Re: [RFC] v2 Documentation: dependencies specification in Bitbake and Debian

["Cirujano Cuesta, Silvano" <silvano.cirujano-cuesta@siemens.com>]

Reposting my first message on this thread adding the patch for mailing list review.


After my first try a couple of weeks ago, I'm coming back with a new try to improve the documentation.

I've taken Claudius feedback into account (thank you again for your very valuable input!).
And I let two colleagues of mine with a similar superficial/deep knowledge of ISAR review it.

Right now it's everything in a single MarkDown document, but I'd rather put the section "Dependency specification variables" into a separate document.
I keep it together right now to simplify the reviews.

For those of you who like to have some comfort in life, I've pushed the document to a fork of ISAR in my personal area.
That way you can take advantage of MarkDown conversion to HTML and rendering going to [1] ;-)

[1] https://github.com/Silvanoc/isar/blob/silvano/dependencies-documentation/doc/dependencies.md

Signed-off-by: Silvano Cirujano Cuesta <silvano.cirujano-cuesta@siemens.com>
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+# Dependency management in ISAR
+
+## Introduction
+
+The typical goal of an ISAR build is to obtain:
+
+1. a disk image containing one or more partition images (all optional),
+1. containing one or more Debian root filesystems with
+1. certain Debian packages installed.
+
+Reaching this goal relies on the availability of the above listed elements on *runtime*, the specification of the **[runtime environments](#runtime-environment-specification)** helps the different
build tools ensuring the availability of the mentioned elements on runtime.
+
+Usually some of the runtime dependencies have to be built, and it happens in special environments. These **[buildtime environments](#buildtime-environment-specification)** help the different build
tools preparing the environment for building the runtime dependencies.
+
+The differentiation between buildtime and runtime environments is very important.
+The fact that ISAR uses Debian both for buildtime and runtime environments might mislead some people.
+In general the names of the variables being used to specify those dependencies aren't supporting this differentiation, but difficulting it.
+
+Not only the build environments have to be specified, but also the **[build process](#build-process-specification)** that takes care of building and integrating both the runtime dependencies and the
buildtime dependencies.
+
+## Runtime environment specification
+
+As mentioned above, ISAR can build [partitioned disk images](#disk-image) or only [root filesystems](#root-filesystem).
+
+Let's first analyze how the content of the different runtime containers can be specified from bigger to smaller container.
+
+```
++---------------------------------------------------------------------------------------------------+
+|                                                                                                   |
+|    DISK IMAGE                                                                                     |
+|                                                                                                   |
+|   +------------+   +-----------------------------------+   +------------------------------+       |
+|   |            |   |                                   |   |                              ||      |
+|   | Bootloader |   | Bootable partition                |   | Other partitions             |||     |
+|   |            |   |                                   |   |                              ||||    |
+|   |            |   | +------------------------------+  |   |                              ||||    |
+|   |            |   | |                              |  |   |                              ||||    |
+|   |            |   | |  ROOT FILESYSTEM             |  |   |                              ||||    |
+|   +------------+   | |                              |  |   |                              ||||    |
+|                    | |    +-------------------+     |  |   |                              ||||    |
+|                    | |    |                   | |   |  |   |                              ||||    |
+|                    | |    |  DEBIAN PACKAGES  | |   |  |   |                              ||||    |
+|                    | |    |                   | |   |  |   |                              ||||    |
+|                    | |    |                   | |   |  |   |                              ||||    |
+|                    | |    |                   | |   |  |   |                              ||||    |
+|                    | |    +-------------------+ |   |  |   |                              ||||    |
+|                    | |     ---------------------+   |  |   |                              ||||    |
+|                    | |                              |  |   +------------------------------+|||    |
+|                    | +------------------------------+  |    -------------------------------+||    |
+|                    |                                   |    --------------------------------+|    |
+|                    +-----------------------------------+     --------------------------------+    |
+|                                                                                                   |
++---------------------------------------------------------------------------------------------------+
+```
+
+### Disk image
+
+A disk image depends on:
