Use systemd/elogind to set brightness if possible

Since version 243, systemd-logind supports setting the brightness of an
"leds" or "backlight" kernel class device [1]. This method call requires
no privileges to call, but a caller can only change the brightness on
sessions that are currently active, and they must own the session.

Using systemd/elogind for unprivileged brightness changes is a nice
feature to have since it allows to changes brightness without giving
explicit permissions to users via groups or setting SUID bit.

[1] https://github.com/systemd/systemd/commit/2a66c2a1eda48433cb7f4ac6e9b9abcf4bd41bf3

configure.ac

 AC_ARG_WITH([udev],
 	AS_HELP_STRING([--with-udev@<:@=PATH@:>@], [use udev instead of SUID root, optional rules.d path]),
 	[udev=$withval], [udev=no])
+AC_ARG_WITH([systemd],
+	AS_HELP_STRING([--with-systemd], [use systemd-logind for unprivileged brightness changes]),
+	[], [systemd=no])
+AC_ARG_WITH([elogind],
+	AS_HELP_STRING([--with-elogind], [use elogind for unprivileged brightness changes]),
+	[], [elogind=no])
 
 AC_MSG_CHECKING(for udev rules.d)
 AS_IF([test "x$udev" != "xno"], [
 	AC_MSG_RESULT([disabled, classic SUID root mode])
 ])
 
+AC_MSG_CHECKING(for systemd)
+AS_IF([test "x$systemd" != "xno"], [
+	AC_CHECK_LIB([systemd], [sd_bus_call_method])
+],[
+	AC_MSG_RESULT([disabled])
+])
+
+AC_MSG_CHECKING(for elogind)
+AS_IF([test "x$elogind" != "xno"], [
+	AC_CHECK_LIB([elogind], [sd_bus_call_method])
+],[
+	AC_MSG_RESULT([disabled])
+])
+
 # Allow classic SUID root behavior if udev rule is not used
 AM_CONDITIONAL(UDEV,    [test "x$udev" != "xno"])
 AM_CONDITIONAL(CLASSIC, [test "x$udev"  = "xno"])

src/helpers.c

+#include "light.h"
 #include "helpers.h"
 
 #include <stdio.h>
 #include <errno.h> // errno
 #include <libgen.h> // dirname 
 
+#if HAVE_LIBSYSTEMD
+#include <systemd/sd-bus.h>
+#elif HAVE_LIBELOGIND
+#include <elogind/sd-bus.h>
+#endif
 
 bool light_file_read_uint64(char const *filename, uint64_t *val)
 {
     return mkdir(dir, mode);
 }
 
+#if HAVE_LIBSYSTEMD || HAVE_LIBELOGIND
+bool light_logind_set_brightness(light_device_target_t *target, uint64_t in_value) {
+    sd_bus *bus = NULL;
+    sd_bus_message *message = NULL;
+    sd_bus_error error = SD_BUS_ERROR_NULL;
+
+    int ret = sd_bus_default_system(&bus);
+
+    if (ret < 0) {
+        LIGHT_ERR("Failed to open D-Bus connection: %s", strerror(-ret));
+        return false;
+    }
+
+    ret = sd_bus_call_method(
+        bus,
+        "org.freedesktop.login1",
+        "/org/freedesktop/login1/session/auto",
+        "org.freedesktop.login1.Session",
+        "SetBrightness",
+        &error,
+        &message,
+        "ssu",
+        target->device->name,
+        target->name,
+        in_value
+    );
+
+    if (ret < 0) {
+        LIGHT_ERR("Failed to set brightness: %s", error.message);
+    }
+
+    sd_bus_error_free(&error);
+    sd_bus_message_unref(message);
+    sd_bus_unref(bus);
+    return ret >= 0;
+}
+#endif  // HAVE_LIBSYSTEMD || HAVE_LIBELOGIND

src/helpers.h

 
 int light_mkpath(char *dir, mode_t mode);
 
+#if HAVE_LIBSYSTEMD || HAVE_LIBELOGIND
+bool light_logind_set_brightness(light_device_target_t *target, uint64_t in_value);
+#endif

src/impl/sysfs.c

     if(!light_file_write_uint64(data->brightness, in_value))
     {
         LIGHT_ERR("failed to write to sysfs device");
+
+#if HAVE_LIBSYSTEMD || HAVE_LIBELOGIND
+        return light_logind_set_brightness(target, in_value);
+#endif
         return false;
     }