/*! \page dependencies Dependencies

Dependencies provide a way for a package builder to require other

packages or capabilities to be installed before or simultaneously

with one another. These can be used to require a python interpreter

for a python based application for example. RPM ensures dependencies

are satisfied whenever packages are installed, erased, or upgraded.

\section dependencies_package Requiring Packages

To require the packages python and perl, use:

\verbatim

	Requires: python perl

\endverbatim

in the spec file. Note that "Requires python, perl" would work as well. If you

needed to have a very recent version of python but any version of perl,

\verbatim

	Requires: python >= 1.3, perl

\endverbatim

would do the trick. Again, the ',' in the line is optional.  Instead of

'>=', you may also use '<', '>', '<=', or '='.  Spaces are required

around the numeric operator to separate the operator from the package name.

The full syntax for specifying a dependency on an epoch, version and release

is

\verbatim

	[epoch:]version[-release]

\endverbatim

where

\verbatim

	epoch	(optional) number, with assumed default of 0 if not supplied

	version	(required) can contain any character except '-'

	release	(optional) can contain any character except '-'

\endverbatim

For example,

\verbatim

	Requires: perl >= 9:5.00502-3

\endverbatim

specifies

\verbatim

	epoch=9

	version=5.00502

	release=3

\endverbatim

The epoch (if present) is a monotonically increasing integer, neither the

version or the release can contain the '-' hyphen character, and the dependency

parser does not permit white space within a definition.  Unspecified epoch

and releases are assumed to be zero, and are interpreted as "providing all"

or "requiring any" value.

The release tag is usually incremented every time a package is rebuilt for

any reason, even if the source code does not change. For example, changes

to the specfile, compiler(s) used to build the package, and/or dependency

changes should all be tracked by incrementing the release.  The version number,

on the other hand, is usually set by the developer or upstream maintainer,

and should not be casually modified by the packager.

Version numbering should be kept simple so that it is easy to determine the

version ordering for any set of packages.  If the packager needs to separate

a release from all other releases that came before it, then the epoch, the

most significant part of package ordering, can be changed.

The algorithm that RPM uses to determine the version ordering of

packages is simple and developers are encouraged not to rely on the

details of its working.  Developers should keep their numbering scheme

simple so any reasonable ordering algorithm would work.  The version

comparison algorithm is in the routine rpmvercmp() and it is just a segmented

strcmp(3).  First, the boundaries of the segments are found using

isdigit(3)/isalpha(3).  Each segment is then compared in order with the

right most segment being the least significant.  The alphabetical

portions are compared using a lexical graphical ascii ordering, the

digit segments strip leading zeroes and compare the strlen before

doing a strcmp. If both numerical strings are equal, the longer string

is larger.  Notice that the algorithm has no knowledge of decimal fractions,

and perl-5.6 is "older" than perl-5.00503 because the number 6 is less than

the number 503.

The concept of "newer" used by rpm to determine when a package should be

upgraded can be broken if version format changes oddly, such as when the

version segments cannot be meaningfully compared.

Example of a bad format change: 2.1.7Ax to 19980531

\verbatim

  The date may be the older version, but it is numerically greater

  2 so it is considered newer :(

\endverbatim

Example of a bad increment: 2.1.7a to 2.1.7A

\verbatim

  The 'a' (ASCII 97) is compared against 'A' (ASCII 65), making 2.1.7a

  the newer version.

\endverbatim

Stick to major.minor.patchlevel using numbers for each if you can.

Keeps life simple :-)

If a Requires: line needs to include an epoch in the comparison, then

the line should be written like

\verbatim

	Requires: somepackage = 23:version

\endverbatim

You can't continue a "Requires: " line. If you have multiple

"Requires: " lines then the package requires all packages mentioned on

all of the lines to be installed.

\section dependencies_prereqs Prereqs

Prereqs are different from requires only in that a PreReq is guaranteed

to be installed before the package that contains the PreReq.  PreReq's

are used only to order packages, otherwise PreReq's are exactly the same

as a Requires: dependency.

