Add systemd integration via a template unit

Provide systemd support for mounting the gpiod-sysfs-proxy compatibility
filesystem.

The package now installs a single template unit, gpiod-sysfs-proxy@.service,
with an instance name of the systemd-escaped mountpoint (e.g.
gpiod-sysfs-proxy@run-gpio.service). No instance is enabled by default so
the admin opts into the mountpoint they want:

    systemctl enable --now gpiod-sysfs-proxy@run-gpio.service
    systemctl enable --now gpiod-sysfs-proxy@sys-class-gpio.service

Exposing /sys/class/gpio on a kernel where the sysfs GPIO interface is
disabled needs the missing 'gpio' directory overlaid onto /sys/class
first. The run-gpio-sys.mount and sys-class.mount units handle that and
an instance-specific drop-in pulls them in only for the sys-class-gpio
instance. PartOf= ties their lifetime to that instance for clean
teardown and ConditionPathExists=!/sys/class/gpio makes them no-ops
when the directory already exists. Other instances never touch the
overlay.

Fixes: #6
Signed-off-by: Christopher Obbard <christopher.obbard@linaro.org>

Author: obbardc

README.md

@@ -31,6 +31,47 @@ For a complete list of available command-line options, please run:
 gpiod-sysfs-proxy --help
 ```
 
+## Integration
+
+### systemd
+
+The package installs a systemd template unit:
+
+```
+gpiod-sysfs-proxy@.service
+```
+
+No instance is enabled by default. The instance name is the
+systemd-escaped mountpoint. To expose the compatibility filesystem at
+`/run/gpio`:
+
+```
+systemctl enable --now gpiod-sysfs-proxy@run-gpio.service
+```
+
+or, to mount over `/sys/class/gpio` (only works when that directory already
+exists, i.e. the kernel sysfs GPIO interface is enabled):
+
+```
+systemctl enable --now gpiod-sysfs-proxy@sys-class-gpio.service
+```
+
+You can generate the escaped instance name for any path with:
+
+```
+systemd-escape --path /run/gpio
+systemd-escape --path /sys/class/gpio
+```
+
+The `sys-class-gpio` instance also works on a kernel where sysfs GPIO support
+is disabled (so `/sys/class/gpio` does not exist): an instance-specific drop-in
+pulls in the bundled `run-gpio-sys.mount` and `sys-class.mount` units, which
+overlay the missing `gpio` directory onto `/sys/class` before the proxy starts
+and tear it back down when the instance is stopped. Nothing else enables those
+mounts, and they are skipped when `/sys/class/gpio` already exists. See the
+[Non-existent `/sys/class/gpio`](#non-existent-sysclassgpio) caveat below for
+the underlying mechanism.
+
 ## Caveats
 
 Due to how FUSE works, there are certain limitations to the level of

pyproject.toml

@@ -22,6 +22,24 @@ authors = [
 Homepage = "https://github.com/brgl/gpiod-sysfs-proxy"
 Issues = "https://github.com/brgl/gpiod-sysfs-proxy/issues"
 
+[tool.setuptools.data-files]
+# Install the systemd template unit but do not enable any instance by default.
+# The admin enables the mountpoint they want, e.g.:
+#   systemctl enable --now gpiod-sysfs-proxy@run-gpio.service
+#
+# The .mount units provide the /sys/class overlay needed to expose
+# /sys/class/gpio on kernels where the sysfs GPIO interface is disabled. They
+# are inert until pulled in: only the sys-class-gpio instance requires them
+# (via the drop-in below) and they carry ConditionPathExists=!/sys/class/gpio.
+"lib/systemd/system" = [
+  "share/gpiod-sysfs-proxy@.service",
+  "share/run-gpio-sys.mount",
+  "share/sys-class.mount",
+]
+"lib/systemd/system/gpiod-sysfs-proxy@sys-class-gpio.service.d" = [
+  "share/gpiod-sysfs-proxy@sys-class-gpio.service.d/overlay.conf",
+]
+
 [build-system]
 requires = ["setuptools", "wheel"]
 build-backend = "setuptools.build_meta"

share/gpiod-sysfs-proxy@.service

@@ -0,0 +1,32 @@
+# SPDX-License-Identifier: CC0-1.0
+# SPDX-FileCopyrightText: 2024 Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
+
+# Template unit: the instance name is the systemd-escaped mountpoint, e.g.
+#
+#   systemctl enable --now gpiod-sysfs-proxy@run-gpio.service
+#   systemctl enable --now gpiod-sysfs-proxy@sys-class-gpio.service
+#
+# Generate the instance name from a path with:
+#
+#   systemd-escape --path /run/gpio
+#   systemd-escape --path /sys/class/gpio
+#
+# Inside the unit %I expands back to the unescaped mountpoint path.
+#
+# Mounting over /sys/class/gpio only works when that directory already exists
+# (i.e. the kernel sysfs GPIO interface is enabled). To expose it on a kernel
+# where sysfs GPIO is disabled, see the overlay .mount example units shipped
+# alongside this file.
+
+[Unit]
+Description=User-space, libgpiod-based GPIO sysfs compatibility layer at %I
+
+[Service]
+RuntimeDirectory=gpio
+Type=simple
+ExecStart=/usr/bin/gpiod-sysfs-proxy %I -f -o allow_other -o default_permissions
+ExecStop=/bin/umount %I
+Restart=always
+
+[Install]
+WantedBy=multi-user.target

share/gpiod-sysfs-proxy@sys-class-gpio.service.d/overlay.conf

@@ -0,0 +1,14 @@
+# SPDX-License-Identifier: CC0-1.0
+# SPDX-FileCopyrightText: 2024 Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
+
+# Instance-specific drop-in: only the sys-class-gpio instance needs the
+# /sys/class overlay. Pulling the mounts in here (rather than in the template)
+# keeps other instances, e.g. gpiod-sysfs-proxy@run-gpio.service, free of it.
+#
+# The mount units carry ConditionPathExists=!/sys/class/gpio, so on a kernel
+# where the sysfs GPIO interface is enabled they become no-ops and the proxy
+# simply mounts over the existing directory.
+
+[Unit]
+Requires=run-gpio-sys.mount sys-class.mount
+After=run-gpio-sys.mount sys-class.mount

share/run-gpio-sys.mount

@@ -0,0 +1,14 @@
+# SPDX-License-Identifier: CC0-1.0
+# SPDX-FileCopyrightText: 2024 Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
+
+[Unit]
+Description=Remount of sysfs for gpiod-sysfs-proxy
+PartOf=gpiod-sysfs-proxy@sys-class-gpio.service
+ConditionPathExists=!/sys/class/gpio
+
+[Mount]
+DirectoryMode=0700
+What=sysfs
+Where=/run/gpio/sys
+Type=sysfs
+Options=nosuid,nodev,noexec

share/sys-class.mount

@@ -0,0 +1,17 @@
+# SPDX-License-Identifier: CC0-1.0
+# SPDX-FileCopyrightText: 2024 Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
+
+[Unit]
+Description=Overlay on top of /sys/class adding the gpio class directory
+Before=gpiod-sysfs-proxy@sys-class-gpio.service
+After=run-gpio-sys.mount
+PartOf=gpiod-sysfs-proxy@sys-class-gpio.service
+ConditionPathExists=!/sys/class/gpio
+
+[Mount]
+RuntimeDirectory=gpio/class/gpio
+DirectoryMode=0755
+What=overlay
+Where=/sys/class
+Type=overlay
+Options=upperdir=/run/gpio/class,lowerdir=/run/gpio/sys/class,workdir=/run/gpio/work,ro,nosuid,nodev,noexec,relatime