

                        ============================
                        Package management on Genode
                        ============================


                               Norman Feske



Motivation and inspiration
##########################

The established system-integration work flow with Genode is based on
the 'run' tool, which automates the building, configuration, integration,
and testing of Genode-based systems. Whereas the run tool succeeds in
overcoming the challenges that come with Genode's diversity of kernels and
supported hardware platforms, its scalability is somewhat limited to
appliance-like system scenarios: The result of the integration process is
a system image with a certain feature set. Whenever requirements change,
the system image is replaced with a new created image that takes those
requirements into account. In practice, there are two limitations of this
system-integration approach:

First, since the run tool implicitly builds all components required for a
system scenario, the system integrator has to compile all components from
source. E.g., if a system includes a component based on Qt5, one needs to
compile the entire Qt5 application framework, which induces significant
overhead to the actual system-integration tasks of composing and configuring
components.

Second, general-purpose systems tend to become too complex and diverse to be
treated as system images. When looking at commodity OSes, each installation
differs with respect to the installed set of applications, user preferences,
used device drivers and system preferences. A system based on the run tool's
work flow would require the user to customize the run script of the system for
each tweak. To stay up to date, the user would need to re-create the
system image from time to time while manually maintaining any customizations.
In practice, this is a burden, very few end users are willing to endure.

The primary goal of Genode's package management is to overcome these
scalability limitations, in particular:

* Alleviating the need to build everything that goes into system scenarios
  from scratch,
* Facilitating modular system compositions while abstracting from technical
  details,
* On-target system update and system development,
* Assuring the user that system updates are safe to apply by providing the
  ability to easily roll back the system or parts thereof to previous versions,
* Securing the integrity of the deployed software,
* Fostering a federalistic evolution of Genode systems,
* Low friction for existing developers.

The design of Genode's package-management concept is largely influenced by Git
as well as the [https://nixos.org/nix/ - Nix] package manager. In particular
the latter opened our eyes to discover the potential that lies beyond the
package management employed in state-of-the art commodity systems. Even though
we considered adapting Nix for Genode and actually conducted intensive
experiments in this direction (thanks to Emery Hemingway who pushed forward
this line of work), we settled on a custom solution that leverages Genode's
holistic view on all levels of the operating system including the build system
and tooling, source structure, ABI design, framework API, system
configuration, inter-component interaction, and the components itself. Whereby
Nix is designed for being used on top of Linux, Genode's whole-systems view
led us to simplifications that eliminated the needs for Nix' powerful features
like its custom description language.


Nomenclature
############

When speaking about "package management", one has to clarify what a "package"
in the context of an operating system represents. Traditionally, a package
is the unit of delivery of a bunch of "dumb" files, usually wrapped up in
a compressed archive. A package may depend on the presence of other
packages. Thereby, a dependency graph is formed. To express how packages fit
with each other, a package is usually accompanied with meta data
(description). Depending on the package manager, package descriptions follow
certain formalisms (e.g., package-description language) and express
more-or-less complex concepts such as versioning schemes or the distinction
between hard and soft dependencies.

Genode's package management does not follow this notion of a "package".
Instead of subsuming all deliverable content under one term, we distinguish
different kinds of content, each in a tailored and simple form. To avoid the
clash of the notions of the common meaning of a "package", we speak of
"archives" as the basic unit of delivery. The following subsections introduce
the different categories.
Archives are named with their version as suffix, appended via a slash. The
suffix is maintained by the author of the archive. The recommended naming
scheme is the use of the release date as version suffix, e.g.,
'report_rom/2017-05-14'.


Raw-data archives
=================

A raw-data archive contains arbitrary data that is - in contrast to executable
binaries - independent from the processor architecture. Examples are
configuration data, game assets, images, or fonts. The content of raw-data
archives is expected to be consumed by components at runtime. It is not
relevant for the build process for executable binaries. Each raw-data
archive contains merely a collection of data files. There is no meta data.


