apt/doc/cache.sgml

<!doctype debiandoc system>
<!-- -*- mode: sgml; mode: fold -*- -->
<book>
<title>APT Cache File Format</title>

<author>Jason Gunthorpe <email>jgg@debian.org</email></author>
<version>$Id: cache.sgml,v 1.1 1998/07/02 02:58:12 jgg Exp $</version>

<abstract>
This document describes the complete implementation and format of the APT
Cache file. The APT Cache file is a way for APT to parse and store a 
large number of package files for display in the UI. It's primary design 
goal is to make display of a single package in the tree very fast by 
pre-linking important things like dependencies and provides.

The specification doubles as documentation for one of the in-memory 
structures used by the package library and the APT GUI.

</abstract>

<copyright>
Copyright &copy; Jason Gunthorpe, 1997.
<p>
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.

<p>
For more details, on Debian GNU/Linux systems, see the file
/usr/doc/copyright/GPL for the full license.
</copyright>

<toc sect>

<chapt>Introduction
<!-- Purpose		                                               {{{ -->
<!-- ===================================================================== -->
<sect>Purpose

<p>
This document describes the implementation of an architecture
dependent binary cache file. The goal of this cache file is two fold, 
firstly to speed loading and processing of the package file array and
secondly to reduce memory consumption of the package file array.

<p>
The implementation is aimed at an environment with many primary package
files, for instance someone that has a Package file for their CD-ROM, a
Package file for the latest version of the distribution on the CD-ROM and a
package file for the development version. Always present is the information
contained in the status file which might be considered a separate package
file.

<p>
Please understand, this is designed as a -CACHE FILE- it is not ment to be
used on any system other than the one it was created for. It is not ment to
be authoritative either, ie if a system crash or software failure occures it
must be perfectly acceptable for the cache file to be in an inconsistant
state. Furthermore at any time the cache file may be erased without losing
any information.

<p>
Also the structures and storage layout is optimized for use by the APT
GUI and may not be suitable for all purposes. However it should be possible 
to extend it with associate cache files that contain other information.

<p>
To keep memory use down the cache file only contains often used fields and
fields that are inexepensive to store, the Package file has a full list of
fields. Also the client may assume that all items are perfectly valid and
need not perform checks against their correctness. Removal of information
from the cache is possible, but blanks will be left in the file, and
unused strings will also be present. The recommended implementation is to
simply rebuild the cache each time any of the data files change. It is
possible to add a new package file to the cache without any negative side
effects.

<sect1>Note on Pointer access
<p>
Every item in every structure is stored as the index to that structure.
What this means is that once the files is mmaped every data access has to
go through a fixup stage to get a real memory pointer. This is done
by taking the tndex, multiplying it by the type size and then adding
it to the start address of the memory block. This sounds complex, but
in C it is a single array dereference. Because all items are aligned to 
their size and indexs are stored as multiples of the size of the structure
the format is immediately portable to all possible architectures - BUT the
generated files are -NOT-.

<p>
This scheme allows code like this to be written:
<example>
  void *Map = mmap(...);
  Package *PkgList = (Package *)Map;
  Header *Head = (Header *)Map;
  char *Strings = (char *)Map;
  cout << (Strings + PkgList[Head->HashTable[0]]->Name) << endl;
</example>
<p>
Notice the lack of casting or multiplication. The net result is to return
the name of the first package in the first hash bucket, without error
checks. 

<p>
The generator uses allocation pools to group similarly sized structures in
large blocks to eliminate any alignment overhead. The generator also
assures that no structures overlap and all indexes are unique. Although
at first glance it may seem like there is the potential for two structures
to exist at the same point the generator never allows this to happen.
(See the discussion of free space pools)
                                                                  <!-- }}} -->

<chapt>Structures
<!-- Header		                                               {{{ -->
<!-- ===================================================================== -->
<sect>Header
<p>
This is the first item in the file.
<example>
   struct Header
   {
      // Signature information
      unsigned long Signature;
      short MajorVersion;
      short MinorVersion;
      bool Dirty;
       
      // Size of structure values
      unsigned short HeaderSz;
      unsigned short PackageSz;
      unsigned short PackageFileSz;
      unsigned short VersionSz;
      unsigned short DependencySz;
      unsigned short ProvidesSz;

      // Structure counts
      unsigned long PackageCount;
      unsigned long VersionCount;
      unsigned long DependsCount;
      unsigned long PackageFileCount;
      
      // Offsets
      unsigned long FileList;              // PackageFile
      unsigned long StringList;            // StringItem
      
      // Pool structures
      unsigned long PoolStart[6];
      unsigned long PoolSize[6];
      unsigned long PoolAln[6];
      
      // Package name lookup
      unsigned long HashTable[512];        // Package
   };
</example>
<taglist>
<tag>Signature<item>
This must contain the hex value 0x98FE76DC which is designed to verify
that the system loading the image has the same byte order and byte size as
the system saving the image

<tag>MajorVersion
<tag>MinorVersion<item>
These contain the version of the cache file, currently 0.2.

