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diff --git a/doc/dpkg-tech.dbk b/doc/dpkg-tech.dbk new file mode 100644 index 0000000..62c5879 --- /dev/null +++ b/doc/dpkg-tech.dbk @@ -0,0 +1,870 @@ +<?xml version="1.0" encoding="UTF-8"?> +<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN" + "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd" [ +<!ENTITY % aptent SYSTEM "apt.ent"> %aptent; +<!ENTITY % aptverbatiment SYSTEM "apt-verbatim.ent"> %aptverbatiment; +<!ENTITY % aptvendor SYSTEM "apt-vendor.ent"> %aptvendor; +]> + +<book lang="en"> + +<title>dpkg technical manual</title> + +<bookinfo> + +<authorgroup> + <author> + <personname>Tom Lees</personname><email>tom@lpsg.demon.co.uk</email> + </author> +</authorgroup> + +<releaseinfo>Version &apt-product-version;</releaseinfo> + +<abstract> +<para> +This document describes the minimum necessary workings for the APT dselect +replacement. It gives an overall specification of what its external interface +must look like for compatibility, and also gives details of some internal +quirks. +</para> +</abstract> + +<copyright><year>1997</year><holder>Tom Lees</holder></copyright> + +<legalnotice> +<title>License Notice</title> +<para> +APT and this document are free software; you can redistribute them and/or +modify them under the terms of the GNU General Public License as published by +the Free Software Foundation; either version 2 of the License, or (at your +option) any later version. +</para> +<para> +For more details, on Debian systems, see the file +/usr/share/common-licenses/GPL for the full license. +</para> +</legalnotice> + +</bookinfo> + +<chapter id="ch1"><title>Quick summary of dpkg's external interface</title> + +<section id="control"><title>Control files</title> +<para> +The basic dpkg package control file supports the following major features:- +</para> +<itemizedlist> +<listitem> +<para> +5 types of dependencies:- +</para> +<itemizedlist> +<listitem> +<para> +Pre-Depends, which must be satisfied before a package may be unpacked +</para> +</listitem> +<listitem> +<para> +Depends, which must be satisfied before a package may be configured +</para> +</listitem> +<listitem> +<para> +Recommends, to specify a package which if not installed may severely limit the +usefulness of the package +</para> +</listitem> +<listitem> +<para> +Suggests, to specify a package which may increase the productivity of the +package +</para> +</listitem> +<listitem> +<para> +Conflicts, to specify a package which must NOT be installed in order for the +package to be configured +</para> +</listitem> +<listitem> +<para> +Breaks, to specify a package which is broken by the package and which should +therefore not be configured while broken +</para> +</listitem> +</itemizedlist> +<para> +Each of these dependencies can specify a version and a dependency on that +version, for example "<= 0.5-1", "== 2.7.2-1", etc. The comparators +available are:- +</para> +<itemizedlist> +<listitem> +<para> +"<<" - less than +</para> +</listitem> +<listitem> +<para> +"<=" - less than or equal to +</para> +</listitem> +<listitem> +<para> +">>" - greater than +</para> +</listitem> +<listitem> +<para> +">=" - greater than or equal to +</para> +</listitem> +<listitem> +<para> +"==" - equal to +</para> +</listitem> +</itemizedlist> +</listitem> +<listitem> +<para> +The concept of "virtual packages", which many other packages may provide, +using the Provides mechanism. An example of this is the "httpd" virtual +package, which all web servers should provide. Virtual package names may be +used in dependency headers. However, current policy is that virtual packages +do not support version numbers, so dependencies on virtual packages with +versions will always fail. +</para> +</listitem> +<listitem> +<para> +Several other control fields, such as Package, Version, Description, Section, +Priority, etc., which are mainly for classification purposes. The package +name must consist entirely of lowercase characters, plus the characters '+', +'-', and '.'