Emacs crashes when using Ediff

[Frederik Fouvry <fouvry@CoLi.Uni-SB.DE>]

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In GNU Emacs 21.3.1 (i686-pc-linux-gnu, X toolkit, Xaw3d scroll bars)
 of 2004-02-19 on cc.at.coli.uni-sb.de
configured using `configure  --prefix=/export/black --with-xpm --with-jpeg --with-tiff --with-gif --with-png --with-x --with-gcc --with-pop --with-sound'
Important settings:
  value of $LC_ALL: nil
  value of $LC_COLLATE: POSIX
  value of $LC_CTYPE: nil
  value of $LC_MESSAGES: nil
  value of $LC_MONETARY: de_DE@euro
  value of $LC_NUMERIC: nil
  value of $LC_TIME: nil
  value of $LANG: en_GB
  locale-coding-system: iso-latin-1
  default-enable-multibyte-characters: t

Please describe exactly what actions triggered the bug
and the precise symptoms of the bug:

I have been merging two directories with Tools > Merge > Directories ...
It happens quite frequently (roughly two to three times an hour)
that Emacs crashes during this operation.  I cannot tell
if it only happens on a specific action or not.

I have attached gdb to it, and this is the output:

[Initialisation and loading messages were removed.]

Program received signal SIGSEGV, Segmentation fault.
0x08113d24 in mem_delete_fixup (x=0x8264780) at alloc.c:3076
3076    alloc.c: No such file or directory.
        in alloc.c
(gdb) bt
#0  0x08113d24 in mem_delete_fixup (x=0x8264780) at alloc.c:3076
#1  0x08113cc8 in mem_delete (z=0x8f19090) at alloc.c:3012
#2  0x08111fde in lisp_free (block=0x918d888) at alloc.c:632
#3  0x0811572b in gc_sweep () at alloc.c:5270
#4  0x081144a4 in Fgarbage_collect () at alloc.c:4194
#5  0x08126f48 in Ffuncall (nargs=2, args=0xbfffd750) at eval.c:2599
#6  0x0814e2c4 in Fbyte_code (bytestr=147373852, vector=1, 
    maxdepth=-1073752240) at bytecode.c:716
#7  0x08127164 in funcall_lambda (fun=1224132872, nargs=3, 
    arg_vector=0xbfffd864) at eval.c:2851
#8  0x08126d82 in Ffuncall (nargs=4, args=0xbfffd860) at eval.c:2716
#9  0x0814e2c4 in Fbyte_code (bytestr=136929868, vector=3, 
    maxdepth=-1073751968) at bytecode.c:716
#10 0x08127164 in funcall_lambda (fun=1224859392, nargs=3, 
    arg_vector=0xbfffd984) at eval.c:2851
#11 0x08126d82 in Ffuncall (nargs=4, args=0xbfffd980) at eval.c:2716
#12 0x0814e2c4 in Fbyte_code (bytestr=151138556, vector=3, 
    maxdepth=-1073751680) at bytecode.c:716
#13 0x08127164 in funcall_lambda (fun=1224763112, nargs=4, 
    arg_vector=0xbfffdab4) at eval.c:2851
#14 0x08126d82 in Ffuncall (nargs=5, args=0xbfffdab0) at eval.c:2716
#15 0x0814e2c4 in Fbyte_code (bytestr=151137716, vector=4, 
    maxdepth=-1073751376) at bytecode.c:716
---Type <return> to continue, or q <return> to quit--- 
#16 0x08127164 in funcall_lambda (fun=1224335896, nargs=2, 
    arg_vector=0xbfffdbe4) at eval.c:2851
#17 0x08126d82 in Ffuncall (nargs=3, args=0xbfffdbe0) at eval.c:2716
#18 0x0814e2c4 in Fbyte_code (bytestr=147376460, vector=2, 
    maxdepth=-1073751072) at bytecode.c:716
#19 0x08127164 in funcall_lambda (fun=1224336120, nargs=1, 
    arg_vector=0xbfffdd04) at eval.c:2851
#20 0x08126d82 in Ffuncall (nargs=2, args=0xbfffdd00) at eval.c:2716
#21 0x0814e2c4 in Fbyte_code (bytestr=147430620, vector=1, 
    maxdepth=-1073750784) at bytecode.c:716
#22 0x08127164 in funcall_lambda (fun=1225088952, nargs=1, 
    arg_vector=0xbfffde24) at eval.c:2851
#23 0x08126d82 in Ffuncall (nargs=2, args=0xbfffde20) at eval.c:2716
#24 0x0814e2c4 in Fbyte_code (bytestr=146498508, vector=1, 
    maxdepth=-1073750496) at bytecode.c:716
#25 0x08127164 in funcall_lambda (fun=1224501712, nargs=0, 
    arg_vector=0xbfffdf44) at eval.c:2851
#26 0x08126d82 in Ffuncall (nargs=1, args=0xbfffdf40) at eval.c:2716
#27 0x0814e2c4 in Fbyte_code (bytestr=147299372, vector=0, 
    maxdepth=-1073750208) at bytecode.c:716
#28 0x08127164 in funcall_lambda (fun=1225024632, nargs=2, 
    arg_vector=0xbfffe064) at eval.c:2851
#29 0x08126d82 in Ffuncall (nargs=3, args=0xbfffe060) at eval.c:2716
---Type <return> to continue, or q <return> to quit---
#30 0x0814e2c4 in Fbyte_code (bytestr=146595700, vector=2, 
    maxdepth=-1073749920) at bytecode.c:716
#31 0x08127164 in funcall_lambda (fun=1225606472, nargs=1, 
    arg_vector=0xbfffe1a4) at eval.c:2851
#32 0x08126d82 in Ffuncall (nargs=2, args=0xbfffe1a0) at eval.c:2716
#33 0x08122e7b in Fcall_interactively (function=419288668, 
    record_flag=405215452, keys=1210580568) at callint.c:797
#34 0x080dbe3e in Fcommand_execute (cmd=-1728194980, record_flag=405215452, 
    keys=405215452, special=405215452) at keyboard.c:9250
#35 0x080d222e in command_loop_1 () at keyboard.c:1661
#36 0x081252fe in internal_condition_case (bfun=0x80d1eb0 <command_loop_1>, 
    handlers=405311796, hfun=0x80d1ad8 <cmd_error>) at eval.c:1267
#37 0x080d1d8a in command_loop_2 () at keyboard.c:1245
#38 0x08124eb9 in internal_catch (tag=151791368, 
    func=0x80d1d6c <command_loop_2>, arg=405215452) at eval.c:1030
#39 0x080d1d40 in command_loop () at keyboard.c:1224
#40 0x080d18a8 in recursive_edit_1 () at keyboard.c:950
#41 0x080d19c4 in Frecursive_edit () at keyboard.c:1006
#42 0x080d0457 in main (argc=1, argv=0xbfffe944, envp=0xbfffe94c)
    at emacs.c:1547
#43 0x42015967 in __libc_start_main () from /lib/i686/libc.so.6
(gdb) 


