docs: add ARCHITECTURE.md with code map

Initial and coarse version of a code map, useful for people getting
started and looking at the repository for the first time.

2 files changed, 121 insertions, 25 deletions

diff --git a/docs/ARCHITECTURE.md b/docs/ARCHITECTURE.md
new file mode 100644
index 0000000000..c631933651
--- /dev/null
+++ b/docs/ARCHITECTURE.md
@@ -0,0 +1,121 @@
+---
+title: systemd repository architecture
+category: Contributing
+layout: default
+---
+
+# Code Map
+
+This section will attempt to provide a high-level overview of the various
+components of the systemd repository.
+
+# Source Code
+
+Directories in `src/` provide the implementation of all daemons, libraries and
+command-line tools shipped by the project. There are many, and more are
+constantly added, so we will not enumerate them all here - the directory
+names are self-explanatory.
+
+## Shared Code
+
+You might wonder what kind of common code belongs in `src/shared/` and what
+belongs in `src/basic/`. The split is like this: anything that is used to
+implement the public shared object we provide (sd-bus, sd-login, sd-id128,
+nss-systemd, nss-mymachines, nss-resolve, nss-myhostname, pam_systemd), must
+be located in `src/basic` (those objects are not allowed to link to
+libsystemd-shared.so). Conversely, anything which is shared between multiple
+components and does not need to be in `src/basic/`, should be in
+`src/shared/`.
+
+To summarize:
+
+`src/basic/`
+- may be used by all code in the tree
+- may not use any code outside of `src/basic/`
+
+`src/libsystemd/`
+- may be used by all code in the tree, except for code in `src/basic/`
+- may not use any code outside of `src/basic/`, `src/libsystemd/`
+
+`src/shared/`
+- may be used by all code in the tree, except for code in `src/basic/`,
+`src/libsystemd/`, `src/nss-*`, `src/login/pam_systemd.*`, and files under
+`src/journal/` that end up in `libjournal-client.a` convenience library.
+- may not use any code outside of `src/basic/`, `src/libsystemd/`, `src/shared/`
+
+## PID 1
+
+Code located in `src/core/` implements the main logic of the systemd system (and user)
+service manager.
+
+BPF helpers written in C and used by PID 1 can be found under `src/core/bpf/`.
+
+## UDEV
+
+Sources for the udev daemon and command-line tool (single binary) can be found under
+`src/udev/`.
+
+## Unit Tests
+
+Source files found under `src/test/` implement unit-level testing, mostly for
+modules found in `src/basic/` and `src/shared/`, but not exclusively. Each test
+file is compiled in a standalone binary that can be run to exercise the
+corresponding module. While most of the tests can be ran by any user, some
+require privileges, and will attempt to clearly log about what they need
+(mostly in the form of effective capabilities). These tests are self-contained,
+and generally safe to run on a host without side effects.
+
+Ideally, every module in `src/basic/` and `src/shared/` should have a corresponding
+unit test under `src/test/`, which exercises every helper function.
+
+# Integration Tests
+
+Sources in `test/` implement system-level testing for executables, libraries and
+daemons that are shipped by the project. They require privileges to run, and
+are not safe to execute directly on a host. By default they will build an image
+and run the test under it via `QEMU` or `systemd-nspawn`.
+
+Most of those tests should be able to run via `systemd-nspawn`, which is orders of
+magnitude faster than `QEMU`, but some tests require privileged operations like
+using `dm-crypt` or `loopdev`. They are clearly marked if that is the case.
+
+See `test/README.testsuite` for more specific details.
+
+# HWDB
+
+Rules built in the static `HWDB` database shipped by the project can be found
+under `hwdb.d/`. Some of these files are updated automatically, some are filled
+by contributors.
+
+# Documentation
+
+## systemd.io
+
+Markdown files found under `docs/` are automatically published on the
+[systemd.io](https://systemd.io) website using Github Pages. A minimal unit test
+to ensure the formatting doesn't have errors is included in the
+`meson test -C build/ github-pages` run as part of the CI.
+
+## MAN pages
+
+Manpages for binaries and libraries, and the DBUS interfaces, can be found under
+`man/` and should ideally be kept in sync with changes to the corresponding
+binaries and libraries.
+
+## Translations
+
+Translations files for binaries and daemons, provided by volunteers, can be found
+under `po/` in the usual format. They are kept up to date by contributors and by
+automated tools.
+
+# System Configuration files and presets
+
+Presets (or templates from which they are generated) for various daemons and tools
+can be found under various directories such as `factory/`, `modprobe.d/`, `network/`,
+`presets/`, `rules.d/`, `shell-completion/`, `sysctl.d/`, `sysusers.d/`, `tmpfiles.d/`.
+
+# Utilities for Developers
+
+`tools/`, `coccinelle/`, `.github/`, `.semaphore/`, `.lgtm/`, `.mkosi/` host various
+utilities and scripts that are used by maintainers and developers. They are not
+shipped or installed.
diff --git a/docs/CODING_STYLE.md b/docs/CODING_STYLE.md
index e4b0d91833..2f9f80f778 100644
--- a/docs/CODING_STYLE.md
+++ b/docs/CODING_STYLE.md
@@ -135,31 +135,6 @@ layout: default
   global in global variables, for example data parsed from command lines, see
   below.
 
-- You might wonder what kind of common code belongs in `src/shared/` and what
-  belongs in `src/basic/`. The split is like this: anything that is used to
-  implement the public shared object we provide (sd-bus, sd-login, sd-id128,
-  nss-systemd, nss-mymachines, nss-resolve, nss-myhostname, pam_systemd), must
-  be located in `src/basic` (those objects are not allowed to link to
-  libsystemd-shared.so). Conversely, anything which is shared between multiple
-  components and does not need to be in `src/basic/`, should be in
-  `src/shared/`.
-
-  To summarize:
-
-  `src/basic/`
-  - may be used by all code in the tree
-  - may not use any code outside of `src/basic/`
-
-  `src/libsystemd/`
-  - may be used by all code in the tree, except for code in `src/basic/`
-  - may not use any code outside of `src/basic/`, `src/libsystemd/`
-
-  `src/shared/`
-  - may be used by all code in the tree, except for code in `src/basic/`,
-    `src/libsystemd/`, `src/nss-*`, `src/login/pam_systemd.*`, and files under
-    `src/journal/` that end up in `libjournal-client.a` convenience library.
-  - may not use any code outside of `src/basic/`, `src/libsystemd/`, `src/shared/`
-
 - Our focus is on the GNU libc (glibc), not any other libcs. If other libcs are
   incompatible with glibc it's on them. However, if there are equivalent POSIX
   and Linux/GNU-specific APIs, we generally prefer the POSIX APIs. If there