+
+1. a bootloader (optional) and
+1. one or more partitions, containing one of them a [root filesystem](#root-filesystem).
+
+The ISAR way to **specify** them is through *WKS files* (OpenEmbedded KickStart files).
+ISAR doesn't change it in any significant way, therefore the corresponding [Yocto documentation][wks] provides the best reference.
+
+### Root filesystem
+
+A root filesystem depends on:
+
+1. a minimal set of files (the result of `debootstrap`) and
+1. additional files and configurations obtained from *Debian packages*.
+
+The ISAR way to **specify** them is quite complex and involves *ISAR variables* that can be found in configuration files or recipes and artifacts and metadata being provided by *Debian packages*.
+
+See following sections for more information about how to specify the content of the root filesystem:
+
+* [ISAR `IMAGE_INSTALL` variable](#isar-image_install-variable)
+* [ISAR `IMAGE_PREINSTALL` variable](#isar-image_preinstall-variable)
+* [ISAR `DEBIAN_DEPENDS` variable](#isar-debian_depends-variable)
+* [Debian `Depends` control-file value](#debian-depends-control-file-value)
+
+## Buildtime environment specification
+
+The creation of the [above mentioned runtime units](#runtime-environment-specification) (disk images, bootloaders, root filesystems, packages, ...) takes place in different environments.
+
+These environments have to be specified.
+
+### Buildchroot
+
+The *Debian packages* are built in the so-called *buildchroot* filesystem.
+
+It's a Debian chroot that is used to build custom Debian packages required by the **[root filesystem](#root-filesystem)**.
+This build environment depends on the presence of certain artifacts (compilers, configurations, linkers, ...).
+
+The ISAR way to **specify** them is quite complex and involves *ISAR variables* that can be found in configuration files or recipes and artifacts and metadata being provided by *Debian packages*.
+
+See following sections for more information about how to specify the content of the chroot:
+
+* [ISAR `IMAGER_INSTALL` variable](#isar-image_install-variable)
+* [ISAR `BUILDCHROOT_PREINSTALL` variable](#isar-buildchroot_preinstall-variable)
+* [Debian `Build-Depends` control-file value](#debian-build-depends-control-file-value)
+
+If a disk image is being created, the same environment is being used for building the image.
+For this usage additional dependencies might appear. See the [imager](#imager) section for more information.
+
+### Imager
+
+The *disk images* are built in the so-called *imager*.
+
+It's a Debian chroot that is used to build **[disk images](#disk-image)**.
+This build environment depends on the presence of certain artifacts (compilers, configurations, linkers, ...).
+Although conceptually the imager doesn't have anything to do with the buildchroot, in the implementation the same chroot is being used for both tasks.
+
+The ISAR way to **specify** them involves *ISAR variables* that can be found in configuration files or recipes.
+
+See following sections for more information about how to specify the content of the chroot:
+
+* [ISAR `WIC_IMAGER_INSTALL` variable](#isar-wic_imager_install-variable)
+* [ISAR `IMAGER_BUILD_DEPS` variable](#isar-imager_build_deps-variable)
+
+### Target filesystem
+
+It's a Debian root filesystem that results from creating a minimalistic Debian skeleton via `debootstrap` and installing Debian packages on it, resulting in the [root filesystem](#root-filesystem).
+
+There's no need to **specify** this build environment, since it's the [root filesystem of the runtime environment](#root-filesystem).
+
+## Build process specification
+
+ISAR relies on a subset of Bitbake to take care of the build process.
+Therefore the ISAR way to **specify** the different tasks involved in the process is Bitbake recipes with some *Bitbake variables* (well documented ) and some *ISAR variables* (some of them are only
syntactic sugar).
+
+See following sections for more information about how to specify inter-tasks dependencies:
+
+* [Bitbake `DEPENDS` variable](#bitbake-depends-variable)
+* [ISAR `IMAGE_INSTALL` variable](#isar-image_install-variable)
+* [ISAR `IMAGER_BUILD_DEPS` variable](#isar-imager_build_deps-variable)
+
+## Mapping: Variables <=> Build environment and Stages
+
+| Variable | Environment | Stage |
+|:-------- |:-----------------:|:-----:|
+| [`DEPENDS`](#bitbake-depends-variable) | | All |
+| [`IMAGE_INSTALL`](#isar-image_install-variable) | [Root filesystem](#root-filesystem) | [Root filesystem](#root-filesystem) preparation and build |
+| [`IMAGE_PREINSTALL`](#isar-image_preinstall-variable) | [Root filesystem](#root-filesystem) | |
+| [`DEBIAN_DEPENDS`](#isar-debian_depends-variable) | [Root filesystem](#root-filesystem) | |
+| [`Depends` control-file](#debian-depends-control-file-value) | [Root filesystem](#root-filesystem) | |
+| [`IMAGE_TRANSIENT_PACKAGES`](#isar-image_transient_packages-variable) | [Root filesystem](#root-filesystem) | [Root filesystem](#root-filesystem) preparation and build |
+| [`BUILDCHROOT_PREINSTALL`](#isar-buildchroot_preinstall-variable) | [Buildchroot](#buildroot) | |
+| [`Build-Depends` control-file](#debian-build-depends-control-file-value) | [Buildchroot](#buildroot) | |
+| [`IMAGER_BUILD_DEPS`](#isar-imager_build_deps-variable) | | [Imager](#imager) preparation |
+| [`IMAGER_INSTALL`](#isar-imager_install-variable) | [Imager](#imager) | |
+| [`WIC_IMAGER_INSTALL`](#isar-wic_imager_install-variable) | [Imager](#imager) | |
+
+## Dependencies specification variables
+
