Monday, 27 October 2008

Making relocatable packages with JHBuild

I have been revisiting an experiment I began back in March with building GNOME with JHBuild. I wanted to see if it would be practical to use JHBuild to package all of GNOME with Zero-Install. The main issue with packaging anything with Zero-Install is relocatability.

If you build and install an autotools-based package with

./configure --prefix=/FOO && make install
the resulting files installed under /FOO will often have the pathname /FOO embedded in them, sometimes in text files, other times compiled into libraries and executables. This is a problem for Zero-Install because it runs under a normal user account and wants to install files into a user's home directory under ~/.cache. Currently if a program is to be packaged with Zero-Install it must be relocatable via environment variables. Compiling pathnames in is no good (at least without an environment variable override) because you don't know in advance where the program will be installed.

I found a few cases where pathnames get compiled in:

  • text: pkg-config .pc files
  • text: libtool .la files
  • text: shell scripts generated from .in files, such as gtkdocize, intltoolize and gettextize
  • binary: rpaths added by libtool

It is possible to handle these individual cases. Zero-Install's make-headers tool will fix up pkg-config .pc files. libtool .la files can apparently just be removed on Linux without any adverse effects. libtool could be modified to not use rpaths (unfortunately --disable-rpath doesn't seem to work), which are overridden by LD_LIBRARY_PATH anyway. gtkdocize et al could be modified. But that sounds like a lot of work. I'd like to get something working first.

In revisiting this I hoped that the only cases that would matter would be text files. It would be easy to do a search and replace inside text files to relocate packages. The idea would be to build with

/configure --prefix=/FAKEPREFIX
make install DESTDIR=/tempdest
and then rewrite /FAKEPREFIX to (say) /home/fred/realprefix. In a text file, changing the length of a pathname and changing the size of the file usually doesn't matter, but doing this to an ELF executable would completely screw the executable up. This search-and-replace trick would be a hack, but it would be worth trying.

It turned out that Evince (which I was using as a test case) embeds the pathname /FAKEPREFIX/share/applications/evince.desktop in its own executable, and if this file doesn't exist, it segfaults on startup.

Then it occurred to me that I could rewrite filenames inside binary files without changing the length of the filename: just pad the filenames out to a fixed length at the start.

So the idea now is to build with something like:

/configure --prefix=/home/bob/builddir/gtk-XXXXXXX
make install
and, when installing the files on another machine, rewrite

Just make sure you start off with enough padding to allow the package to be relocated to any path a user is likely to use in practice.

This is even hackier than rewriting filenames inside text files, but it's very simple!

This is partly inspired by Nix, which does something similar, but with a bit more complexity. Nix will install a package under (something like) /nix/store/<hash>, where <hash> is (roughly) a cryptographic hash of the binary package's contents. But packages like Evince contain embedded filenames referring to their own contents, so Nix will build it with:

./configure --prefix=/nix/store/<made-up-hash>
make install
where <made-up-hash> is chosen randomly. Afterwards, the output is rewritten to replace <made-up-hash> with the real hash of the output, but there is some cleverness to discount <made-up-hash> from affecting the real hash.

(Earlier versions of Nix used the hash of the build input to identify packages rather than the hash of the build output. This avoided the need to do rewriting but didn't allow a package's contents to be verified based on its hash name.)

The fact that Nix uses this scheme successfully indicates that filename rewriting in binaries works, and filenames are not being checksummed or compressed or encoded in weird ways, which is good.

My plan now is:

  • Extend JHBuild to build packages into fixed-length prefixes and produce Zero-Install feeds. My work so far is on this Bazaar branch.
  • Extend Zero-Install to do filename rewriting inside files in order to relocate packages.