API archive
===========

An API archive has the structure of a Genode source-code repository. It may
contain all the typical content of such a source-code repository such as header
files (in the _include/_ subdirectory), source codes (in the _src/_
subdirectory), library-description files (in the _lib/mk/_ subdirectory), or
ABI symbols (_lib/symbols/_ subdirectory). At the top level, a LICENSE file is
expected that clarifies the license of the contained source code. There is no
meta data contained in an API archive.

An API archive is meant to provide _ingredients_ for building components. The
canonical example is the public programming interface of a library (header
files) and the library's binary interface in the form of an ABI-symbols file.
One API archive may contain the interfaces of multiple libraries. For example,
the interfaces of libc and libm may be contained in a single "libc" API
archive because they are closely related to each other. Conversely, an API
archive may contain a single header file only. The granularity of those
archives may vary. But they have in common that they are used at build time
only, not at runtime.


Source archive
==============

Like an API archive, a source archive has the structure of a Genode
source-tree repository and is expected to contain all the typical content of
such a source repository along with a LICENSE file. But unlike an API archive,
it contains descriptions of actual build targets in the form of Genode's usual
'target.mk' files.

In addition to the source code, a source archive contains a file
called 'used_apis', which contains a list of API-archive names with each
name on a separate line. For example, the 'used_apis' file of the 'report_rom'
source archive looks as follows:

! base/2017-05-14
! os/2017-05-13
! report_session/2017-05-13

The 'used_apis' file declares the APIs needed to incorporate into the build
process when building the source archive. Hence, they represent _build-time_
_dependencies_ on the specific API versions.

A source archive may be equipped with a top-level file called 'api' containing
the name of exactly one API archive. If present, it declares that the source
archive _implements_ the specified API. For example, the 'libc/2017-05-14'
source archive contains the actual source code of the libc and libm as well as
an 'api' file with the content 'libc/2017-04-13'. The latter refers to the API
implemented by this version of the libc source package (note the differing
versions of the API and source archives)


Binary archive
==============

A binary archive contains the build result of the equally-named source archive
when built for a particular architecture. That is, all files that would appear
at the _<build-dir>/bin/_ subdirectory when building all targets present in
the source archive. There is no meta data present in a binary archive.

A binary archive is created out of the content of its corresponding source
archive and all API archives listed in the source archive's 'used_apis' file.
Note that since a binary archive depends on only one source archive, which
has no further dependencies, all binary archives can be built independently
from each other.
For example, a libc-using application needs the source code of the
application as well as the libc's API archive (the libc's header file and
ABI) but it does not need the actual libc library to be present.


Package archive
===============

A package archive contains an 'archives' file with a list of archive names
that belong together at runtime. Each listed archive appears on a separate line.
For example, the 'archives' file of the package archive for the window
manager 'wm/2017-05-31' looks as follows:

! genodelabs/raw/wm/2017-05-31
! genodelabs/src/wm/2017-05-31
! genodelabs/src/report_rom/2017-05-31
! genodelabs/src/decorator/2017-05-31
! genodelabs/src/floating_window_layouter/2017-05-31

In contrast to the list of 'used_apis' of a source archive, the content of
the 'archives' file denotes the origin of the respective archives
("genodelabs"), the archive type, followed by the versioned name of the
archive.

An 'archives' file may specify raw archives, source archives, or package
archives (as type 'pkg'). It thereby allows the expression of _runtime
dependencies_. If a package archive lists another package archive, it inherits
the content of the listed archive. This way, a new package archive may easily
customize an existing package archive.

A package archive does not specify binary archives directly as they differ
between the architecture and are already referenced by the source archives.

In addition to an 'archives' file, a package archive is expected to contain
a 'README' file explaining the purpose of the collection.


Depot structure
###############

Archives are stored within a directory tree called _depot/_. The depot
is structured as follows:

! <user>/pubkey
! <user>/download
! <user>/src/<name>/<version>/
! <user>/api/<name>/<version>/
! <user>/raw/<name>/<version>/
! <user>/pkg/<name>/<version>/
! <user>/bin/<arch>/<src-name>/<src-version>/

The <user> stands for the origin of the contained archives. For example, the
official archives provided by Genode Labs reside in a _genodelabs/_
subdirectory. Within this directory, there is a 'pubkey' file with the
user's public key that is used to verify the integrity of archives downloaded
from the user. The file 'download' specifies the download location as an URL.

