OpenBSD::Intro - Introduction to the pkg tools internals
use OpenBSD::PackingList; ...
Note that the "OpenBSD::" namespace of perl modules is not limited to package tools, but also includes pkg-config(1) and makewhatis(8) support modules. This document only covers package tools material.
The design of the package tools revolves around a few central ideas:
Design modules that manipulate some notions in a consistent way, so that they can be used by the package tools proper, but also with a high-level API that's useful for anything that needs to manipulate packages. This was validated by the ease with which we can now update packing-lists, check for conflicts, and check various properties of our packages.
Try to be as safe as possible where installation and update operations are concerned. Cut up operations into small subsets which yields frequent safe intermediate points where the machine is completely functional.
Traditional package tools often rely on the following model: take a snapshot of the system, try to perform an operation, and roll back to a stable state if anything goes wrong.
Instead, OpenBSD package tools take a computational approach: record semantic information in a useful format, pre-compute as much as can be about an operation, and only perform the operation when we have proved that (almost) nothing can go wrong. As far as possible, the actual operation happens on the side, as a temporary scaffolding, and we only commit to the operation once most of the work is over.
Keep high-level semantic information instead of recomputing it all the time, but try to organize as much as possible as plain text files. Originally, it was a bit of a challenge: trying to see how much we could get away with, before having to define an actual database format. Turns out we do not need a database format, or even any cache on the ftp server.
Avoid copying files all over the place. Hence the OpenBSD::Ustar(3p) module that allows package tools to manipulate tarballs directly without having to extract them first in a staging area.
All the package tools use the same internal perl modules, which gives them some consistency about fundamental notions.
It is highly recommended to try to understand packing-lists and packing elements first, since they are the core that unlocks most of the package tools.
- packing-lists and elements
- Each package consists of a list of objects (mostly files, but there are
some other abstract structures, like new user accounts, or stuff to do
when the package gets installed). They are recorded in a
OpenBSD::PackingList(3p), the module offers everything needed to
manipulate packing-lists. The packing-list format has a text
representation, which is documented in pkg_create(1). Internally,
packing-lists are heavily structured. Objects are reordered by the
internals of OpenBSD::PackingList(3p), and there are some standard
filters defined to gain access to some commonly used information
(dependencies and conflicts mostly) without having to read and parse the
whole packing-list. Each object is an OpenBSD::PackingElement(3p),
which is an abstract class with lots of children classes. The use of
packing-lists most often combines two classic design patterns: one uses
Visitor to traverse a packing-list and perform an operation on all its
elements (this is where the order is important, and why some stuff like
user creation will `bubble up' to the beginning of the list), allied to
Template Method: the operation is often not determined for a basic
OpenBSD::PackingElement(3p), but will make more sense to an
OpenBSD::PackingElement::FileObject(3p) or similar. Packing-list
objects have an "automatic visitor" property: if a method is not
defined for the packing-list proper, but exists for packing elements, then
invoking the method on the packing-list will traverse it and apply the
method to each element. For instance, package installation happens through
the following snippet:
where "install_and_progress" is defined at the packing element level, and invokes "install" and shows a progress bar if needed.
- package names and specs
- Package names and specifications for package names have a specific format, which is described in packages-specs(7). Package specs are objects created in OpenBSD::PkgSpec(3p), which are then compared to objects created in OpenBSD::PackageName(3p). Both classes contain further functions for high level manipulation of names and specs. There is also a framework to organize searches based on OpenBSD::Search(3p) objects. Specifications are structured in a specific way, which yields a shorthand for conflict handling through OpenBSD::PkgCfl(3p), allows the package system to resolve dependencies in OpenBSD::Dependencies(3p) and to figure out package updates in OpenBSD::Update(3p).
- sources of packages
- Historically, OpenBSD::PackageInfo(3p) was used to get to the list of installed packages and grab information. This is now part of a more generic framework OpenBSD::PackageRepository(3p), which interacts with the search objects to allow you to access packages, be they installed, on the local machines, or distant. Once a package is located, the repository yields a proxy object called OpenBSD::PackageLocation(3p) that can be used to gain further info. (There are still shortcuts for installed packages for performance and simplicity reasons.)
