On X11, don't require XSETTINGS

We could fail to setup property watcher and fail to start, thus
don't require XSETTINGS to work.

Fixes: df8805c0 (On X11, reload DPI on _XSETTINGS_SETTINGS)

src/platform_impl/linux/x11/util/randr.rs

@@ -38,11 +38,13 @@ impl XConnection {
     // Retrieve DPI from Xft.dpi property
     pub fn get_xft_dpi(&self) -> Option<f64> {
         // Try to get it from XSETTINGS first.
-        match self.xsettings_dpi() {
+        if let Some(xsettings_screen) = self.xsettings_screen() {
-            Ok(Some(dpi)) => return Some(dpi),
+            match self.xsettings_dpi(xsettings_screen) {
-            Ok(None) => {}
+                Ok(Some(dpi)) => return Some(dpi),
-            Err(err) => {
+                Ok(None) => {}
-                log::warn!("failed to fetch XSettings: {err}");
+                Err(err) => {
+                    log::warn!("failed to fetch XSettings: {err}");
+                }
             }
         }
 

src/platform_impl/linux/x11/xdisplay.rs

@@ -59,7 +59,7 @@ pub(crate) struct XConnection {
     randr_version: (u32, u32),
 
     /// Atom for the XSettings screen.
-    xsettings_screen: xproto::Atom,
+    xsettings_screen: Option<xproto::Atom>,
 
     pub latest_error: Mutex<Option<XError>>,
     pub cursor_cache: Mutex<HashMap<Option<CursorIcon>, ffi::Cursor>>,
@@ -108,21 +108,6 @@ impl XConnection {
         // Get the default screen.
         let default_screen = unsafe { (xlib.XDefaultScreen)(display) } as usize;
 
-        // Fetch the _XSETTINGS_S[screen number] atom.
-        let xsettings_screen = xcb
-            .intern_atom(false, format!("_XSETTINGS_S{}", default_screen).as_bytes())
-            .map_err(|e| XNotSupported::XcbConversionError(Arc::new(e)))?;
-
-        // Fetch the other atoms.
-        let atoms = Atoms::new(&xcb)
-            .map_err(|e| XNotSupported::XcbConversionError(Arc::new(e)))?
-            .reply()
-            .map_err(|e| XNotSupported::XcbConversionError(Arc::new(e)))?;
-        let xsettings_screen = xsettings_screen
-            .reply()
-            .map_err(|e| XNotSupported::XcbConversionError(Arc::new(e)))?
-            .atom;
-
         // Load the database.
         let database = resource_manager::new_from_default(&xcb)
             .map_err(|e| XNotSupported::XcbConversionError(Arc::new(e)))?;
@@ -134,23 +119,16 @@ impl XConnection {
             .reply()
             .expect("failed to query XRandR version");
 
-        // Get PropertyNotify events from the XSETTINGS window.
+        let xsettings_screen = Self::new_xsettings_screen(&xcb, default_screen);
-        // TODO: The XSETTINGS window here can change. In the future, listen for DestroyNotify on this window
+        if xsettings_screen.is_none() {
-        // in order to accomodate for a changed window here.
+            log::warn!("error setting XSETTINGS; Xft options won't reload automatically")
-        let selector_window = xcb
+        }
-            .get_selection_owner(xsettings_screen)
+
+        // Fetch atoms.
+        let atoms = Atoms::new(&xcb)
             .map_err(|e| XNotSupported::XcbConversionError(Arc::new(e)))?
             .reply()
-            .map_err(|e| XNotSupported::XcbConversionError(Arc::new(e)))?
+            .map_err(|e| XNotSupported::XcbConversionError(Arc::new(e)))?;
-            .owner;
-        xcb.change_window_attributes(
-            selector_window,
-            &xproto::ChangeWindowAttributesAux::new()
-                .event_mask(xproto::EventMask::PROPERTY_CHANGE),
-        )
-        .map_err(|e| XNotSupported::XcbConversionError(Arc::new(e)))?
-        .check()
-        .map_err(|e| XNotSupported::XcbConversionError(Arc::new(e)))?;
 
         Ok(XConnection {
             xlib,
@@ -170,6 +148,37 @@ impl XConnection {
         })
     }
 
+    fn new_xsettings_screen(xcb: &XCBConnection, default_screen: usize) -> Option<xproto::Atom> {
+        // Fetch the _XSETTINGS_S[screen number] atom.
+        let xsettings_screen = xcb
+            .intern_atom(false, format!("_XSETTINGS_S{}", default_screen).as_bytes())
+            .ok()?
+            .reply()
+            .ok()?
+            .atom;
+
+        // Get PropertyNotify events from the XSETTINGS window.
+        // TODO: The XSETTINGS window here can change. In the future, listen for DestroyNotify on this window
+        // in order to accomodate for a changed window here.
+        let selector_window = xcb
+            .get_selection_owner(xsettings_screen)
+            .ok()?
+            .reply()
+            .ok()?
+            .owner;
+
+        xcb.change_window_attributes(
+            selector_window,
+            &xproto::ChangeWindowAttributesAux::new()
+                .event_mask(xproto::EventMask::PROPERTY_CHANGE),
+        )
+        .ok()?
+        .check()
+        .ok()?;
+
+        Some(xsettings_screen)
+    }
+
     /// Checks whether an error has been triggered by the previous function calls.
     #[inline]
     pub fn check_errors(&self) -> Result<(), XError> {
@@ -258,7 +267,7 @@ impl XConnection {
 
     /// Get the atom for Xsettings.
     #[inline]
-    pub fn xsettings_screen(&self) -> u32 {
+    pub fn xsettings_screen(&self) -> Option<xproto::Atom> {
         self.xsettings_screen
     }
 }

src/platform_impl/linux/x11/xsettings.rs

@@ -4,13 +4,13 @@
 //!
 //! [here]: https://github.com/derat/xsettingsd
 
-use super::{atoms::*, XConnection};
-
-use x11rb::protocol::xproto::ConnectionExt;
-
 use std::iter;
 use std::num::NonZeroUsize;
 
+use x11rb::protocol::xproto::{self, ConnectionExt};
+
+use super::{atoms::*, XConnection};
+
 type Result<T> = core::result::Result<T, ParserError>;
 
 const DPI_NAME: &[u8] = b"Xft/DPI";
@@ -20,13 +20,16 @@ const BIG_ENDIAN: u8 = b'B';
 
 impl XConnection {
     /// Get the DPI from XSettings.
-    pub(crate) fn xsettings_dpi(&self) -> core::result::Result<Option<f64>, super::X11Error> {
+    pub(crate) fn xsettings_dpi(
+        &self,
+        xsettings_screen: xproto::Atom,
+    ) -> core::result::Result<Option<f64>, super::X11Error> {
         let atoms = self.atoms();
 
         // Get the current owner of the screen's settings.
         let owner = self
             .xcb_connection()
-            .get_selection_owner(self.xsettings_screen())?
+            .get_selection_owner(xsettings_screen)?
             .reply()?;
 
         // Read the _XSETTINGS_SETTINGS property.