. Fields can extend across multiple lines - on the second and +subsequent lines, there is a space at the beginning instead of a field name +and a ':'. Empty lines must consist of the text " .", which will be ignored, +as will the initial space for other continuation lines. This feature is +usually only used in the Description field. +</para> +</listitem> +</itemizedlist> +</section> + +<section id="s1.2"><title>The dpkg status area</title> +<para> +The "dpkg status area" is the term used to refer to the directory where dpkg +keeps its various status files (GNU would have you call it the dpkg shared +state directory). This is always, on Debian systems, /var/lib/dpkg. However, +the default directory name should not be hard-coded, but #define'd, so that +alteration is possible (it is available via configure in dpkg 1.4.0.9 and +above). Of course, in a library, code should be allowed to override the +default directory, but the default should be part of the library (so that +the user may change the dpkg admin dir simply by replacing the library). +</para> +<para> +Dpkg keeps a variety of files in its status area. These are discussed later +on in this document, but a quick summary of the files is here:- +</para> +<itemizedlist> +<listitem> +<para> +available - this file contains a concatenation of control information from all +the packages which dpkg knows about. This is updated using the dpkg commands +"--update-avail <file>", "--merge-avail <file>", and +"--clear-avail". +</para> +</listitem> +<listitem> +<para> +status - this file contains information on the following things for every +package:- +</para> +<itemizedlist> +<listitem> +<para> +Whether it is installed, not installed, unpacked, removed, failed +configuration, or half-installed (deconfigured in favour of another package). +</para> +</listitem> +<listitem> +<para> +Whether it is selected as install, hold, remove, or purge. +</para> +</listitem> +<listitem> +<para> +If it is "ok" (no installation problems), or "not-ok". +</para> +</listitem> +<listitem> +<para> +It usually also contains the section and priority (so that dselect may classify +packages not in available) +</para> +</listitem> +<listitem> +<para> +For packages which did not initially appear in the "available" file when they +were installed, the other control information for them. +</para> +</listitem> +</itemizedlist> +<para> +The exact format for the "Status:" field is: +</para> +<screen> + Status: Want Flag Status +</screen> +<para> +Where <replaceable>Want</replaceable> may be one of +<emphasis>unknown</emphasis>, <emphasis>install</emphasis>, +<emphasis>hold</emphasis>, <emphasis>deinstall</emphasis>, +<emphasis>purge</emphasis>. <replaceable>Flag</replaceable> may +be one of <emphasis>ok</emphasis>, <emphasis>reinstreq</emphasis>. +<replaceable>Status</replaceable> may +be one of <emphasis>not-installed</emphasis>, <emphasis>config-files</emphasis>, +<emphasis>half-installed</emphasis>, <emphasis>unpacked</emphasis>, +<emphasis>half-configured</emphasis> and <emphasis>installed</emphasis>. +The states are as follows:- +</para> +<variablelist> +<varlistentry> +<term>not-installed</term> +<listitem> +<para> +No files are installed from the package, it has no config files left, it +uninstalled cleanly if it ever was installed. +</para> +</listitem> +</varlistentry> +<varlistentry> +<term>unpacked</term> +<listitem> +<para> +The basic files have been unpacked (and are listed in +/var/lib/dpkg/info/[package].list. There are config files present, but the +postinst script has _NOT_ been run. +</para> +</listitem> +</varlistentry> +<varlistentry> +<term>half-configured</term> +<listitem> +<para> +The package was installed and unpacked, but the postinst script failed in some +way. +</para> +</listitem> +</varlistentry> +<varlistentry> +<term>installed</term> +<listitem> +<para> +All files for the package are installed, and the configuration was also +successful. +</para> +</listitem> +</varlistentry> +<varlistentry> +<term>half-installed</term> +<listitem> +<para> +An attempt was made to remove the package but there was a failure in the +prerm script. +</para> +</listitem> +</varlistentry> +<varlistentry> +<term>config-files</term> +<listitem> +<para> +The package was "removed", not "purged". The config files are left, but +nothing else. +</para> +</listitem> +</varlistentry> +</variablelist> +<para> +The two last items are only left in dpkg for compatibility - they are +understood by it, but never written out in this form. +</para> +<para> +Please see the dpkg source code, <literal>lib/parshelp.c</literal>, +<emphasis>statusinfos</emphasis>, <emphasis>eflaginfos</emphasis> and +<emphasis>wantinfos</emphasis> for more details. +</para> +</listitem> +<listitem> +<para> +info - this directory contains files from the control archive of every +package currently installed. They are installed with a prefix of +"<packagename>.". In addition to this, it also contains a file +called <package>.list for every package, which contains a list +of files. Note also that the control file is not