backlight: run without a backlight device instead of panicking (0.4.3)

A config carrying a !backlight widget on a machine with nothing under
/sys/class/backlight (a laptop config on a desktop) hit
expect("did not find backlight device") in Backlight::new, which with
panic = "abort" killed wldash before a window appeared.

The device path is now Option<PathBuf>. A missing device, or a
configured `device` name that does not exist, prints one line on stderr
and leaves the widget in place so layout indices stay aligned: it
renders empty, skips sysfs reads and writes, and ignores clicks.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

CHANGELOG.md

@@ -1,5 +1,9 @@
 # Changelog
 
+## 0.4.3 (2026-10-05)
+
+- Backlight: a config with a `!backlight` widget on a machine without `/sys/class/backlight` entries (or with a `device` name that does not exist) panicked at start ("did not find backlight device"), fatal with `panic = "abort"`. The widget now warns once on stderr, renders empty and ignores clicks, so one config can be shared between a laptop and a desktop.
+
 ## 0.4.2 (2026-10-03)
 
 - Fonts: a `font_paths` entry pointing at a file that does not exist on this machine made the font-loader thread panic and abort wldash ("Font file not found"). Pinned paths that are missing now fall back to fontconfig by name, with one line on stderr; a font that cannot be resolved at all exits with a clear message instead of a backtrace.

Cargo.lock

@@ -1071,7 +1071,7 @@ checksum = "dff9641d1cd4be8d1a070daf9e3773c5f67e78b4d9d42263020c057706765c04"
 
 [[package]]
 name = "wldash"
-version = "0.4.2"
+version = "0.4.3"
 dependencies = [
  "calloop",
  "chrono",

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "wldash"
-version = "0.4.2"
+version = "0.4.3"
 edition = "2024"
 
 [profile.release]

PKGBUILD

@@ -4,7 +4,7 @@
 # project.uhhm.no (namespace bl). pkgver is the version; the tag has
 # to agree with it and with Cargo.toml.
 pkgname=wldash
-pkgver=0.4.2
+pkgver=0.4.3
 pkgrel=1
 pkgdesc="Dashboard/launcher/control-panel for wlroots-style Wayland compositors (layer-shell)"
 arch=('x86_64' 'aarch64')

src/widgets/backlight.rs

@@ -26,7 +26,11 @@ fn write_file_as_u64(path: &Path, value: u64) -> Result<(), Error> {
 }
 
 pub struct Backlight {
-    device_path: PathBuf,
+    /// `None` when no backlight exists on this machine: the widget then
+    /// renders empty and ignores clicks instead of taking wldash down.
+    /// Configs are shared between hosts, so a laptop config running on a
+    /// desktop must not panic here.
+    device_path: Option<PathBuf>,
     cur: u64,
     max: u64,
     dirty: bool,
@@ -46,10 +50,13 @@ fn find_device() -> Option<PathBuf> {
 
 impl Backlight {
     fn update(&mut self) {
-        if let Ok(cur) = read_file_as_u64(self.device_path.join("brightness").as_path()) {
+        let Some(dev) = &self.device_path else {
+            return;
+        };
+        if let Ok(cur) = read_file_as_u64(dev.join("brightness").as_path()) {
             self.cur = cur;
         }
-        if let Ok(max) = read_file_as_u64(self.device_path.join("max_brightness").as_path()) {
+        if let Ok(max) = read_file_as_u64(dev.join("max_brightness").as_path()) {
             self.max = max;
         }
     }
@@ -58,11 +65,30 @@ impl Backlight {
     #[allow(clippy::new_ret_no_self)]
     pub fn new(path: Option<&str>, fm: &mut FontMap, font: &'static str, size: f32) -> BarWidget {
         let device_path = match path {
-            Some(path) => Some(Path::new("/sys/class/backlight").join(path)),
-            None => find_device(),
+            Some(path) => {
+                let p = Path::new("/sys/class/backlight").join(path);
+                if p.is_dir() {
+                    Some(p)
+                } else {
+                    eprintln!(
+                        "wldash: backlight device {:?} not found, widget disabled",
+                        p
+                    );
+                    None
+                }
+            }
+            None => {
+                let p = find_device();
+                if p.is_none() {
+                    eprintln!(
+                        "wldash: no backlight device under /sys/class/backlight, widget disabled"
+                    );
+                }
+                p
+            }
         };
         let mut dev = Backlight {
-            device_path: device_path.expect("did not find backlight device"),
+            device_path,
             cur: 0,
             max: 0,
             dirty: true,
@@ -77,9 +103,13 @@ impl Backlight {
     }
 
     pub fn set(&mut self, brightness: f32) {
+        let Some(dev) = &self.device_path else {
+            return;
+        };
         let val = (self.max as f32 * brightness.clamp(0.0, 1.0)) as u64;
-        let _ = write_file_as_u64(self.device_path.join("brightness").as_path(), val);
-        if let Ok(cur) = read_file_as_u64(self.device_path.join("brightness").as_path()) {
+        let brightness_path = dev.join("brightness");
+        let _ = write_file_as_u64(&brightness_path, val);
+        if let Ok(cur) = read_file_as_u64(&brightness_path) {
             self.cur = cur;
         }
     }
@@ -93,12 +123,14 @@ impl BarWidgetImpl for Backlight {
         self.dirty
     }
     fn value(&mut self) -> f32 {
+        self.dirty = false;
+        if self.device_path.is_none() {
+            return 0.0;
+        }
         if self.max == 0 || self.cur > self.max {
-            // what.
+            // Sysfs handed us nonsense; show full rather than a bogus ratio.
             return 1.0;
         }
-
-        self.dirty = false;
         self.cur as f32 / self.max as f32
     }
     fn color(&self) -> Color {