rpm/doc/manual/tsort

/*! \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.

*/