Subsuming archives in a subdirectory that correspond to their the origin
(user) serves two purposes. First, it provides a user-local name space for
versioning archives. E.g., there might be two versions of a
'nitpicker/2017-04-15' source archive, one by "genodelabs" and one by
"nfeske". However, since each version resides under its origin's subdirectory,
version-naming conflicts between different origins cannot happen. Second, by
allowing multiple archive origins in the depot side-by-side, package archives
may incorporate archives of different origins, which fosters the goal of a
federalistic development, where contributions of different origins can be
easily combined.

The actual archives are stored in the subdirectories named after the archive
types ('raw', 'api', 'src', 'bin', 'pkg'). Archives contained in the _bin/_
subdirectories are further subdivided in the various architectures (like
'x86_64', or 'arm_v7').


Depot management
################

The tools for managing the depot content reside under the _tool/depot/_
directory. When invoked without arguments, each tool prints a brief
description of the tool and its arguments.

Unless stated otherwise, the tools are able to consume any number of archives
as arguments. By default, they perform their work sequentially. This can be
changed by the '-j<N>' argument, where <N> denotes the desired level of
parallelization. For example, by specifying '-j4' to the _tool/depot/build_
tool, four concurrent jobs are executed during the creation of binary archives.


Downloading archives
====================

The depot can be populated with archives in two ways, either by creating
the content from locally available source codes as explained by Section
[Automated extraction of archives from the source tree], or by downloading
ready-to-use archives from a web server.

In order to download archives originating from a specific user, the depot's
corresponding user subdirectory must contain two files:

:_pubkey_: contains the public key of the GPG key pair used by the creator
  (aka "user") of the to-be-downloaded archives for signing the archives. The
  file contains the ASCII-armored version of the public key.

:_download_: contains the base URL of the web server where to fetch archives
  from. The web server is expected to mirror the structure of the depot.
  That is, the base URL is followed by a sub directory for the user,
  which contains the archive-type-specific subdirectories.

If both the public key and the download locations are defined, the download
tool can be used as follows:

! ./tool/depot/download genodelabs/src/zlib/2017-05-31

The tool automatically downloads the specified archives and their
dependencies. For example, as the zlib depends on the libc API, the libc API
archive is downloaded as well. All archive types are accepted as arguments
including binary and package archives. Furthermore, it is possible to download
all binary archives referenced by a package archive. For example, the
following command downloads the window-manager (wm) package archive including
all binary archives for the 32-bit x86 architecture. Downloaded binary
archives are always accompanied with their corresponding source and used API
archives.

! ./tool/depot/download genodelabs/pkg/x86_32/wm/2017-05-31

Archive content is not downloaded directly to the depot. Instead, the
individual archives and signature files are downloaded to a quarantine area in
the form of a _public/_ directory located in the root of Genode's source tree.
As its name suggests, the _public/_ directory contains data that is imported
from or to-be exported to the public. The download tool populates it with the
downloaded archives in their compressed form accompanied with their
signatures.

The compressed archives are not extracted before their signature is checked
against the public key defined at _depot/<user>/pubkey_. If however the
signature is valid, the archive content is imported to the target destination
within the depot. This procedure ensures that depot content - whenever
downloaded - is blessed by a cryptographic signature of its creator.


Building binary archives from source archives
=============================================

With the depot populated with source and API archives, one can use the
_tool/depot/build_ tool to produce binary archives. The arguments have the
form '<user>/bin/<arch>/<src-name>' where '<arch>' stands for the targeted
CPU architecture. For example, the following command builds the 'zlib'
library for the 64-bit x86 architecture. It executes four concurrent jobs
during the build process.

! ./tool/depot/build genodelabs/bin/x86_64/zlib/2017-05-31 -j4

Note that the command expects a specific version of the source archive as
argument. The depot may contain several versions. So the user has to decide,
which one to build.