\section dependencies_virtual Virtual Packages

Sometimes you need to make sure the system your package is being installed 

on has a package which provides a certain capability, even though you don't

care what specific package provides it. For example, sendmail won't work

properly unless a local delivery agent (lda) is present. You can ensure that

one is installed like this:

\verbatim

	Requires: lda

\endverbatim

This will match either a package called lda (as mentioned above), or any

package which contains:

\verbatim

	Provides: lda

\endverbatim

in its .spec file. No version numbers may be used with virtual packages.

Virtual packages are often used to supply file dependencies such as /bin/sh

on machines that are only partly managed by rpm. A virtual package with

\verbatim

	Provides: /bin/sh

\endverbatim

differs from a package that has /bin/sh in the %files list in that the

package can be safely removed without removing /bin/sh.

\section dependencies_automatic Automatic Dependencies

To reduce the amount of work required by the package builder, RPM scans

the file list of a package when it is being built. Any files in the file

list which require shared libraries to work (as determined by ldd) cause

that package to require the shared library.

For example, if your package contains /bin/vi, RPM will add dependencies 

for both libtermcap.so.2 and libc.so.5. These are treated as virtual

packages, so no version numbers are used.

A similar process allows RPM to add Provides information automatically. Any

shared library in the file list is examined for its soname (the part of

the name which must match for two shared libraries to be considered

equivalent) and that soname is automatically provided by the package. For

example, the libc-5.3.12 package has provides information added for

libm.so.5 and libc.so.5. We expect this automatic dependency generation

to eliminate the need for most packages to use explicit Requires: lines.

\section dependencies_custom Custom Automatic Dependency

The automatic dependency programs are found via macro expansion.  Thus

sites can very the amount of dependency processing that are performed

locally, by changing the executable/script which is run.  Dependency

processing can even be changed on a per-package basis if the macros are

defined in the spec file. To allow for maximum configurability the

dependency programs are shell scripts which can be duplicated and edited

for site specific needs.

The macros: %__find_provides, %__find_prereq, %__find_requires,

%__find_conflicts, %__find_obsoletes, if they exist, are expanded to

the name of a program to exec. For each package, the program receives

the glob'ed %files manifest on stdin and returns dependencies on stdout. The

discovered dependencies are parsed exactly as if they were found after

\verbatim

	Provides:

	PreReq:

	Requires:

	Conflicts:

	Obsoletes:

\endverbatim

tokens in a spec file (i.e. the same parser is used), so items look like

(comments added)

\verbatim

	/bin/sh			# file existence

	libc.so.6		# soname existence

	foo <= 1:2.3-4		# versioned package

	perl5(Apache) <= 1.2	# versioned namespace

\endverbatim

The default rpm configuration has only

	%__find_provides	/usr/lib/rpm/find-provides

	%__find_requires	/usr/lib/rpm/find-requires

which can be overridden (or even undefined) within a spec file.

\section dependencies_interpreters Interpreters and Shells

Modules for interpreted languages like perl and tcl impose additional

dependency requirements on packages. A script written for an interpreter

often requires language specific modules to be installed in order to execute

correctly. In order to automatically detect language specific modules, each

interpreter may have its own find-provides and find-requires. To prevent

module name collisions between interpreters, module names are enclosed within

parentheses and a conventional interpreter specific identifier is prepended:

\verbatim

  Provides: perl(MIME-Base64), perl(Mail-Header)-1-09

  Requires: perl(Carp), perl(IO-Wrap) = 4.5

\endverbatim

The output of a per-interpreter find-requires (notice in this example the

first requirement is a package and the rest are language specific modules)

\verbatim

	Mail-Header >= 1.01

	perl(Carp) >= 3.2

	perl(IO-Wrap) == 4.5 or perl(IO-Wrap)-4.5

\endverbatim

the output from find-provides is

\verbatim

	Foo-0.9

	perl(Widget)-0-1

\endverbatim

The per-interpreter automatic module detectors will normally be located in

\verbatim

	/usr/lib/rpm/{perl,tcl}/find-{provides,requires}

with appropriate per-interpreter hooks into

\verbatim

	/usr/lib/rpm/find-{provides,requires}

\endverbatim

@todo per-interpreter dependency generators are not located in subdirectories.