<tag>Dirty<item>
Dirty is true if the cache file was opened for reading, the client expects
to have written things to it and have not fully synced it. The file should
be erased and rebuilt if it is true.

<tag>HeaderSz
<tag>PackageSz
<tag>PackageFileSz
<tag>VersionSz
<tag>DependencySz
<tag>ProvidesSz<item>
*Sz contains the sizeof() that particular structure. It is used as an
extra consistancy check on the structure of the file.

If any of the size values do not exactly match what the client expects then
the client should refuse the load the file.

<tag>PackageCount
<tag>VersionCount
<tag>DependsCount
<tag>PackageFileCount<item>
These indicate the number of each structure contianed in the cache. 
PackageCount is especially usefull for generating user state structures. 
See Package::Id for more info.

<tag>FileList<item>
This contains the index of the first PackageFile structure. The PackageFile
structures are singely linked lists that represent all package files that
have been merged into the cache.

<tag>StringList<item>
This contains a list of all the unique strings (string item type strings) in 
the cache. The parser reads this list into memory so it can match strings 
against it.

<tag>PoolStart
<tag>PoolSize
<tag>PoolAln<item>
The Pool structures manage the allocation pools that the generator uses.
Start indicates the first byte of the pool, Size is the number of bytes
remaining in the pool and Aln (alignment) is the structure size of the pool. 
An Aln of 0 indicates the slot is empty. There should be the same number of
slots as there are structure types. The generator stores this information 
so future additions can make use of any unused pool blocks.

<tag>HashTable<item>
HashTable is a hash table that provides indexing for all of the packages.
Each package name is inserted into the hash table using the following has
function:
<example>
   unsigned long Hash(string Str)
   {
      unsigned long Hash = 0;
      for (const char *I = Str.begin(); I != Str.end(); I++)
         Hash += *I * ((Str.end() - I + 1));
      return Hash % _count(Head.HashTable);
   }
</example>
<p>
By iterating over each entry in the hash table it is possible to iterate over 
the entire list of packages. Hash Collisions are handled with a singely linked
list of packages based at the hash item. The linked list contains only 
packages that macth the hashing function.

</taglist>
                                                                  <!-- }}} -->
<!-- Package		                                               {{{ -->
<!-- ===================================================================== -->
<sect>Package
<p>
This contians information for a single unique package. There can be any
number of versions of a given package. Package exists in a singly
linked list of package records starting at the hash index of the name in 
the Header->HashTable.
<example>
   struct Pacakge
   {
      // Pointers
      unsigned long Name;              // Stringtable
      unsigned long VersionList;       // Version
      unsigned long TargetVer;         // Version
      unsigned long CurrentVer;        // Version
      unsigned long TargetDist;        // StringTable (StringItem)
      unsigned long Section;           // StringTable (StringItem)
      
      // Linked lists 
      unsigned long NextPackage;       // Package
      unsigned long RevDepends;        // Dependency
      unsigned long ProvidesList;      // Provides
       
      // Install/Remove/Purge etc
      unsigned char SelectedState;     // What
      unsigned char InstState;         // Flags
      unsigned char CurrentState;      // State

      // Unique ID for this pkg
      unsigned short ID;
      unsigned short Flags;
   };
</example>   

<taglist>
<tag>Name<item>
Name of the package.

<tag>VersionList<item>
Base of a singely linked list of version structures. Each structure 
represents a unique version of the package. The version structures
contain links into PackageFile and the original text file as well as
detailed infromation about the size and dependencies of the specific
package. In this way multiple versions of a package can be cleanly handled
by the system. Furthermore, this linked list is guarenteed to be sorted
from Highest version to lowest version with no duplicate entries.

<tag>TargetVer
<tag>CurrentVer<item>
This is an index (pointer) to the sub version that is being targeted for
upgrading. CurrentVer is an index to the installed version, either can be
0.

<tag>TargetDist<item>
This indicates the target distribution. Automatic upgrades should not go
outside of the specified dist. If it is 0 then the global target dist should 
be used. The string should be contained in the StringItem list.