- package sets
- Each operation (installation, removal, or replacement of packages) is cut up into small atomic operations, in order to guarantee maximal stability of the installed system. The package tools will try really hard to only deal with one or two packages at a time, in order to minimize combinatorial complexity, and to have a maximal number of safe points, where an update operation can stop without hosing the whole system. An update set is simply a minimal bag of packages, with old packages that are going to be removed, new packages that are going to replace them, and an area to record related ongoing computations. The old set may be empty, the new set may be empty, and in all cases, the update set shall be small (as small as possible). We have already met with update situations where dependencies between packages invert (A-1.0 depends on B-1.0, but B-0.0 depends on A-0.0), or where files move between packages, which in theory will require update-sets with two new packages that replace two old packages. We still cheat in a few cases, but in most cases, pkg_add(1) will recognize those situations, and merge updatesets as required. pkg_delete(1) also uses package sets, but a simpler variation, known as delete sets. Some update operations may produce inter-dependent packages, and those will have to be deleted together, instead of one after another. OpenBSD::UpdateSet(3p) contains the code for both UpdateSets and DeleteSets for historical reasons.
- updater and tracker
- PackageSets contain some initial information, such as a package name to
install, or a package location to update.
This information will be completed incrementally by a "OpenBSD::Update" updater object, which is responsible for figuring out how to update each element of an updateset, if it is an older package, or to resolve a hint to a package name to a full package location.
In order to avoid loops, a "OpenBSD::Tracker" tracker object keeps track of all the package name statuses: what's queued for update, what is uptodate, or what can't be updated.
pkgdelete(1) uses a simpler tracker, which is currently located inside the OpenBSD::PkgDelete(3p) code.
- dependency information
- Dependency information exists at three levels: first, there are source
specifications within ports. Then, those specifications turn into binary
specifications with more constraints when the package is built by
pkg_create(1), and finally, they're matched against lists of
installed objects when the package is installed, and recorded as lists of
inter-dependencies in the package system.
At the package level, there are currently two types of dependencies: package specifications, that establish direct dependencies between packages, and shared libraries, that are described below.
Normal dependencies are shallow: it is up to the package tools to figure out a whole dependency tree throughout top-level dependencies. None of this is hard-coded: this a prerequisite for flavored packages to work, as we do not want to depend on a specific package if something more generic will do.
At the same time, shared libraries have harsher constraints: a package won't work without the exact same shared libraries it needs (same major number, at least), so shared libraries are handled through a want/provide mechanism that walks the whole dependency tree to find the required shared libraries.
Dependencies are just a subclass of the packing-elements, rooted at the "OpenBSD::PackingElement::Depend" class.
A specific "OpenBSD::Dependencies::Solver" object is used for the resolution of dependencies (see OpenBSD::Dependencies(3p), the solver is mostly a tree-walker, but there are performance considerations, so it also caches a lot of information and cooperates with the "OpenBSD::Tracker". Specificities of shared libraries are handled by OpenBSD::SharedLibs(3p). In particular, the base system also provides some shared libraries which are not recorded within the dependency tree.
Lists of inter-dependencies are recorded in both directions (RequiredBy/Requiring). The OpenBSD::RequiredBy(3p) module handles the subtleties (removing duplicates, keeping things ordered, and handling pretend operations).
- Some items may be recorded multiple times within several packages (mostly directories, users and groups). There is a specific OpenBSD::SharedItems(3p) module which handles these. Mostly, removal operations will scan all packing-lists at high speed to figure out shared items, and remove stuff that's no longer in use.
- virtual file system
- Most package operations will lead to the installation and removal of some files. Everything is checked beforehand: the package system must verify that no new file will erase an existing file, or that the file system won't overflow during the package installation. The package tools also have a "pretend" mode where the user can check what will happen before doing an operation. All the computations and caching are handled through the OpenBSD::Vstat(3p) module, which is designed to hide file system oddities, and to perform addition/deletion operations virtually before doing them for real.