Notice that shell dependencies will require that all %post et al scriptlets

be processed by the find-requires. Since a shell script depends on all the

programs which it runs.

\section dependencies_installing Installing and Erasing Packages with Dependencies

For the most part, dependencies should be transparent to the user. However,

a few things will change.

First, when packages are added or upgraded, all of their dependencies

must be satisfied. If they are not, an error message like this appears:

\verbatim

    failed dependencies:

	    libICE.so.6  is needed by somepackage-2.11-1

	    libSM.so.6  is needed by somepackage-2.11-1

	    libc.so.5  is needed by somepackage-2.11-1

\endverbatim

Similarly, when packages are removed, a check is made to ensure that 

no installed packages will have their dependency conditions break due to

the packages being removed. If you wish to turn off dependency checking for 

a particular command, use the --nodeps flag.

\section dependencies_conflicts Conflicts

While conflicts were implemented in earlier versions of RPM they never 

worked properly until RPM 2.3.4 (well, we hope they work properly now

anyway).

Conflicts allow a package to say it won't work with another package (or

virtual package) installed on the system. For example, qmail doesn't work

(w/o custom setup) on machines with sendmail installed. The qmail spec file

may codify this with a line like:

\verbatim

	Conflicts: sendmail

\endverbatim

The syntax of the "Conflicts" tag is identical to the syntax of the Requires

tag and conflict checking may be overridden by using the --nodeps flag.

\section dependencies_querying Querying for Dependencies

Two new query information selection options are now available. The first, 

--provides, prints a list of all of the capabilities a package provides. 

The second, --requires, shows the other packages that a package requires

to be installed, along with any version number checking.

There are also two new ways to search for packages. Running a query with 

--whatrequires \<item\> queries all of the packages that require \<item\>. 

Similarly, running --whatprovides \<item\> queries all of the packages that 

provide the \<item\> virtual package. Note that querying for package that 

provides "python" will not return anything, as python is a package, not

a virtual package.

\section dependencies_verifying Verifying Dependencies

As of RPM 2.2.2, -V (aka --verify) verifies package dependencies

by default. You can tell rpm to ignore dependencies during system

verification with the --nodeps. If you want RPM to verify just dependencies

and not file attributes (including file existence), use the --nofiles

flag. Note that "rpm -Va --nofiles --nodeps" will not verify anything at

all, nor generate an error message.

\section dependencies_branching Branching Version

It is quite common to need to branch a set of sources in version

control. It is not so obvious how those branches should be represented

in the package version numbers. Here is one solution.

You have a bag of features that are injected into a package in a

non-ordered fashion, and you want to have the package

name-version-release be able to:

\verbatim

	1) identify the "root version" of the source code.

	2) identify the handful of features that are in that

	   branch of the package.

	3) preserve sufficient ordering so that packages upgrade

	   without the use of --oldpackage.

\endverbatim

A simple (but possibly not adequate) scheme to achieve this is:

\verbatim

	Name: foo

	Version: <the "root version" of the source code>

	Release: <release instance>.<branch>

\endverbatim

where the release instance is something like YYYYMMDD or some linear

record of the number of builds with the current tar file, it is used

to preserve ordering when necessary.

Another alternative scheme might be:

\verbatim

	Name: foo

	Epoch: <branch>

	Version: <the branch specific version of the code>

	Release: <release instance>

\endverbatim

\section dependencies_build Build dependencies

The following dependencies are available at build time.  These are

similar to the install time version but these apply only during

package creation and are specified in the specfile not in the binary

package.

\verbatim

	BuildRequires:

	BuildConflicts:

	BuildPreReq:

\endverbatim

*/