+### Bitbake `DEPENDS` variable
+
+This is a **Bitbake** variable that specifies **Bitbake inter-task dependencies**.
+
+Since it specifies the recipes that another recipe depend upon, this variable can be specified in **package recipes** and **image recipes**.
+
+### ISAR `IMAGE_INSTALL` variable
+
+This is originally a **Bitbake** variable that has conceptually the same goal in ISAR as in Yocto.
+In the case of ISAR it specifies at the same time **Bitbake inter-task dependencies** and **image custom Debian package dependencies**.
+
+It basically specifies that a **[root filesystem](#root-filesystem)** (the prefix 'IMAGE' can be misleading here) requires a custom Debian package.
+But since custom Debian packages are being built from package recipes, this variable also specifies that an image recipe depends on package recipes.
+
+For this "magic" to work both the Debian package and the package recipe have to have the same name.
+Something else than this 1:1 behaviour (e.g. a recipe creating multiple Debian binary packages out of a single Debian source package) requires some hacking out of the scope of this document.
+
+This variable can be found in **configuration files** and **image recipes**.
+
+### ISAR `IMAGE_PREINSTALL` variable
+
+This is an **ISAR-only** variable that specifies **image upstream Debian package dependencies**.
+
+It specifies that a **[root filesystem](#root-filesystem)** requires certain *upstream Debian packages*, being "upstream Debian packages" ready built packages that can be downloaded from a Debian
package repository.
+
+This variable can be found in **configuration files** and **image recipes**.
+
+### ISAR `DEBIAN_DEPENDS` variable
+
+This is an **ISAR-only** variable that specifies **Debian inter-package dependencies** and can be used only on package recipes inheriting from the **`dpkg-raw` class**.
+
+It lists the Debian binary packages that a custom Debian package depends upon.
+It can be used for both upstream and custom packages.
+It results in an entry in the corresponding **[Debian `Depends` control-file value](#debian-depends-control-file-value)**.
+
+A common usage is for creating meta-packages that only keep dependencies on further packages synchronized.
+
+This variable can be found in **package recipes**.
+
+### ISAR `IMAGE_TRANSIENT_PACKAGES` variable
+
+This is an **ISAR-only** variable that specifies **image custom Debian package dependencies** and **Bitbake inter-task dependencies**.
+
+This is a variable that is rarely needed and only for a very specific use case!
+That is when a package is needed in the **[root filesystem](#root-filesystem)** during the **root filesystem build stage**, but not in the final **[root filesystem](#root-filesystem)**.
+Meaning that the installation is only transient, therefore its name.
+
+This variable can be found in **configuration files** and **image recipes**.
+
+### Debian `Depends` control-file value
+
+This is an **Debian** variable that specifies **Debian inter-package dependencies**.
+
+`Depends` specifies the Debian packages that are required in the [runtime environment](#runtime-environment-specification) by the package declaring the dependencies.
+
+It relies on standard Debian mechanisms, therefore the corresponding [official Debian documentation][pkg-rels] provides the best reference.
+
+### ISAR `BUILDCHROOT_PREINSTALL` variable
+
+This is an **ISAR-only** variable that specifies **buildchroot Debian package dependencies**.
+
+It specifies that **[buildchroot](#buildroot)** requires certain *Debian packages*.
+
+This variable can be found in **buildchroot recipes**.
+
+### Debian `Build-Depends` control-file value
+
+This is an **Debian** variable that specifies **Debian inter-package dependencies**.
+
+`Build-Depends` specifies the Debian packages that are required in the [buildtime environment](#buildtime-environment-specification) to build the package declaring the dependencies.
+
+It relies on standard Debian mechanisms, therefore the corresponding [official Debian documentation][pkg-rels] provides the best reference.
+
+### ISAR `IMAGER_BUILD_DEPS` variable
+
+This is an **ISAR-only** variable that specifies **Bitbake inter-task dependencies**.
+
+It has exactly the same effect as `DEPENDS`.
+It simply provides syntactic sugar to separate dependencies on recipes for building the runtime environment from dependencies on recipes for building the **[imager](#imager)**.
+
+This variable can be found in **configuration files**.
+
+### ISAR `IMAGER_INSTALL` variable
+
+This is an **ISAR-only** variable that specifies **imager Debian package dependencies**.
+
+It specifies that certain *Debian packages* have to be installed on the **[imager](#imager)**.
+
+This variable can be found typically in **configuration files**, although it could also be part of **image recipes**.
+
+### ISAR `WIC_IMAGER_INSTALL` variable
+
+This is an **ISAR-only** variable that specifies **imager Debian package dependencies**.
+
+It specifies that an **[imager](#imager)** requires certain *Debian packages* so that WIC can build the disk images (e.g. `parted`).
+
+This variable can be found typically in **configuration files**, although it could also be part of **image recipes**.
+
+[wks]: https://www.yoctoproject.org/docs/current/ref-manual/ref-manual.html#ref-kickstart "OpenEmbedded Kickstart (.wks) Reference"
+[pkg-rels]: https://www.debian.org/doc/debian-policy/ch-relationships.html "Debian policy: Declaring relationships between packages"
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