copied into here; it +is instead found as part of status or available. +</para> +</listitem> +<listitem> +<para> +methods - this directory is reserved for "method"-specific files - each +"method" has a subdirectory underneath this directory (or at least, +it can have). In addition, there is another subdirectory "mnt", where +misc. filesystems (floppies, CD-ROMs, etc.) are mounted. +</para> +</listitem> +<listitem> +<para> +alternatives - directory used by the "update-alternatives" program. It +contains one file for each "alternatives" interface, which contains +information about all the needed symlinked files for each alternative. +</para> +</listitem> +<listitem> +<para> +diversions - file used by the "dpkg-divert" program. Each diversion takes +three lines. The first is the package name (or ":" for user diversion), the +second the original filename, and the third the diverted filename. +</para> +</listitem> +<listitem> +<para> +updates - directory used internally by dpkg. This is discussed later, in the +section <xref linkend="updates"/>. +</para> +</listitem> +<listitem> +<para> +parts - temporary directory used by dpkg-split +</para> +</listitem> +</itemizedlist> +</section> + +<section id="s1.3"><title>The dpkg library files</title> +<para> +These files are installed under /usr/lib/dpkg (usually), but +/usr/local/lib/dpkg is also a possibility (as Debian policy dictates). Under +this directory, there is a "methods" subdirectory. The methods subdirectory in +turn contains any number of subdirectories for each general method processor +(note that one set of method scripts can, and is, used for more than one of +the methods listed under dselect). +</para> +<para> +The following files may be found in each of these subdirectories:- +</para> +<itemizedlist> +<listitem> +<para> +names - One line per method, two-digit priority to appear on menu at +beginning, followed by a space, the name, and then another space and +the short description. +</para> +</listitem> +<listitem> +<para> +desc.<name> - Contains the long description displayed by dselect +when the cursor is put over the <name> method. +</para> +</listitem> +<listitem> +<para> +setup - Script or program which sets up the initial values to be used +by this method. Called with first argument as the status area directory +(/var/lib/dpkg), second argument as the name of the method (as in the +directory name), and the third argument as the option (as in the names file). +</para> +</listitem> +<listitem> +<para> +install - Script/program called when the "install" option of dselect is run +with this method. Same arguments as for setup. +</para> +</listitem> +<listitem> +<para> +update - Script/program called when the "update" option of dselect is +run. Same arguments as for setup/install. +</para> +</listitem> +</itemizedlist> +</section> + +<section id="s1.4"><title>The "dpkg" command-line utility</title> + +<section id="s1.4.1"><title>"Documented" command-line interfaces</title> +<para> +As yet unwritten. You can refer to the other manuals for now. See +<citerefentry><refentrytitle>dpkg</refentrytitle><manvolnum>8</manvolnum></citerefentry>. +</para> +</section> + +<section id="s1.4.2"><title>Environment variables which dpkg responds to</title> +<itemizedlist> +<listitem> +<para> +SHELL - used to determine which shell to run. +</para> +</listitem> +<listitem> +<para> +CC - used as the C compiler to call to determine the target architecture. The +default is "gcc". +</para> +</listitem> +<listitem> +<para> +PATH - dpkg checks that it can find at least the following files in the path +when it wants to run package installation scripts, and gives an error if it +cannot find all of them:- +</para> +<itemizedlist> +<listitem> +<para> +ldconfig +</para> +</listitem> +<listitem> +<para> +start-stop-daemon +</para> +</listitem> +<listitem> +<para> +install-info +</para> +</listitem> +<listitem> +<para> +update-rc.d +</para> +</listitem> +</itemizedlist> +</listitem> +</itemizedlist> +</section> + +<section id="s1.4.3"><title>Assertions</title> +<para> +The dpkg utility itself is required for quite a number of packages, even if +they have been installed with a tool totally separate from dpkg. The reason +for this is that some packages, in their pre-installation scripts, check that +your version of dpkg supports certain features. This was broken from the +start, and it should have actually been a control file header "Dpkg-requires", +or similar. What happens is that the configuration scripts will abort or +continue according to the exit code