After the tool is finished, the freshly built binary archive can be found in
the depot within the _genodelabs/bin/<arch>/<src>/<version>/_ subdirectory.
Only the final result of the built process is preserved. In the example above,
that would be the _zlib.lib.so_ library.

For debugging purposes, it might be interesting to inspect the intermediate
state of the build. This is possible by adding 'KEEP_BUILD_DIR=1' as argument
to the build command. The binary's intermediate build directory can be
found besides the binary archive's location named with a '.build' suffix.

By default, the build tool won't attempt to rebuild a binary archive that is
already present in the depot. However, it is possible to force a rebuild via
the 'REBUILD=1' argument.


Publishing archives
===================

Archives located in the depot can be conveniently made available to the public
using the _tool/depot/publish_ tool. Given an archive path, the tool takes
care of determining all archives that are implicitly needed by the specified
one, wrapping the archive's content into compressed tar archives, and signing
those.

As a precondition, the tool requires you to possess the private key that
matches the _depot/<you>/pubkey_ file within your depot. The key pair should
be present in the key ring of your GNU privacy guard.

To publish archives, one needs to specify the specific version to publish.
For example:

! ./tool/depot/publish <you>/pkg/wm/2017-05-31

The command checks that the specified archive and all dependencies are present
in the depot. It then proceeds with the archiving and signing operations. For
the latter, the pass phrase for your private key will be requested. The
publish tool prints the information about the processed archives, e.g.:

! publish /.../public/<you>/pkg/wm/2017-05-30.tar.xz
! publish /.../public/<you>/src/decorator/2017-05-30.tar.xz
! publish /.../public/<you>/src/floating_window_layouter/2017-05-30.tar.xz
! publish /.../public/<you>/src/report_rom/2017-05-30.tar.xz
! publish /.../public/<you>/src/wm/2017-05-30.tar.xz
! publish /.../public/<you>/raw/wm/2017-05-30.tar.xz
! publish /.../public/<you>/api/base/2017-05-30.tar.xz
! publish /.../public/<you>/api/framebuffer_session/2017-05-30.tar.xz
! publish /.../public/<you>/api/gems/2017-05-30.tar.xz
! publish /.../public/<you>/api/input_session/2017-05-30.tar.xz
! publish /.../public/<you>/api/nitpicker_gfx/2017-04-24.tar.xz
! publish /.../public/<you>/api/nitpicker_session/2017-05-30.tar.xz
! publish /.../public/<you>/api/os/2017-05-30.tar.xz
! publish /.../public/<you>/api/report_session/2017-05-30.tar.xz
! publish /.../public/<you>/api/scout_gfx/2017-04-24.tar.xz

According to the output, the tool populates a directory called _public/_
at the root of the Genode source tree with the to-be-published archives.
The content of the _public/_ directory is now ready to be copied to a
web server, e.g., by using rsync.


Automated extraction of archives from the source tree
#####################################################

Genode users are expected to populate their local depot with content obtained
via the _tool/depot/download_ tool. However, Genode developers need a way to
create depot archives locally in order to make them available to users. Thanks
to the _tool/depot/extract_ tool, the assembly of archives does not need to be
a manual process. Instead, archives can be conveniently generated out of the
source codes present in the Genode source tree and the _contrib/_ directory.

However, the granularity of splitting source code into archives, the
definition of what a particular API entails, and the relationship between
archives must be augmented by the archive creator as this kind of information
is not present in the source tree as is. This is where so-called "archive
recipes" enter the picture. An archive recipe defines the content of an
archive. Such recipes can be located at an _recipes/_ subdirectory of any
source-code repository, similar to how port descriptions and run scripts
are organized. Each _recipe/_ directory contains subdirectories for the
archive types, which, in turn, contain a directory for each archive. The
latter is called a _recipe directory_.

