[syslog-ng] Quick question about libsyslog-ng.so.0

[Balazs Scheidler]

On Sun, 2011-08-21 at 12:48 +0200, Gergely Nagy wrote:
> Hi!
> 
> As part of packaging syslog-ng, and splitting it into smaller packages,
> I face the problem of having to create a devel package. Now, I could go
> the easy way (as I've done with the current set of Debian packages) and
> create a syslog-ng-core package, which would contain libsyslog-ng.so.0,
> the plugins and the rest, and a syslog-ng-dev package with the headers
> and other tools.
> 
> This could work, but the downside is that on an automated buildd, one
> does not neccessarily want to replace the running syslogd (even if
> that's already syslog-ng, which it is in my case) just to be able to
> build a package.
> 
> So I was thinking about splitting the libsyslog-ng.so library out into
> its own package. And herein lies the problem! The SONAME of the library
> is currently not set, thus, libtool defaults to libsyslog-ng.so.0. The
> problem with this, is that the SONAME will remain the same, even if the
> library breaks its ABI underneath. Which will be a problem for any third
> party module linked against this library: a new version of syslog-ng
> comes out, all of them will break at runtime, in various interesting
> ways, perhaps even after days of working, when the module runs over an
> incompatible change.
> 
> This can be worked around on the packaging level, by ensuring that a new
> version of the libsyslog-ng package will conflict with any older
> versions, but... that's a pain to maintain, and very fragile.
> 
> So instead, I'd like to propose a different way: use the -release flag
> of Libtool. This will make the library's SONAME contain syslog-ng's
> version aswell, thus, every upgrade will use a different soname (like
> libsyslog-ng-3.3.0beta2.so), and modules will break at startup time,
> which is much better, as we don't trip over unexpected changes.
> 
> It is also tremendously easier to package a semi-private library this
> way. (Though, this has a few downsides as well from a packaging point of
> view, but considerably less.)
> 
> So here I am, seeking input on the issue!
> 
> By the way, a third option also exists: maintaining ABI and API
> compatibility, and only bumping the SONAME where something did really
> change incompatibly. This would be the best option, but this also takes
> a lot more work to maintain properly (I'm happy to help with this,
> though, I'm already doing this for libmongo-client; but on the other
> hand, libsyslog-ng really is something syslog-ng specific, and not a
> general purpose library, so it might not worth the trouble to go this
> far).

I was thinking about this issue just the other day, and I was wondering
if it'd make sense to move libsyslog-ng.so to $prefix/lib/syslog-ng, as
it is more a private library. But this would require setting
LD_LIBRARY_PATH or -rpath in the syslog-ng binary. So this was a no-go.

Also, the .so only needs to exist because of linker pecularities. If we
focused on Linux only, it'd not be needed at all. (symbols could get
resolved through the main program only, but this doesn't work on non-ELF
platforms, and could have problems on some ELF platforms, like Solaris).

-release would help, since even if it is included in $prefix/lib, it'd
clearly be visible that it is release specific.

However the same problem applies to modules too: there's only a single
syslog-ng plugin directory, and if some plugins are compiled against a
different core, they'd get into the same dir.

What about doing this:

  * libsyslog-ng.so is linked with -release $VERSION
  * the module directory (plus search path) is set to
    $prefix/lib/syslog-ng/$VERSION

This way, the set of modules that would actually work would be cleanly
separated.

Except for the -release change, the module directory could be used this
way by merely specifying options for the configure script by
"debian/rules". 

I would like to overwrite binaries if one compiles from source (except
maybe libsyslog-ng.so).

-- 
Bazsi