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