Another problem with Edit/merge is that Emacs gives an error
"trying to set a constant symbol" on moving to the next area (or
a message to that effect).  When going back and immediately
return to the same position (i.e. typing `p' `n') the message is
not repeated.

Recent input:
<down> <down> <down> <down> <down> <down> <right> <right> 
<right> <right> <right> <right> <return> <down> <down> 
<down> <down> <down> <down> <down> <up> <up> <up> <up> 
<down> <down> l <down> <left> <left> <return> <down> 
<down> <down> <down> <down> <down> <down> <down> <down> 
<down> <down> <down> <down> <down> <down> <down> <down> 
<down> <down> <down> <down> <down> <down> <down> <down> 
<down> <down> <down> <down> <down> n <down> <next> 
<down> <down> <down> <down> <down> <down> <down> <down> 
<down> <down> <down> <down> <down> <down> <down> <down> 
<down> <down> <down> <down> <down> <down> <down> <f8> 
C-g <help-echo> <help-echo> <help-echo> <help-echo> 
<help-echo> <help-echo> <help-echo> <help-echo> <menu-bar> 
<help-menu> <report-emacs-bug>

Recent messages:
Mark set
Loading emacsbug...done
byte-code: Quit
Composing main Info directory...
Mark set
Composing main Info directory...done
unzipping emacs.gz...done
unzipping emacs-1.gz...done
Mark saved where search started
unzipping emacs-27.gz...done
keyboard-quit: Quit