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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.
*/
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