Recipe directory
----------------

The recipe directory is named after the archive _omitting the archive version_
and contains at least one file named _hash_. This file defines the version
of the archive along with a hash value of the archive's content
separated by a space character. By tying the version name to a particular hash
value, the _extract_ tool is able to detect the appropriate points in time
whenever the version should be increased due to a change of the archive's
content.

API, source, and raw-data archive recipes
-----------------------------------------

Recipe directories for API, source, or raw-data archives contain a
_content.mk_ file that defines the archive content in the form of make
rules. The content.mk file is executed from the archive's location within
the depot. Hence, the contained rules can refer to archive-relative files as targets.
The first (default) rule of the content.mk file is executed with a customized
make environment:

:GENODE_DIR: A variable that holds the path to root of the Genode source tree,
:REP_DIR: A variable with the path to source code repository where the recipe
  is located
:port_dir: A make function that returns the directory of a port within the
  _contrib/_ directory. The function expects the location of the
  corresponding port file as argument, for example, the 'zlib' recipe
  residing in the _libports/_ repository may specify '$(REP_DIR)/ports/zlib'
  to access the 3rd-party zlib source code.

Source archive recipes contain simplified versions of the 'used_apis' and
(for libraries) 'api' files as found in the archives. In contrast to the
depot's counterparts of these files, which contain version-suffixed names,
the files contained in recipe directories omit the version suffix. This
is possible because the extract tool always extracts the _current_ version
of a given archive from the source tree. This current version is already
defined in the corresponding recipe directory.

Package-archive recipes
-----------------------

The recipe directory for a package archive contains the verbatim content of
the to-be-created package archive except for the _archives_ file. All other
files are copied verbatim to the archive. The content of the recipe's
_archives_ file may omit the version information from the listed ingredients.
Furthermore, the user part of each entry can be left blank by using '_' as a
wildcard. When generating the package archive from the recipe, the extract
tool will replace this wildcard with the user that creates the archive.


Convenience front-end to the extract, build tools
#################################################

For developers, the work flow of interacting with the depot is most often the
combination of the _extract_ and _build_ tools whereas the latter expects
concrete version names as arguments. The _create_ tool accelerates this common
usage pattern by allowing the user to omit the version names. Operations
implicitly refer to the _current_ version of the archives as defined in
the recipes.

Furthermore, the _create_ tool is able to manage version updates for the
developer. If invoked with the argument 'UPDATE_VERSIONS=1', it automatically
updates hash files of the involved recipes by taking the current date as
version name. This is a valuable assistance in situations where a commonly
used API changes. In this case, the versions of the API and all dependent
archives must be increased, which would be a labour-intensive task otherwise.
If the depot already contains an archive of the current version, the create
tools won't re-create the depot archive by default. Local modifications of
the source code in the repository do not automatically result in a new archive.
To ensure that the depot archive is current, one can specify 'FORCE=1' to
the create tool. With this argument, existing depot archives are replaced by
freshly extracted ones and version updates are detected. When specified for
creating binary archives, 'FORCE=1' normally implies 'REBUILD=1'. To prevent
the superfluous rebuild of binary archives whose source versions remain
unchanged, 'FORCE=1' can be combined with the argument 'REBUILD='.


Accessing depot content from run scripts
########################################

The depot tools are not meant to replace the run tool but rather to complement
it. When both tools are combined, the run tool implicitly refers to "current"
archive versions as defined for the archive's corresponding recipes. This way,
the regular run-tool work flow can be maintained while attaining a
productivity boost by fetching content from the depot instead of building it.

Run scripts can use the 'import_from_depot' function to incorporate archive
content from the depot into a scenario. The function must be called after the
'create_boot_directory' function and takes any number of pkg, src, or raw
archives as arguments. An archive is specified as depot-relative path of the
form '<user>/<type>/name'. Run scripts may call 'import_from_depot'
repeatedly. Each argument can refer to a specific version of an archive or
just the version-less archive name. In the latter case, the current version
(as defined by a corresponding archive recipe in the source tree) is used.

If a 'src' archive is specified, the run tool integrates the content of
the corresponding binary archive into the scenario. The binary archives
are selected according the spec values as defined for the build directory.