of a call to dpkg, which will stop them +from being wrongly configured. +</para> +<para> +These special command-line options, which simply return as true or false are +all prefixed with "--assert-". Here is a list of them (without the prefix):- +</para> +<itemizedlist> +<listitem> +<para> +support-predepends - Returns success or failure according to whether a version +of dpkg which supports predepends properly (1.1.0 or above) is installed, +according to the database. +</para> +</listitem> +<listitem> +<para> +working-epoch - Return success or failure according to whether a version of +dpkg which supports epochs in version properly (1.4.0.7 or above) is installed, +according to the database. +</para> +</listitem> +</itemizedlist> +<para> +Both these options check the status database to see what version of the +"dpkg" package is installed, and check it against a known working version. +</para> +</section> + +<section id="s1.4.4"><title>--predep-package</title> +<para> +This strange option is described as follows in the source code: +</para> +<screen> +/* Print a single package which: + * (a) is the target of one or more relevant predependencies. + * (b) has itself no unsatisfied pre-dependencies. + * If such a package is present output is the Packages file entry, + * which can be massaged as appropriate. + * Exit status: + * 0 = a package printed, OK + * 1 = no suitable package available + * 2 = error + */ +</screen> +<para> +On further inspection of the source code, it appears that what is does is +this:- +</para> +<itemizedlist> +<listitem> +<para> +Looks at the packages in the database which are selected as "install", +and are installed. +</para> +</listitem> +<listitem> +<para> +It then looks at the Pre-Depends information for each of these packages +from the available file. When it find a package for which any of the +pre-dependencies are not satisfied, it breaks from the loop through the +packages. +</para> +</listitem> +<listitem> +<para> +It then looks through the unsatisfied pre-dependencies, and looks for +packages which would satisfy this pre-dependency, stopping on the first +it finds. If it finds none, it bombs out with an error. +</para> +</listitem> +<listitem> +<para> +It then continues this for every dependency of the initial package. +</para> +</listitem> +</itemizedlist> +<para> +Eventually, it writes out the record of all the packages to satisfy the +pre-dependencies. This is used by the disk method to make sure that its +dependency ordering is correct. What happens is that all pre-depending +packages are first installed, then it runs dpkg -iGROEB on the directory, +which installs in the order package files are found. Since pre-dependencies +mean that a package may not even be unpacked unless they are satisfied, it +is necessary to do this (usually, since all the package files are unpacked +in one phase, the configured in another, this is not needed). +</para> +</section> + +</section> + +</chapter> + +<chapter id="ch2"><title>dpkg-deb and .deb file internals</title> +<para> +This chapter describes the internals to the "dpkg-deb" tool, which is used by +"dpkg" as a back-end. dpkg-deb has its own tar extraction functions, which is +the source of many problems, as it does not support long filenames, using +extension blocks. +</para> + +<section id="s2.1"><title>The .deb archive format</title> +<para> +The main principal of the new-format Debian archive (I won't describe the old +format - for that have a look at deb-old.5), is that the archive really is an +archive - as used by "ar" and friends. However, dpkg-deb uses this format +internally, rather than calling "ar". Inside this archive, there are usually +the following members:- +</para> +<itemizedlist> +<listitem> +<para> +debian-binary +</para> +</listitem> +<listitem> +<para> +control.tar.gz +</para> +</listitem> +<listitem> +<para> +data.tar.gz +</para> +</listitem> +</itemizedlist> +<para> +The debian-binary member consists simply of the string "2.0", indicating +the format version. control.tar.gz contains the control files (and scripts), +and the data.tar.gz contains the actual files to populate the filesystem +with. Both tarfiles extract straight into the current directory. Information +on the tar formats can be found in the GNU tar info page. Since dpkg-deb +calls "tar -cf" to build packages, the Debian packages use the GNU extensions. +</para> +</section> + +<section id="s2.2"><title>The dpkg-deb command-line</title> +<para> +dpkg-deb documents itself thoroughly with its '--help' command-line +option. However, I am including a reference to these for +completeness. dpkg-deb supports the following options:- +</para> +<itemizedlist> +<listitem> +<para> +--build (-b) <dir> - builds a .deb archive, takes a directory which +contains all the files as an argument. Note that the directory +<dir>/DEBIAN will be packed separately into the control archive. +</para> +</listitem> +<listitem> +<para> +--contents (-c) <debfile> - Lists the contents of the "data.tar.gz" +member. +</para> +</listitem> +<listitem> +<para> +--control (-e) <debfile> - Extracts the control archive into a directory +called DEBIAN. Alternatively, with another argument, it will extract it into a +different directory. +</para> +</listitem> +<listitem> +<para> +--info (-I) <debfile> - Prints the contents of the "control" file in the +control archive to stdout. Alternatively, giving it other arguments will cause +it to print the contents of those files instead. +</para> +</listitem> +<listitem> +<para> +--field (-f) <debfile> <field> ... - Prints any number of fields +from the "control" file. Giving it extra arguments limits the fields it prints +to only those specified. With no command-line arguments other than a filename, +it is equivalent to -I and just the .deb filename. +</para> +</listitem> +<listitem> +<para> +--extract (-x) <debfile> <dir> - Extracts the data archive of a +debian package under the directory <dir>. +</para> +</listitem> +<listitem> +<para> +--vextract (-X) <debfile> <dir> - Same as --extract, except it +is equivalent of giving tar the '-v' option - it prints the filenames as it +extracts them. +</para> +</listitem> +<listitem> +<para> +--fsys-tarfile <debfile> - This option outputs a gunzip'd version of +data.tar.gz to stdout. +</para> +</listitem> +<listitem> +<para> +--new - sets the archive format to be used to the new Debian format +</para> +</listitem> +<listitem> +<para> +--old - sets the archive format to be used to the old Debian format +</para> +</listitem> +<listitem> +<para> +--debug - Tells dpkg-deb to produce debugging output +</para> +</listitem> +<listitem> +<para> +--nocheck - Tells dpkg-deb not to check the sanity of the control file +</para> +</listitem> +<listitem> +<para> +--help (-h) - Gives a help message +</para> +</listitem> +<listitem> +<para> +--version - Shows the version number +</para> +</listitem> +<listitem> +<para> +--licence/--license (UK/US spellings) - Shows a brief outline of the GPL +</para> +</listitem> +</itemizedlist> + +<section id="s2.2.1"><title>Internal checks used by dpkg-deb when building packages</title> +<para> +Here is a list of the internal checks used by dpkg-deb when building +packages. It is in the order they are done. +</para> +<itemizedlist> +<listitem> +<para> +First, the output Debian archive argument, if it is given, is checked using +stat. If it is a directory, an internal flag is set. This check is only made +if the archive name is specified explicitly on the command-line. If the +argument was not given, the default is the directory name, with ".deb" +appended. +</para> +</listitem> +<listitem> +<para> +Next, the control file is checked, unless the --nocheck flag was specified on +the command-line. dpkg-deb will bomb out if the second argument to --build was +a directory, and --nocheck was specified. Note that dpkg-deb will not be able +to determine the name of the package in this case. In the control file, the +following things are checked:- +</para> +<itemizedlist> +<listitem> +<para> +The package name is checked to see if it contains any invalid characters (see +<xref linkend="control"/> for this). +</para> +</listitem> +<listitem> +<para> +The priority field is checked to see if it uses standard values, and +user-defined values are warned against. However, note that this check is now +redundant, since the control file no longer contains the priority - the +changes file now does this. +</para> +</listitem> +<listitem> +<para> +The control file fields are then checked against the standard list of fields +which appear in control files, and any "user-defined" fields are reported as +warnings. +</para> +</listitem> +<listitem> +<para> +dpkg-deb then checks that the control file contains a valid version number. +</para> +</listitem> +</itemizedlist> +</listitem> +<listitem> +<para> +After this, in the case where a directory was specified to build the .deb file +in, the filename is created as "directory/pkg_ver.deb" or +"directory/pkg_ver_arch.deb", depending on whether the control file contains +an architecture