[Rd] X11/view bug introduced in r90379

[Ben Bolker]

  r90379 (ripley, 2026-08-08, "avoid [-Wunused-but-set-global] warnings 
from clang 23") causes View() to crash, trivially, on my machine with 
(partial) sessionInfo():

R Under development (unstable) (2026-09-11 r90524)
Platform: x86_64-pc-linux-gnu
Running under: Pop!_OS 22.04 LTS

locale:
  [1] LC_CTYPE=en_CA.UTF-8       LC_NUMERIC=C

   The patch below fixes this for me, although I can't say if it is the 
best solution/has no other side effects ...

   I can submit this as a bug report if that's preferred.

   cheers
     Ben Bolker

----

    From a fresh install/R session:

> View(mtcars)

  *** caught segfault ***
address (nil), cause 'memory not mapped'

Traceback:
  1: View(mtcars)

Possible actions:
1: abort (with core dump, if enabled)
2: normal R exit
3: exit R without saving workspace
4: exit R saving workspace

   Claude's diagnosis and patch (lightly edited by me):

The commit commented out the line in src/modules/X11/dataentry.c that 
fills in the fs_list font list. It was treated as dead code because its 
return value, font_set_cnt, was never used. But the next line still 
reads fs_list[0]:

// src/modules/X11/dataentry.c:1966-1971 (at r90590)
       XFontSetExtents *extent = XExtentsOfFontSet(font_set);
//      char **ml;
       ...
//    font_set_cnt = XFontsOfFontSet(font_set, &fs_list, &ml);
       DE->text_offset = 2 + fs_list[0]->max_bounds.descent;

fs_list is a static pointer that is never assigned now, so it is NULL. 
fs_list[0] then dereferences a null pointer and R crashes. This code 
only runs in a multibyte locale (if(mbcslocale)), and your session uses 
LANG=en_CA.UTF-8, so any View() of a data frame hits it.

Fix: put back the call but ignore its return value, which still avoids 
the compiler warning:

       char **ml;
       ...
       XFontsOfFontSet(font_set, &fs_list, &ml);
       DE->text_offset = 2 + fs_list[0]->max_bounds.descent;

Index: src/modules/X11/dataentry.c
===================================================================
--- src/modules/X11/dataentry.c	(revision 90590)
+++ src/modules/X11/dataentry.c	(working copy)
@@ -1964,10 +1964,10 @@
      DE->box_w = twidth + 4;
      if(mbcslocale) {
  	XFontSetExtents *extent = XExtentsOfFontSet(font_set);
-//      char **ml;
+	char **ml;
  	DE->box_h = (extent->max_logical_extent.height)
  	    + (extent->max_logical_extent.height / 5) + 4;
-//	font_set_cnt = XFontsOfFontSet(font_set, &fs_list, &ml);
+	XFontsOfFontSet(font_set, &fs_list, &ml);
  	DE->text_offset = 2 + fs_list[0]->max_bounds.descent;
      } else {
  	DE->box_h = DE->font_info->max_bounds.ascent