software-origins/nixguix: Document origins and link to loaders

Refs. swh/devel/swh-loader-core#4749

docs/user/software-origins/nixguix.rst

@@ -3,17 +3,86 @@
 Nix and Guix
 ============
 
-.. todo::
-   This page is a work in progress.
-
 .. include:: dynamic/nixguix_status.inc
 
-TODO:
+This page documents how |swh| archives source packages from the `GNU Guix`_ and Nix_
+distributions.
+
+Those distributions provide functional package managers with similar properties (e.g.
+transactional, declarative up to the operating system, reproducible, ...). Definition of
+packages is dependent on their respective DSL. As it's not easily parsable nor any
+listing api existed, community effort was done to provide regular extraction of origins
+listing as json manifest.
+
+|swh|'s :py:class:`swh.lister.nixguix.lister.NixGuix` lister queries respectively those
+manifests. As they contain various types of origins, |swh| uses various loaders to
+ingest with those origins, url targeting a:
+
+- **simple file**: The :py:class:`swh.loader.core.loader.ContentLoader`
+  ingests origin of type ``content``.
+- **tarball**: The :py:class:`swh.loader.core.loader.TarballDirectoryLoader`
+  ingests origin with type ``tarball-directory``.
+- :ref:`Svn <user-software-origins-svn>` repository: The
+  :py:class:`swh.loader.svn.loader.SvnLoader` ingests origin with type ``svn``.
+- ** :ref:`Svn <user-software-origins-svn>` repository at a specific revision**: The
+  :py:class:`swh.loader.svn.directory.SvnExportLoader` ingests origins
+  with type ``svn-export``.
+- ** :ref:`Git <user-software-origins-git>` repository**: The
+  :py:class:`swh.loader.git.loader.GitLoader` ingests origin with type ``git``.
+- ** :ref:`Git <user-software-origins-git>` repository at a specific git commit**: The
+  :py:class:`swh.loader.git.directory.GitCheckoutLoader` ingests origin with type
+  ``git-checkout``.
+- ** :ref:`Mercurial <user-software-origins-mercurial>` repository**: The
+  :py:class:`swh.loader.mercurial.loader.HgLoader` ingests origin with type ``hg``.
+- ** :ref:`Mercurial <user-software-origins-mercurial>` repository**: The
+  :py:class:`swh.loader.mercurial.directory.HgCheckoutLoader>` ingests with type
+  ``hg-checkout``.
+
+Origin URLs match each main url provided in the manifest.
+
+For some cases like content or tarball urls, there can be mirror urls provided. They are
+used as fallback artifact retrieval when the main url is no longer available.
+
+No extrinsic nor intrinsic metadata collection is happening on the lister's side.
+
+For some origin visit types (``content``, ``tarball-directory``, ``svn-export``,
+``hg-checkout``, ``git-checkout``), extra intrinsic information like the artifact
+checksums (``standard``, e.g. sha256, or ``nar``, specific intrinsic identifier used by
+`GNU Guix`_ and Nix_, see :py:class:`swh.loader.core.nar.Nar`), are transmitted to the
+loaders.
+
+During their ingestion, if the checksum(s) do not match, the artifact is rejected and
+the visit is marked as ``failed``. If not, the artifact is ingested.
+
+The resulting snapshot of the visit is targeting either a content for the loading of a
+file (visit type ``content``) either a directory for tarball (visit type
+``tarball-directory``) and vcs repository at specific commit (``git-checkout``,
+``svn-export``, ``hg-checkout``). Usual standard snapshot happens for vcs (``git``,
+``svn``, ``hg``) repository ingestion.
+
+Note also that a new entry is recorded in the ExtID table to map the SWHID content (for
+a file) or the SWHID directory (for the other kind) ingested to their their original
+checksum.
+
+Sample:
+
++-----------------+---------------+--------------------------------------------------------------------+-------------+--------------------------------------------+
+| extid_type      | extid_version | extid                                                              | target_type | target                                     |
++-----------------+---------------+--------------------------------------------------------------------+-------------+--------------------------------------------+
+| checksum-sha256 |             1 | \x00001a5b5be28bde9bc8c353afe546d8fe84e49b269a70393c1616957b0e1cce | directory   | \xbe186100480d766ebdf0cfaeac0c90198f4b42e7 |
++-----------------+---------------+--------------------------------------------------------------------+-------------+--------------------------------------------+
+| nar-sha256      |             1 | \x00002584a56a9bce85793515604298f8b3b1e9497e00fc6361a0c2e731c063f3 | directory   | \x1e1ace5b0ef56e188d3cf99059070cc5448d7454 |
++-----------------+---------------+--------------------------------------------------------------------+-------------+--------------------------------------------+
+
+Resources:
+
+* `Gnu Guix git repository <https://git.savannah.gnu.org/cgit/guix.git>`__
+
+* `Nixpkgs git repository <https://github.com/nixos/nixpkgs>`__
+
+* `Remote nixpkgs json manifest <https://nix-community.github.io/nixpkgs-swh/sources-unstable-full.json>`__
+
+- `Remote guix json manifest <https://nix-community.github.io/nixpkgs-swh/>`__
 
-* description of the software origin
-* summary of the lister's algorithm
-* summary of the loader's algorithm
-* URL pattern
-* collect extrinsic metadata?
-* index extrinsic metadata?
-* index intrinsic metadata?
+.. _`GNU Guix`: https://guix.gnu.org/
+.. _Nix: https://nixos.org/