<tag>Section<item>
This indicates the deduced section. It should be "Unknown" or the section
of the last parsed item.

<tag>NextPackage<item>
Next link in this hash item. This linked list is based at Header.HashTable
and contains only packages with the same hash value.

<tag>RevDepends<item>
Reverse Depends is a linked list of all dependencies linked to this package.

<tag>ProvidesList<item>
This is a linked list of all provides for this package name.

<tag>SelectedState
<tag>InstState
<tag>CurrentState<item>
These corrispond to the 3 items in the Status field found in the status
file. See the section on defines for the possible values.
<p>
SelectedState is the state that the user wishes the package to be
in.
<p>
InstState is the installation state of the package. This normally
should be Ok, but if the installation had an accident it may be otherwise.
<p>
CurrentState indicates if the package is installed, partially installed or
not installed.

<tag>ID<item>
ID is a value from 0 to Header->PackageCount. It is a unique value assigned
by the generator. This allows clients to create an array of size PackageCount
and use it to store state information for the package map. For instance the
status file emitter uses this to track which packages have been emitted 
already.

<tag>Flags<item>
Flags are some usefull indicators of the package's state. 

</taglist>

                                                                  <!-- }}} -->
<!-- PackageFile	                                               {{{ -->
<!-- ===================================================================== -->
<sect>PackageFile
<p>
This contians information for a single package file. Package files are
referenced by Version structures. This is a singly linked list based from
Header.FileList
<example>
   struct PackageFile
   {
      // Names
      unsigned long FileName;        // Stringtable
      unsigned long Version;         // Stringtable
      unsigned long Distribution;    // Stringtable
      unsigned long Size;            
      
      // Linked list
      unsigned long NextFile;        // PackageFile
      unsigned short ID;
      unsigned short Flags;
      time_t mtime;                  // Modification time
   };
</example>   
<taglist>

<tag>FileName<item>
Refers the the physical disk file that this PacakgeFile represents.

<tag>Version<item>
Version is the given version, ie 1.3.1, 2.4_revision_1 etc.

<tag>Distribution<item>
Distribution is the symbolic name for this PackageFile, hamm,bo,rexx etc

<tag>Size<item>
Size is provided as a simple check to ensure that the package file has not
been altered.

<tag>ID<item>
See Package::ID.

<tag>Flags<item>
Provides some flags for the PackageFile, see the section on defines.

<tag>mtime<item>
Modification time for the file at time of cache generation.

</taglist>

                                                                  <!-- }}} -->
<!-- Version		                                               {{{ -->
<!-- ===================================================================== -->
<sect>Version
<p>
This contians the information for a single version of a package. This is a
singley linked list based from Package.Versionlist.

<p>
The version list is always sorted from highest version to lowest version by
the generator. Also there may not be any duplicate entries in the list (same 
VerStr).

<example>
   struct Version
   {
      unsigned long VerStr;            // Stringtable
      unsigned long File;              // PackageFile
      unsigned long Section;           // StringTable (StringItem)
      
      // Lists
      unsigned long NextVer;           // Version
      unsigned long DependsList;       // Dependency
      unsigned long ParentPkg;         // Package
      unsigned long ProvidesList;      // Provides
     
      unsigned long Offset;
      unsigned long Size;
      unsigned long InstalledSize;
      unsigned short ID;
      unsigned char Priority;
   };
</example>
<taglist>

<tag>VerStr<item>
This is the complete version string.

<tag>File<item>
References the PackageFile that this version came out of. File can be used
to determine what distribution the Version applies to. If File is 0 then
this is a blank version. The structure should also have a 0 in all other
fields excluding VerStr and Possibly NextVer.

<tag>Section<item>
This string indicates which section it is part of. The string should be
contained in the StringItem list.

<tag>NextVer<item>
Next step in the linked list.

<tag>DependsList<item>
This is the base of the dependency list.

<tag>ParentPkg<item>
This links the version to the owning package, allowing reverse dependencies
to determine the package.

<tag>ProvidesList<item>
Head of the linked list of Provides::NextPkgProv, forward provides.

<tag>Offset<item>
The byte offset of the first line of this item in the specified
PackageFile

<tag>Size
<tag>InstalledSize<item>
The archive size for this version. For debian this is the size of the .deb
file. Installed size is the uncompressed size for this version

<tag>ID<item>
See Package::ID.

