[Date Prev][Date Next] [Thread Prev][Thread Next] [Date Index] [Thread Index]

Perl symbol problem - release critical (Re: Bug#489132)

Raphael Hertzog writes ("Bug#489132: lenny release notes, upgrade dpkg first"):
> To work-around a problem that can happen in the perl 5.10 upgrade (see
> #479711), the perl scripts contained in dpkg (update-alternatives,
> dpkg-divert) have been modified... but for the work-around to be used, the
> new dpkg must obviously be installed first, before the dist-upgrade.

I don't think this is the right solution.  To be honest I'm just
astonished at this situation, which is terrible.  It is the
consequence of a mistake in the Debian Perl policy - a mistake which
has caused trouble on every previous upgrade, too.

Here is a summary of the problem:

  Perl extensions (XS modules) are not compatible across Perl versions
  due to ABI changes.  For this reason Perl upstream put the Perl
  version number in the paths at which modules are installed.  The
  Debian Perl maintainer has decided not to do this.

  As a result, if you try to load a Perl extension from a script when
  the versions of the Perl interpreter (in perl-base) and the module
  (in a different package) are incompatible, you are trying to load a
  library .so with an incompatible ABI.

  As it happens, because of lazy symbol resolution, this is detected
  very late: after Perl thinks it has loaded the module, the runtime
  linker finds a missing symbol and has no option but to kill the

  (If it weren't for that, then loading the library would fail; the
  Essential scripts which are trying to load the module will then fall
  back, so the system would remain functional in a basic way and could

  As a result, it is possible for a situation to arise where Essential
  scripts in the dpkg package (and presumably in other packages) don't
  work, without any of the dependencies having been violated.  Unless
  you're an expert, once your system is in this state you're hosed.

Some observations and opinions:

* This problem is clearly release critical.  I don't think documenting
  a release critical bug of this severity in the release notes is
  acceptable.  Furthermore, the proposed workaround is very cumbersome
  due to the necessary installation ordering.

* The Debian Perl maintainer's decision to remove the Perl version
  number from the module path is clearly wrong.  Here is what upstream
  have to say, and the Debian Perl maintainer's explanation:
  This explanation is basically that `Debian's dependency system
  means that it will work anyway'.  This is
    (a) not a reason to deviate from upstream - at best it is only a
        lack of a reason not to deviate;
    (b) false.
  That it is false can be seen from the fact that a problem like this
  has happened for the last three releases: #158835, #278417,
  and now #479711.  (There may be other reports of course.)

* Suppressing lazy symbol resolution may work in this case, but it is
  not correct.  ABI changes may result in random crashes due to
  different structure sizes and do not necessarily involve missing
  symbols - so the problem may go undetected.  If we think that we
  want to fix it in etch->lenny by suppressing lazy symbol resolution,
  we need to:
    (a) check what the actual ABI differences are and that either
        there aren't any others besides missing symbols, or that
        every module will definitely fail to load
    (b) regard this as a workaround and do something sensible next

* One of the Perl upstream commenters in #479711 suggests that the
  answer is to use a `pre-inst dependency' which apparently none of
  the submitters have realised is what dpkg already has and calls
  Pre-Depends.  However, a Pre-Depends doesn't solve this problem
  because there is no correct order to upgrade the packages:
  regardless of whether you upgrade Perl first, or the modules first,
  something may break.

* The fundamental problem is that there are currently some Perl
  module packages in lenny which whose dependencies are not violated
  by unpacking them into an etch system, but which will break the
  execution of essential packages.  This definitely cannot be fixed
  without changing at least those Perl module packages (because
  an etch system will be willing to install the broken Perl module
  packages right now, and the only thing currently stopping it doing
  so is that lenny isn't released).

Possible solutions that I see for lenny:

1. Reinsert the Perl version number in the Perl module packages.
   This is the correct long-term solution but involves at least
   rebuilding about 300 packages.

2. Find out which modules are used in this way by Essential packages.
   Arrange somehow for those modules to fail at `require' when loaded
   with Perl 5.8 from etch.  This might involve rebuilding only
   those modules.

3. Make the lenny Perl 5.10 package _also_ look in the directory with
   5.10 in the name.  Change the module(s) used by Essential packages
   to put their modules in that directory.  Make the lenny Perl 5.10
   package suppress RTLD_LAZY always.  (Specialisation of the above.)

4. Tell everyone in the release notes that it's hideously broken,
   and give them an error-prone 6-rune recipe for upgrading, which if
   not followed will break their system.  This is as requested in
   #489132.  This is I think hopeless.  What is the point of Debian if
   it can't manage to get an upgrade right ?

Perhaps someone has some other ideas.


Reply to: