Assertion in slap_listener

While trying to reproduce Ubuntu Bug #921489 on a more modern release (Natty) I seem to have triggered an assertion in slap_listener.

The test I am using is four threads of

while true; do ldapsearch -Y GSSAPI -H ldap://ldap.example.com uid=user > /dev/null ; if [ $? -ne 0 ]; then sleep 2; fi ; done

within three minutes the server fails printing the following message

slapd: /build/buildd/openldap-2.4.23/servers/slapd/daemon.c:1846: slap_listener: Assertion `((slap_daemon.sd_index[(sfd)]) == -1)' failed.

gdb output:

Core was generated by `/usr/sbin/slapd -d 0 -h ldap:/// ldaps:/// -f /local/sukat/conf/slapd.conf'.
Program terminated with signal 6, Aborted.
#0 0x00007f4872755d05 in raise (sig=6) at ../nptl/sysdeps/unix/sysv/linux/raise.c:64
64 ../nptl/sysdeps/unix/sysv/linux/raise.c: No such file or directory.
        in ../nptl/sysdeps/unix/sysv/linux/raise.c
(gdb) bt
#0 0x00007f4872755d05 in raise (sig=6) at ../nptl/sysdeps/unix/sysv/linux/raise.c:64
#1 0x00007f4872759ab6 in abort () at abort.c:92
#2 0x00007f487274e7c5 in __assert_fail (assertion=0x7f4874de9290 "((slap_daemon.sd_index[(sfd)]) == -1)", file=<value optimised out>, line=1846,
    function=<value optimised out>) at assert.c:81
#3 0x00007f4874d402b3 in slap_listener (sl=0x7f48757b42a0) at /build/buildd/openldap-2.4.23/servers/slapd/daemon.c:1846
#4 0x00007f4874d402d9 in slap_listener_thread (ctx=<value optimised out>, ptr=0x7f48757b42a0) at /build/buildd/openldap-2.4.23/servers/slapd/daemon.c:2040
#5 0x00007f48748a92da in ldap_int_thread_pool_wrapper (xpool=0x7f48757f0a30) at /build/buildd/openldap-2.4.23/libraries/libldap_r/tpool.c:685
#6 0x00007f4872abcd8c in start_thread (arg=0x7f4755ee2700) at pthread_create.c:304
#7 0x00007f487280804d in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:112
#8 0x0000000000000000 in ?? ()

Architecture: amd64
Date: Fri Jan 27 14:54:13 2012
Dependencies:
  adduser 3.112+nmu1ubuntu5
  coreutils 8.5-1ubuntu6
  debconf 1.5.36ubuntu4
  debconf-i18n 1.5.36ubuntu4
  debianutils 3.4.3ubuntu1
  dpkg 1.16.0~ubuntu7.1 [modified: sbin/start-stop-daemon]
  gcc-4.5-base 4.5.2-8ubuntu4
  libacl1 2.2.49-4ubuntu2
  libattr1 1:2.4.44-2ubuntu3
  libbz2-1.0 1.0.5-6ubuntu1.11.04.1
  libc-bin 2.13-0ubuntu13
  libc6 2.13-0ubuntu13
  libcomerr2 1.41.14-1ubuntu3
  libdb4.8 4.8.30-5ubuntu2
  libgcc1 1:4.5.2-8ubuntu4
  libgcrypt11 1.4.6-4ubuntu2
  libgdbm3 1.8.3-9ubuntu1
  libgnutls26 2.8.6-1ubuntu2
  libgpg-error0 1.10-0.2ubuntu1
  libgssapi-krb5-2 1.8.3+dfsg-5ubuntu2.2
  libk5crypto3 1.8.3+dfsg-5ubuntu2.2
  libkeyutils1 1.4-4ubuntu2
  libkrb5-3 1.8.3+dfsg-5ubuntu2.2
  libkrb5support0 1.8.3+dfsg-5ubuntu2.2
  libldap-2.4-2 2.4.23-6ubuntu6.1
  liblocale-gettext-perl 1.05-6
  libltdl7 2.2.6b-2ubuntu3
  liblzma2 5.0.0-2
  libncurses5 5.7+20101128-1
  libpam-modules 1.1.2-2ubuntu8.4
  libpam-modules-bin 1.1.2-2ubuntu8.4
  libpam0g 1.1.2-2ubuntu8.4
  libperl5.10 5.10.1-17ubuntu4.1
  libreadline6 6.2-0ubuntu1
  libsasl2-2 2.1.23.dfsg1-5ubuntu3
  libselinux1 2.0.96-1ubuntu2
  libslp1 1.2.1-7.8ubuntu1
  libtasn1-3 2.7-1ubuntu1
  libtext-charwidth-perl 0.04-6
  libtext-iconv-perl 1.7-2
  libtext-wrapi18n-perl 0.06-7
  libwrap0 7.6.q-19ubuntu1
  lsb-base 4.0-0ubuntu11
  multiarch-support 2.13-0ubuntu13
  ncurses-bin 5.7+20101128-1
  odbcinst 2.2.14p2-2ubuntu1
  odbcinst1debian2 2.2.14p2-2ubuntu1
  passwd 1:4.1.4.2+svn3283-3ubuntu1
  perl 5.10.1-17ubuntu4.1
  perl-base 5.10.1-17ubuntu4.1
  perl-modules 5.10.1-17ubuntu4.1
  psmisc 22.13-1
  readline-common 6.2-0ubuntu1
  sed 4.2.1-8
  sensible-utils 0.0.6ubuntu2
  tzdata 2011n-0ubuntu0.11.04
  unixodbc 2.2.14p2-2ubuntu1
  xz-utils 5.0.0-2
  zlib1g 1:1.2.3.4.dfsg-3ubuntu3
DistroRelease: Ubuntu 11.04
Package: slapd 2.4.23-6ubuntu6.1
PackageArchitecture: amd64
ProblemType: Bug
ProcEnviron:
  SHELL=/bin/bash
  PATH=(custom, no user)
  LANG=en_GB.UTF-8
  LANGUAGE=en_GB:en
  LC_CTYPE=en_GB.UTF-8
ProcVersionSignature: Ubuntu 2.6.38-13.54-generic 2.6.38.8
SourcePackage: openldap
Tags: natty
Uname: Linux 2.6.38-13-generic x86_64
UpgradeStatus: No upgrade log present (probably fresh install)