bug#70989: 29.3; Calling isearch-forward when using Unifont throws error "Invalid font name"

[Rodrigo Morales]

Po Lu <luangruo@yahoo.com> writes:

> Rodrigo, would you please let us know which font driver this font is
> loaded by?  Please type M-x describe-char RET with point on a character
> in the default font, and respond with the output of the Help buffer.

I emptied the file =~/.config/emacs/init.el= and inserted the
following sexp.

#+HEADER: :tangle ~/.config/emacs/init.el
#+BEGIN_SRC elisp
(add-to-list 'default-frame-alist '(font . "Unifont"))
#+END_SRC

I launched emacs without using the =-Q= flag so that
=~/.config/emacs/init.el= is read. I switched to the buffer
=*scratch*= buffer because the buffer =*GNU Emacs*= used other
fonts. See 
[[http://web.archive.org/web/20240517150825/http://0x0.st/XPqZ.png][this 
screenshot]] (note that modeline uses =Unifont= but the
buffer =*GNU Emacs*= uses other fonts). The =*scratch*= buffer used
Unifont.

Here's the content of =*Help*= buffer which was opened when I run =M-x
describe-char= in the =;= which is shown as the first character in the
=*scratch*= buffer.

#+BEGIN_SRC text
             position: 1 of 145 (0%), column: 0
            character: ; (displayed as ;) (codepoint 59, #o73, #x3b)
              charset: ascii (ASCII (ISO646 IRV))
code point in charset: 0x3B
               script: latin
               syntax: <        which means: comment
             category: .:Base, a:ASCII, l:Latin, r:Roman
             to input: type "C-x 8 RET 3b" or "C-x 8 RET SEMICOLON"
          buffer code: #x3B
            file code: #x3B (encoded by coding system utf-8-unix)
              display: by this font (glyph code):
    
ftcrhb:-GNU-Unifont-medium-normal-normal-Sans-Serif-16-*-*-*-c-80-iso10646-1 
(#x3C)

Character code properties: customize what to show
  name: SEMICOLON
  general-category: Po (Punctuation, Other)
  decomposition: (59) (';')

There are text properties here:
  face                 font-lock-comment-delimiter-face
  fontified            t

[back]
#+END_SRC

Feel free to ask for more information if needed.