Add sysfs fallback for screen brightness

Some drivers/displays don't support the xbacklight stuff, so fall back
to sysfs if needed.  This usually will require some extra permissions
setup on the user's part.

README.md

@@ -146,3 +146,26 @@ resuming.
 
 The settings component is a standalone GTK app that presents a settings
 dialog, which reads from and writes to your configuration file.
+
+#### Screen Brightness
+
+If you enable the setting that allows `bscreensaver` to make your screen
+brightness keys work while the screen is locked, this should hopefully
+work without further intervention, if your display driver supports the
+`XBACKLIGHT` protocol.  If not, `bscreensaver` will attempt to use the
+backlight controls in sysfs, which probably will not work without extra
+setup, as those controls are usually only accessible to root.  You can
+give yourself (well, anyone in the `video` group) access by adding a
+udev rules file, say at `/etc/udev/rules.d/90-backlight.rules`:
+
+```
+SUBSYSTEM=="backlight", ACTION=="add", \
+  RUN+="/bin/chgrp video /sys/class/backlight/%k/brightness", \
+  RUN+="/bin/chmod g+w /sys/class/backlight/%k/brightness"
+```
+
+You may need to restart to get those settings applied, or you can just
+run those commands yourself (replacing `%k` with whatever directories
+happen to be located there).  Your user account will also need to be in
+the `video` group; if it isn't, you'll probably need to log out and in
+again before any changes take effect.

locker/src/monitor.rs

@@ -1,5 +1,6 @@
 use log::{debug, error, warn};
-use std::{cmp, thread, time::Duration};
+use nix::unistd::{access, AccessFlags};
+use std::{cmp, thread, time::Duration, path::PathBuf, fs, io::Write};
 use xcb::{x, randr, Xid};
 
 use bscreensaver_util::{BSCREENSAVER_WM_CLASS, create_atom, destroy_cursor, destroy_pixmap, destroy_gc, destroy_window};
@@ -7,8 +8,13 @@ use bscreensaver_util::{BSCREENSAVER_WM_CLASS, create_atom, destroy_cursor, dest
 const BACKLIGHT_ATOM_NAME: &[u8] = b"Backlight";
 const BACKLIGHT_FALLBACK_ATOM_NAME: &[u8] = b"BACKLIGHT";
 
+enum BacklightLocation {
+    XBacklight(x::Atom),
+    Sysfs(PathBuf),
+}
+
 struct BacklightControl {
-    property: x::Atom,
+    location: BacklightLocation,
     min_level: i32,
     max_level: i32,
     step: u32,
@@ -361,9 +367,11 @@ impl<'a> Monitor<'a> {
     }
 
     fn get_current_brightness(&self, backlight_control: &BacklightControl) -> anyhow::Result<Option<u32>> {
+        match &backlight_control.location {
+            BacklightLocation::XBacklight(property) => {
                 let cookie = self.conn.send_request(&randr::GetOutputProperty {
                     output: self.properties.output,
-            property: backlight_control.property,
+                    property: *property,
                     r#type: x::ATOM_INTEGER,
                     long_offset: 0,
                     long_length: 4,
@@ -377,16 +385,28 @@ impl<'a> Monitor<'a> {
                 } else {
                     Ok(None)
                 }
+            },
+            BacklightLocation::Sysfs(path) => Ok(Some(fs::read_to_string(path)?.trim().parse::<u32>()?)),
+        }
     }
 
     fn set_brightness(&self, backlight_control: &BacklightControl, level: i32) -> anyhow::Result<()> {
+        match &backlight_control.location {
+            BacklightLocation::XBacklight(property) => {
                 self.conn.send_and_check_request(&randr::ChangeOutputProperty {
                     output: self.properties.output,
-            property: backlight_control.property,
+                    property: *property,
                     r#type: x::ATOM_INTEGER,
                     mode: x::PropMode::Replace,
                     data: &[level as u32],
-        })?;
+                })?
+            },
+            BacklightLocation::Sysfs(path) => {
+                let mut f = fs::OpenOptions::new().write(true).open(path)?;
+                let level_str = level.to_string();
+                f.write_all(level_str.as_bytes())?;
+            },
+        }
         Ok(())
     }
 }
@@ -513,28 +533,57 @@ fn create_blank_cursor(conn: &xcb::Connection, root: x::Window) -> xcb::Result<x
     Ok(blank_cursor)
 }
 
-fn find_backlight_control(conn: &xcb::Connection, output: randr::Output) -> xcb::Result<Option<BacklightControl>> {
+fn find_backlight_control(conn: &xcb::Connection, output: randr::Output) -> anyhow::Result<Option<BacklightControl>> {
+    fn calc_step(min: i32, max: i32) -> u32 {
+        cmp::min((max - min) as u32, cmp::max(10, (max - min) / 10) as u32)
+    }
+
     for prop_name in [BACKLIGHT_ATOM_NAME, BACKLIGHT_FALLBACK_ATOM_NAME] {
         let property = create_atom(conn, prop_name)?;
         let cookie = conn.send_request(&randr::QueryOutputProperty {
             output,
             property,
         });
-        let reply = conn.wait_for_reply(cookie)?;
+        if let Ok(reply) = conn.wait_for_reply(cookie) {
             let values = reply.valid_values();
             if reply.range() && values.len() == 2 {
                 let min_level = values[0];
                 let max_level = values[1];
                 let range = max_level - min_level;
                 if range > 0 {
+                    debug!("Found xbacklight control");
                     return Ok(Some(BacklightControl {
-                    property,
+                        location: BacklightLocation::XBacklight(property),
                         min_level,
                         max_level,
-                    step: cmp::min(range as u32, cmp::max(10, (max_level - min_level) / 10) as u32),
+                        step: calc_step(min_level, max_level),
                     }));
                 }
             }
         }
+    }
+
+    if let Ok(entries) = fs::read_dir("/sys/class/backlight") {
+        for entry in entries {
+            let entry = entry?;
+            let mut path = entry.path();
+            path.push("brightness");
+            if path.exists() && access(&path, AccessFlags::W_OK).is_ok() {
+                let mut max_path = entry.path();
+                max_path.push("max_brightness");
+                let max_level = fs::read_to_string(max_path)?.trim().parse::<i32>()?;
+                let mut cur_path = entry.path();
+                cur_path.push("brightness");
+                debug!("Found sysfs backlight control at {:?}", cur_path);
+                return Ok(Some(BacklightControl {
+                    location: BacklightLocation::Sysfs(cur_path),
+                    min_level: 0,
+                    max_level,
+                    step: calc_step(0, max_level),
+                }));
+            }
+        }
+    }
+
     Ok(None)
 }