- framework for user interaction
- Most commands are now implemented as perl modules, with
pkg(1) requiring the correct module
"M", and invoking
All those commands use a class derived from "OpenBSD::State" for user interaction. Among other things, "OpenBSD::State" provides for printable, translatable messages, consistent option handling and usage messages.
All commands that provide a progress meter use the derived module "OpenBSD::AddCreateDelete", which contains a derived state class "OpenBSD::AddCreateDelete::State", and a main command class "OpenBSD::AddCreateDelete", with consistent options.
Eventually, this will allow third party tools to simply override the user interface part of "OpenBSD::State"/"OpenBSD::ProgressMeter" to provide alternate displays.
There are three basic operations: package addition (installation), package removal (deinstallation), and package replacement (update).
These operations are achieved through repeating the correct operations on all elements of a packing-list.
For package addition, pkg_add(1) first checks that everything is correct, then runs through the packing-list, and extracts element from the archive.
For package deletion, pkg_delete(1) removes elements from the packing-list, and marks `common' stuff that may need to be unregistered, then walks quickly through all installed packages and removes stuff that's no longer used (directories, users, groups...)
Package replacement is more complicated. It relies on package names and conflict markers.
In normal usage, pkg_add(1) installs only new stuff, and checks that all files in the new package don't already exist in the file system. By convention, packages with the same stem are assumed to be different versions of the same package, e.g., screen-1.0 and screen-1.1 correspond to the same software, and users are not expected to be able to install both at the same time.
This is a conflict.
One can also mark extra conflicts (if two software distributions install the same file, generally a bad idea), or remove default conflict markers (for instance, so that the user can install several versions of autoconf at the same time).
If pkg_add(1) is invoked in replacement mode (-r), it will use conflict information to figure out which package(s) it should replace. It will then operate in a specific mode, where it replaces old package(s) with a new one.
- determine which package to replace through conflict information
- extract the new package 'alongside' the existing package(s) using temporary filenames.
- remove the old package
- finish installing the new package by renaming the temporary files.
Thus replacements will work without needing any extra information besides conflict markers. pkg_add -r will happily replace any package with a conflicting package. Due to missing information (one can't predict the future), conflict markers work both way: packages a and b conflict as soon as a conflicts with b, or b conflicts with a.
Package replacement is the basic operation behind package updates. In your average update, each individual package will be replaced by a more recent one, starting with dependencies, so that the installation stays functional the whole time. Shared libraries enjoy a special status: old shared libraries are kept around in a stub .lib-* package, so that software that depends on them keeps running. (Thus, it is vital that porters pay attention to shared library version numbers during an update.)
An update operation starts with update sets that contain only old packages. There is some specific code (the "OpenBSD::Update" module) which is used to figure out the new package name from the old one.
Note that updates are slightly more complicated than straight replacement: a package may replace an older one if it conflicts with it. But an older package can only be updated if the new package matches (both conflicts and correct pkgpath markers).
In every update or replacement, pkg_add will first try to install or update the quirks package, which contains a global list of exceptions, such as extra stems to search for (allowing for package renames), or packages to remove as they've become part of base OpenBSD.
This search relies on stem names first (e.g., to update package foo-1.0, pkg_add -u will look for foo-* in the PKG_PATH), then it trims the search results by looking more closely inside the package candidates. More specifically, their pkgpath (the directory in the ports tree from which they were compiled). Thus, a package that comes from category/someport/snapshot will never replace a package that comes from category/someport/stable. Likewise for flavors.
Finally, pkg_add -u decides whether the update is needed by comparing the package version and the package signatures: a package will not be downgraded to an older version. A package signature is composed of the name of a package, together with relevant dependency information: all wantlib versions, and all run dependencies versions. pkg_add only replaces packages with different signatures.