<tag>Priority<item>
This is the parsed priority value of the package.
</taglist>

                                                                  <!-- }}} -->
<!-- Dependency		                                               {{{ -->
<!-- ===================================================================== -->
<sect>Dependency
<p>
Dependency contains the information for a single dependency record. The records
are split up like this to ease processing by the client. The base of list
linked list is Version.DependsList. All forms of dependencies are recorded
here including Conflicts, Suggests and Recommends.

<p>
Multiple depends on the same package must be grouped together in 
the Dependency lists. Clients should assume this is always true.

<example>
    struct Dependency
    {
       unsigned long Version;         // Stringtable
       unsigned long Package;         // Package
       unsigned long NextDepends;     // Dependency
       unsigned long NextRevDepends;  // Reverse dependency linking
       unsigned long ParentVer;       // Upwards parent version link
       
       // Specific types of depends
       unsigned char Type;
       unsigned char CompareOp;
       unsigned short ID;
    };
</example>
<taglist>
<tag>Version<item>
The string form of the version that the dependency is applied against.

<tag>Package<item>
The index of the package file this depends applies to. If the package file
does not already exist when the dependency is inserted a blank one (no
version records) should be created.

<tag>NextDepends<item>
Linked list based off a Version structure of all the dependencies in that
version.

<tag>NextRevDepends<item>
Reverse dependency linking, based off a Package structure. This linked list
is a list of all packages that have a depends line for a given package.

<tag>ParentVer<item>
Parent version linking, allows the reverse dependency list to link
back to the version and package that the dependency are for.

<tag>Type<item>
Describes weather it is depends, predepends, recommends, suggests, etc.

<tag>CompareOp<item>
Describes the comparison operator specified on the depends line. If the high
bit is set then it is a logical or with the previous record.

<tag>ID<item>
See Package::ID.

</taglist>

                                                                  <!-- }}} -->
<!-- Provides		                                               {{{ -->
<!-- ===================================================================== -->
<sect>Provides
<p>
Provides handles virtual packages. When a Provides: line is encountered 
a new provides record is added associating the package with a virtual 
package name. The provides structures are linked off the package structures.
This simplifies the analysis of dependencies and other aspects A provides 
refers to a specific version of a specific package, not all versions need to 
provide that provides.

<p>
There is a linked list of provided package names started from each 
version that provides packages. This is the forwards provides mechanism.
<example>
    struct Provides
    {
       unsigned long ParentPkg;        // Package
       unsigned long Version;          // Version
       unsigned long ProvideVersion;   // Stringtable
       unsigned long NextProvides;     // Provides
       unsigned long NextPkgProv;      // Provides
    };
</example>
<taglist>
<tag>ParentPkg<item>
The index of the package that head of this linked list is in. ParentPkg->Name
is the name of the provides.

<tag>Version<item>
The index of the version this provide line applies to.

<tag>ProvideVersion<item>
Each provides can specify a version in the provides line. This version allows
dependencies to depend on specific versions of a Provides, as well as allowing
Provides to override existing packages. This is experimental.

<tag>NextProvides<item>
Next link in the singly linked list of provides (based off package)

<tag>NextPkgProv<item>
Next link in the singly linked list of provides for 'Version'.

</taglist>

                                                                  <!-- }}} -->
<!-- StringItem		                                               {{{ -->
<!-- ===================================================================== -->
<sect>StringItem
<p>
StringItem is used for generating single instances of strings. Some things
like Section Name are are usefull to have as unique tags. It is part of 
a linked list based at Header::StringList.
<example>
   struct StringItem
   {
      unsigned long String;        // Stringtable
      unsigned long NextItem;      // StringItem
   };
</example>
<taglist>
<tag>String<item>
The string this refers to.

<tag>NextItem<item>
Next link in the chain.
</taglist>
                                                                  <!-- }}} -->
<!-- StringTable	                                               {{{ -->
<!-- ===================================================================== -->
<sect>StringTable
<p>
All strings are simply inlined any place in the file that is natural for the
writer. The client should make no assumptions about the positioning of
strings. All stringtable values point to a byte offset from the start of the
file that a null terminated string will begin.
                                                                  <!-- }}} -->
<!-- Defines		                                               {{{ -->
<!-- ===================================================================== -->
<sect>Defines
<p>
Several structures use variables to indicate things. Here is a list of all
of them.

