**Milestone 2: Unification of generating binary R packages across operating
systems, and the refactoring of versioning and dependency resolution**

This work item aimed to unify and generalise the process generation of
binary packages across operating systems such that it is not limited to
macOS and Windows, and to implement a system of binary versioning such
that re-builds of binaries can propagate through dependencies. This is
important to make sure the users can reliably update to the latest built
version with the latest security updates, and unification aids
maintenance and resilience of the code.

**Milestone Results**

The requirements for generalisation of binary packages have been
collected by consulting various vendors of 3rd party packages and their
use. Similarly the requirements for repositories to facilitate
dependency resolution and a way to define the state when a binary needs
to be re-installed even if the source package has not changed have been
devised. The design has been finalised to comprise of three new
features:

1.  Way for [R]{.sans-serif} distributions to define custom native build
    types.

2.  Definition of custom package build types and corresponding
    repository layout specification.

3.  Unified binary package handling with a way to record and utilise
    build date as a way to define when a binary is outdated.

Previously, only the macOS CRAN [R]{.sans-serif} distribution was able
to specify the native binary type, this concept has now been made
generally available to any distribution by supporting the
`R_PLATFORM_PKGTYPE` environment variable which can be set in the
distribution's `Renviron` file (see r89368 in the [R]{.sans-serif}
source SVN).

The repository layout is defined by the `contrib.url()` function where a
binary type has the form `<system>.binary.<build>` which results in the
directory layout of

::: center
`bin/<system>/<build>/contrib/<x.y>`
:::

where `<x.y>` is the [R]{.sans-serif} version without patch level (see
r89369 in the [R]{.sans-serif} source SVN).

The binary package format has been unified to be a compressed `tar`
archive with the possible compressions `bzip2`, `zx` or `zstd`, i.e.,
the corresponding extensions `.tar.bz2`, `.tar.xz`, `.tar.zst` and
`.tar.zstd`. The repository metadata has been augmented accordingly to
provide `File` entry to make the file name deterministic from the
metadata (see r89370 in the [R]{.sans-serif} source SVN).

As a test-case for this new feature the Sonoma build [R]{.sans-serif}
macOS binaries have been used to bypass the legacy binary support and
use the new system by declaring `macos.binary.sonoma-arm64` native
package type and `.tar.zstd` as the package extension. Similarly, the
same can be used for other custom binaries such as the "manylinux"
binaries.

The dependency resolution and update management has been augmented to
record and pass the `Built` field in both the repository and package
metadata. The `old.packages()` (and by proxy `update.packages()`)
function uses the metadata to compare built times in addition the
package versions for binary packages to re-install packages in cases
where a security update has been published in the binary repository (see
r89371 in the [R]{.sans-serif} source SVN).

For the corresponding `NEWS` entries in [R]{.sans-serif} sources see
r89372 in the R source SVN (<https://svn.r-project.org/R/trunk>).