens, so there's only an empty string for
replacement and there's an error because it doesn't pass the test.

* Patch to "make it work"
With this patch I was able to stop having the infinite freeze/stuttering
I think the issue is that :
- when the font does not have a ligature for the matching pattern
- it somehow passes an empty string =""= in this if-statement
- =(if "" 'not-nil)= return ='not-nil= so it goes to the else branch
- =STRING_MULTIBYTE("")= is false because an empty string doesn't have the
  marker
- composite errors and loops

Or maybe the error comes from having a composition-table where a replacement
triggered by =prettify-symbols= occurs before the regex for
=composition-ligature-table= happens, so there's only an empty string for
replacement and there's an error because it doesn't pass the test.

NOTE : I know this is a "dirty" patch from thw warning I got from the compiler
(i.e. using strlen on the =string= variable here is not clean.) but I feel it
conveys what I mean in a better way, and to be honest I don't know what the
clean version is.

NOTE 2 : this patch also shows how I was able to find out which if-branch
triggered. I guess the reason for not displaying the bad string is to avoid
side-effects in the minibuffer.

#+BEGIN_SRC diff :tangle no
diff --git a/src/composite.c b/src/composite.c
index 53e6930b5f..1151721d61 100644
--- a/src/composite.c
+++ b/src/composite.c
@@ -1735,7 +1735,7 @@ Otherwise (for terminal display), FONT-OBJECT
must be a terminal ID, a
   if (NILP (string))
     {
       if (NILP (BVAR (current_buffer, enable_multibyte_characters)))
-       error ("Attempt to shape unibyte text");
+       error ("Attempt to shape unibyte text \"%s\" in non multibyte
buffer", string);
       validate_region (&from, &to);
       frompos = XFIXNAT (from);
       topos = XFIXNAT (to);
@@ -1745,8 +1745,8 @@ Otherwise (for terminal display), FONT-OBJECT
must be a terminal ID, a
     {
       CHECK_STRING (string);
       validate_subarray (string, from, to, SCHARS (string), &frompos, &topos);
-      if (! STRING_MULTIBYTE (string))
-       error ("Attempt to shape unibyte text");
+      if (strlen(string) != 0 && ! STRING_MULTIBYTE (string))
+       error ("Attempt to shape unibyte text \"%s\"", string);
       frombyte = string_char_to_byte (string, frompos);
     }
#+END_SRC

* Question
I guess the only question is : what's supposed to happen when =string= is an
empty lisp string in this condition ?

Gerry AGBOBADA