<sect1>Definitions for Dependency::Type
<p>
<example>
#define pkgDEP_Depends 1
#define pkgDEP_PreDepends 2
#define pkgDEP_Suggests 3
#define pkgDEP_Recommends 4
#define pkgDEP_Conflicts 5
#define pkgDEP_Replaces 6
</example>
</sect1>

<sect1>Definitions for Dependency::CompareOp
<p>
<example>
#define pkgOP_OR 0x10
#define pkgOP_LESSEQ 0x1
#define pkgOP_GREATEREQ 0x2
#define pkgOP_LESS 0x3
#define pkgOP_GREATER 0x4
#define pkgOP_EQUALS 0x5
</example>
The lower 4 bits are used to indicate what operator is being specified and
the upper 4 bits are flags. pkgOP_OR indicates that the next package is
or'd with the current package.
</sect1>

<sect1>Definitions for Package::SelectedState
<p>
<example>
#define pkgSTATE_Unkown 0
#define pkgSTATE_Install 1
#define pkgSTATE_Hold 2
#define pkgSTATE_DeInstall 3
#define pkgSTATE_Purge 4
</example>
</sect1>

<sect1>Definitions for Package::InstState
<p>
<example>
#define pkgSTATE_Ok 0
#define pkgSTATE_ReInstReq 1
#define pkgSTATE_Hold 2
#define pkgSTATE_HoldReInstReq 3
</example>
</sect1>

<sect1>Definitions for Package::CurrentState
<p>
<example>
#define pkgSTATE_NotInstalled 0
#define pkgSTATE_UnPacked 1
#define pkgSTATE_HalfConfigured 2
#define pkgSTATE_UnInstalled 3
#define pkgSTATE_HalfInstalled 4
#define pkgSTATE_ConfigFiles 5
#define pkgSTATE_Installed 6
</example>
</sect1>

<sect1>Definitions for Package::Flags
<p>
<example>
#define pkgFLAG_Auto (1 << 0)
#define pkgFLAG_New (1 << 1)
#define pkgFLAG_Obsolete (1 << 2)
#define pkgFLAG_Essential (1 << 3)
#define pkgFLAG_ImmediateConf (1 << 4)
</example>
</sect1>

<sect1>Definitions for Version::Priority
<p>
Zero is used for unparsable or absent Priority fields.
<example>
#define pkgPRIO_Important 1
#define pkgPRIO_Required 2
#define pkgPRIO_Standard 3
#define pkgPRIO_Optional 4
#define pkgPRIO_Extra 5
</example>
</sect1>

<sect1>Definitions for PackageFile::Flags
<p>
<example>
#define pkgFLAG_NotSource (1 << 0)
</example>
</sect1>

                                                                  <!-- }}} -->

<chapt>Notes on the Generator
<!-- Notes on the Generator 	                                       {{{ -->
<!-- ===================================================================== -->
<p>
The pkgCache::MergePackageFile function is currently the only generator of
the cache file. It implements a conversion from the normal textual package
file into the cache file.

<p>
The generator assumes any package declaration with a
Status: line is a 'Status of the package' type of package declaration. 
A Package with a Target-Version field should also really have a status field.
The processing of a Target-Version field can create a place-holder Version
structure that is empty to refer to the specified version (See Version
for info on what a empty Version looks like). The Target-Version syntax
allows the specification of a specific version and a target distribution. 

<p>
Different section names on different versions is supported, but I 
do not expect to use it. To simplify the GUI it will mearly use the section
in the Package structure. This should be okay as I hope sections do not change
much.

<p>
The generator goes through a number of post processing steps after producing
a disk file. It sorts all of the version lists to be in descending order
and then generates the reverse dependency lists for all of the packages.
ID numbers and count values are also generated in the post processing step.

<p>
It is possible to extend many of the structures in the cache with extra data.
This is done by using the ID member. ID will be a unique number from 0 to
Header->??Count. For example
<example>
struct MyPkgData;
MyPkgData *Data = new MyPkgData[Header->PackageCount];
Data[Package->ID]->Item = 0;
</example>
This provides a one way reference between package structures and user data. To
get a two way reference would require a member inside the MyPkgData structure.

<p>
The generators use of free space pools tend to make the package file quite
large, and quite full of blank space. This could be fixed with sparse files.

                                                                  <!-- }}} -->

<chapt>Future Directions
<!-- Future Directions		                                       {{{ -->
<!-- ===================================================================== -->
<p>
Some good directions to take the cache file is into a cache directory that
contains many associated caches that cache other important bits of
information. (/var/cache/apt, FHS2) 

<p>
Caching of the info/*.list is an excellent place to start, by generating all
the list files into a tree structure and reverse linking them to the package
structures in the main cache file major speed gains in dpkg might be achived.

                                                                  <!-- }}} -->

</book>