Currently, pkg_add -u stops at the first entry in the PKG_PATH from which suitable candidates are found.
BUGS AND LIMITATIONS
There are a few desireable changes that will happen in the future:
- there should be some carefully designed mechanisms to register more `global' processing, to avoid exec/unexec.
LIST OF MODULES
- common operations related to a package addition.
- common operations related to package addition/creation/deletion. Mainly "OpenBSD::ProgressMeter" related.
- common operations used during addition and deletion. Mainly due to the fact that pkg_add(1) will remove packages during updates, and that addition/suppression operations are only allowed to fail at specific times. Most updateset algorithms live there, as does the upper layer framework for handling signals safely.
- additional layer on top of "OpenBSD::Ustar" that matches extra information that the archive format cannot record with a packing-list.
- checks a collision list obtained through "OpenBSD::Vstat" against the full list of installed files, and reports origin of existing files.
- common operations related to package deletion.
- looking up all kind of dependencies. Contains rather complicated caching to speed things up. Interacts with the global tracker object.
- handles signal registration, the exception mechanism, and auto-caching methods. Most I/O operations have moved to "OpenBSD::State".
- Getopt::Std(3p)-like with extra hooks for special options.
- proxy class to go from a package location to an opened package with plist, including state information to cache errors.
- caches uid and gid vs. user names and group names correspondences.
- handles user questions (do not call directly, go through "OpenBSD::State" and derivatives).
- interactions between library objects from packing-lists, library specifications, and matching those against actual lists of libraries (from packages or from the system).
- extends "OpenBSD::LibSpec" for matching during ports builds.
- component for printing information later, to be used by derivative classes of "OpenBSD::State".
- simple parser for mtree(8) specifications.
- code required by pkg_add(1) to handle the removal of old libraries during update.
- handles package meta-information (all the +CONTENTS, +DESCR, etc files)
- proxy for a package, either as a tarball, or an installed package. Obtained through "OpenBSD::PackageRepository".
- central non-OO hub for the normal repository list (should use a singleton pattern instead).
- common operations on package names.
- base class for all package sources. Actual packages instantiate as "OpenBSD::PackageLocation".
- list of package repository, provided as a front to search objects, because searching through a repository list has ld(1)-like semantics (stops at the first repository that matches).
- all the packing-list elements class hierarchy, together with common methods that do not belong elsewhere.
- responsible for reading/writing packing-lists, copying them, comparing them.
- hardcoded paths to external programs and locations.
- OpenBSD::PkgAdd, OpenBSD::PkgCreate, OpenBSD::PkgCheck, OpenBSD::PkgDelete, OpenBSD:PkgInfo
- implements corresponding commands.
- conflict lists handling in an efficient way.
- ad-hoc search for package specifications. External API is stable, but it needs to be updated to use "OpenBSD::PackageName" objects now that they exist.
- handles display of a progress meter when a terminal is available, devolves to nothings otherwise.
- common operations related to package replacement.
- handles requiredby and requiring lists.
- search object for package repositories: specs, stems, and pkgpaths.
- handles items that may be shared by several packages.
- shared library specificities when handled as dependencies.
- handles package signatures and the corresponding version comparison (do not confuse with cryptographic signatures, as handled through "OpenBSD::x509").
- base class to UI and option handling.
- conventions used for substituting variables during pkg_create(1), and related algorithms.
- safe creation of temporary files as a light-weight module that also deals with signal issues.
- tracks all package names through update operations, in order to avoid loops while doing incremental updates.
- incremental computation of package replacements required by an update or installation.
- common operations to all package tools that manipulate update sets.
- simple API that allows for Ustar (new tar) archive manipulation, allowing for extraction and copies on the fly.
- virtual file system (pretend) operations.
- simple interface to the Digest::MD5(3p) and Digest::SHA(3p) modules.
- cryptographic signature through x509 certificates. Mostly calls openssl(1). Note that "OpenBSD::ArcCheck" is vital in ensuring archive meta-info have not been tampered with.