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/*! \page tsort Package ordering in rpm-4.0.1 and later The package ordering algorithm in rpm-4.0.1 has changed. \section tsort_problem The Problem Here's a simple test to illustrate the need for the change (from bugzilla #12327): Assume the minimal 7.0 package manifest in /tmp/list \verbatim /mnt/redhat/comps/dist/7.0/sparc/bash-2.04-11.sparc.rpm /mnt/redhat/comps/dist/7.0.2/sparc/glibc-2.1.94-1.sparc.rpm /mnt/redhat/comps/dist/7.0/sparc/mktemp-1.5-5.sparc.rpm /mnt/redhat/comps/dist/7.0/noarch/basesystem-7.0-2.noarch.rpm /mnt/redhat/comps/dist/7.0/noarch/setup-2.3.4-1.noarch.rpm /mnt/redhat/comps/dist/7.0/noarch/filesystem-2.0.7-1.noarch.rpm /mnt/redhat/comps/dist/7.0/sparc/libtermcap-2.0.8-25.sparc.rpm /mnt/redhat/comps/dist/7.0/noarch/termcap-11.0.1-3.noarch.rpm \endverbatim with database initialization as \verbatim mkdir -p /tmp/ROOT/var/lib/rpm rpm --initdb /tmp/ROOT/var/lib/rpm \endverbatim This command "works" \verbatim rpm -Uvh -r /tmp/ROOT `cat /tmp/list` \endverbatim while this command \verbatim rpm -Uvh -r /tmp/ROOT `tac /tmp/list` \endverbatim fails with \verbatim loop in prerequisite chain: libtermcap bash libtermcap \endverbatim \note The 2nd upgrade reverse orders the packages in the manifest. The problem is that the previous ordering algorithm, basically a very clever implementation of tsort, was sensitive to initial conditions, and the first command "happens" to snip a loop, while the second does not. \section tsort_solution The Solution The current ordering algorithm is exactly tsort from Knuth V1, with one further twist. Since the only way out of a dependency loop is to snip the loop somewhere, rpm uses hints from Requires: dependencies to distinguish co-requisite (these are not needed to install, only to use, a package) from pre-requisite (these are guaranteed to be installed before the package that includes the dependency) relations. There is now syntax in spec files to explicitly specify the source of a Requires: dependency. If, for example, you use grep in %post, then you as a packager would normally add \verbatim PreReq: grep \endverbatim in order to insure that grep was installed before attempted use by the %postun scriptlet. Now the same dependency can be expressed more precisely as \verbatim Requires(post): grep \endverbatim For completeness, here's the complete set of tokens that may be added to Requires: as in the example above: \verbatim "interp", RPMSENSE_INTERP "prereq", RPMSENSE_PREREQ "preun", RPMSENSE_SCRIPT_PREUN "pre", RPMSENSE_SCRIPT_PRE "postun", RPMSENSE_SCRIPT_POSTUN "post", RPMSENSE_SCRIPT_POST "rpmlib", RPMSENSE_RPMLIB "verify", RPMSENSE_SCRIPT_VERIFY \endverbatim Ditto BuildRequires: \verbatim "prep", RPMSENSE_SCRIPT_PREP "build", RPMSENSE_SCRIPT_BUILD "install", RPMSENSE_SCRIPT_INSTALL "clean", RPMSENSE_SCRIPT_CLEAN \endverbatim but let's not go there (yet). For giggles, you can also do stuff like \verbatim Requires(pre,post): /bin/sh \endverbatim By marking dependencies more precisely, rpm can distinguish between an upgrade context (like the use of grep in %post above) and an installed context (like the autogenerated Requires: in a package that includes a script with #!/bin/sh), and that permits rpm to differentiate pre-requisites from co-requisites while doing package ordering. Here's what cures the libtermcap <-> bash loop: \verbatim Requires(postun): /bin/sh \endverbatim which, since the dependency is clearly not useful or necessary in determining install ordering, is safely ignored. \section tsort_sideeffects Side Effects One of the side effects of changing the package install ordering, is that there are a handful of new loops that are detected. Here's what I found looking at supported Red Hat releases: \verbatim ghostscript-fonts ghostscript /* 7.0 only */ pango-gtkbeta-devel pango-gtkbeta XFree86 Mesa compat-glibc db2 compat-glibc db1 pam initscripts kernel initscripts initscripts sysklogd /* 6.2 */ egcs-c++ libstdc++ /* 6.1 */ pilot-link-devel pilot-link /* 5.2 */ pam pamconfig \endverbatim Why are there new loops? Because tsort is trying to use all of the dependency relations for ordering, while the previous tsort ignored all Requires: from added packages. Except for the "well known" libtermcap <-> bash loop (which is just wrong), all of the other dependencies are simply not needed in an upgrade context to perform package ordering. Please note that all of the known to cause loop dependencies listed above are, for now, explicitly ignored when determining package install ordering. \section tsort_summary Summary So what does this all mean? Basically not much, unless you find yourself trying to specify dependencies amongst a set of packages correctly and happen to create a dependency loop. And, before you start adding the new-fangled syntax to packages, please remember that rpm will almost certainly be auto-generating fine-grained dependencies for %post et al scriptlets pretty soon. Truly, rpm needs to make packaging easier, not provide Yet More Complicated Syntax in spec files. With thanks to Ken Estes for doing the implementation in bash2 that makes it possible to auto-generate scriptlet dependencies, blame me for the long, slow deployment. */