PostgreSQL's libpq5 package got its symbols file somewhere around version 8.4, so the symbols in there were all marked with version "8.4~" or greater. As I'm working on pgapt.debian.net aiming at providing packages for all upstream-supported PG versions (currently 8.3 and up), this made packages incompatible with 8.3's libpq5.

There didn't seem to be a ready program to feed a series of .deb files which would then run dpkg-gensymbols and build a symbols file, so I wrote this shell script:

#!/bin/sh

set -eu

[ -d tmp ] || mkdir tmp

i=1

for pkg in "$@" ; do
    echo "$pkg"
    test -e "$pkg"
    name=$(dpkg-deb -I "$pkg" | perl -lne 'print $1 if /^ Package: (.+)/')
    version=$(dpkg-deb -I "$pkg" | perl -lne 'print $1 if /^ Version: (.+)/')
    out=$(printf "tmp/%03d_%s" $i "$version")
    dpkg-deb -x "$pkg" "$out"
    dpkg-gensymbols -P"$out" -p"$name" -v"$version" \
        ${oldsymbols:+-I"$oldsymbols"} -O"$out.symbols" | \
        tee "$out.symbols.diff"
    test -s "$out.symbols.diff" || rm "$out.symbols.diff"
    oldsymbols="$out.symbols"
    rm -rf "$out"
    i=$(expr $i + 1)
done

To use it, do the following:

• debsnap -a i386 libpq5
• ls binary-libpq5/*.deb > files
• edit "files" to have proper ordering (~rc versions before releases, remove bpo versions, etc.)
• ./walk-symbols $(cat files)

The highest-numbered *.symbols file in tmp/ will then have symbol information for all packages. I then did some manual post-processing like s/~rc1-1/~/ to get nice (and backportable) version numbers.

Another nice trick (pointed out by jcristau) is to replace the lowest version number of that package (8.2~ here, where libpq changed SONAME) by 0 which will make dpkg-shlibdeps omit the >= version part. (Most packages depending on libpq5 will profit from that.)

I'm still pondering whether this script is non-trivial enough add to devscripts. (The people I asked so far only made comments about the mkdir call...)