field. +</para> +</listitem> +<listitem> +<para> +Next, dpkg-deb checks for the <dir>/DEBIAN directory. It complains if it +doesn't exist, or if it has permissions < 0755, or > 0775. +</para> +</listitem> +<listitem> +<para> +It then checks that all the files in this subdir are either symlinks or plain +files, and have permissions between 0555 and 0775. +</para> +</listitem> +<listitem> +<para> +The conffiles file is then checked to see if the filenames are too +long. Warnings are produced for each that is. After this, it checks +that the package provides initial copies of each of these conffiles, +and that they are all plain files. +</para> +</listitem> +</itemizedlist> +</section> + +</section> + +</chapter> + +<chapter id="ch3"><title>dpkg internals</title> +<para> +This chapter describes the internals of dpkg itself. Although the low-level +formats are quite simple, what dpkg does in certain cases often does not make +sense. +</para> + +<section id="updates"><title>Updates</title> +<para> +This describes the /var/lib/dpkg/updates directory. The function of this +directory is somewhat strange, and seems only to be used internally. A +function called cleanupdates is called whenever the database is scanned. This +function in turn uses +<citerefentry><refentrytitle>scandir</refentrytitle><manvolnum>3</manvolnum></citerefentry>, +to sort the files in this directory. Files who names do not consist entirely +of digits are discarded. dpkg also causes a fatal error if any of the +filenames are different lengths. +</para> +<para> +After having scanned the directory, dpkg in turn parses each file the same way +it parses the status file (they are sorted by the scandir to be in numerical +order). After having done this, it then writes the status information back to +the "status" file, and removes all the "updates" files. +</para> +<para> +These files are created internally by dpkg's "checkpoint" function, and are +cleaned up when dpkg exits cleanly. +</para> +<para> +Judging by the use of the updates directory I would call it a Journal. Inorder +to efficiently ensure the complete integrity of the status file dpkg will +"checkpoint" or journal all of it's activities in the updates directory. By +merging the contents of the updates directory (in order!!) against the original +status file it can get the precise current state of the system, even in the +event of a system failure while dpkg is running. +</para> +<para> +The other option would be to sync-rewrite the status file after each operation, +which would kill performance. +</para> +<para> +It is very important that any program that uses the status file abort if the +updates directory is not empty! The user should be informed to run dpkg +manually (what options though??) to correct the situation. +</para> +</section> + +<section id="s3.2"><title>What happens when dpkg reads the database</title> +<para> +First, the status file is read. This gives dpkg an initial idea of the +packages that are there. Next, the updates files are read in, overriding the +status file, and if necessary, the status file is re-written, and updates files +are removed. Finally, the available file is read. The available file is read +with flags which preclude dpkg from updating any status information from it, +though - installed version, etc., and is also told to record that the packages +it reads this time are available, not installed. +</para> +<para> +More information on updates is given above. +</para> +</section> + +<section id="s3.3"><title>How dpkg compares version numbers</title> +<para> +Version numbers consist of three parts: the epoch, the upstream version, and +the Debian revision. Dpkg compares these parts in that order. If the epochs +are different, it returns immediately, and so on. +</para> +<para> +However, the important part is how it compares the versions which are +essentially stored as just strings. These are compared in two distinct +parts: those consisting of numerical characters (which are evaluated, and +then compared), and those consisting of other characters. When comparing +non-numerical parts, they are compared as the character values (ASCII), +but non-alphabetical characters are considered "greater than" alphabetical +ones. Also note that longer strings (after excluding differences where +numerical values are equal) are considered "greater than" shorter ones. +</para> +<para> +Here are a few examples of how these rules apply:- +</para> +<screen> +15 > 10 +0010 == 10 + +d.r > dsr +32.d.r == 0032.d.r +d.rnr < d.rnrn +</screen> +</section> + +</chapter> + +</book> |