Merge pull request #199 from alexheretic/xwayland-env-var

Add a `WINIT_UNIX_BACKEND` environment variable to all the user to control the choice of x11/wayland backend.

src/lib.rs

@@ -181,6 +181,14 @@ pub struct ButtonId(u32);
 /// Provides a way to retreive events from the windows that were registered to it.
 ///
 /// To wake up an `EventsLoop` from a another thread, see the `EventsLoopProxy` docs.
+///
+/// Usage will result in display backend initialisation, this can be controlled on linux
+/// using an environment variable `WINIT_UNIX_BACKEND`.
+/// > Legal values are `x11` and `wayland`. If this variable is set only the named backend
+/// > will be tried by winit. If it is not set, winit will try to connect to a wayland connection,
+/// > and if it fails will fallback on x11.
+/// >
+/// > If this variable is set with any other value, winit will panic.
 pub struct EventsLoop {
     events_loop: platform::EventsLoop,
 }

src/platform/linux/mod.rs

@@ -2,6 +2,7 @@
 
 use std::collections::VecDeque;
 use std::sync::Arc;
+use std::env;
 
 use {CreationError, CursorState, EventsLoopClosed, MouseCursor, ControlFlow};
 use libc;
@@ -15,6 +16,15 @@ mod dlopen;
 pub mod wayland;
 pub mod x11;
 
+/// Environment variable specifying which backend should be used on unix platform.
+///
+/// Legal values are x11 and wayland. If this variable is set only the named backend
+/// will be tried by winit. If it is not set, winit will try to connect to a wayland connection,
+/// and if it fails will fallback on x11.
+///
+/// If this variable is set with any other value, winit will panic.
+const BACKEND_PREFERENCE_ENV_VAR: &str = "WINIT_UNIX_BACKEND";
+
 #[derive(Clone, Default)]
 pub struct PlatformSpecificWindowBuilderAttributes {
     pub visual_infos: Option<XVisualInfo>,
@@ -24,19 +34,42 @@ pub struct PlatformSpecificWindowBuilderAttributes {
 pub enum UnixBackend {
     X(Arc<XConnection>),
     Wayland(Arc<wayland::WaylandContext>),
-    Error(XNotSupported),
+    Error(Option<XNotSupported>, Option<String>),
 }
 
 lazy_static!(
     pub static ref UNIX_BACKEND: UnixBackend = {
-        if let Some(ctxt) = wayland::WaylandContext::init() {
-            UnixBackend::Wayland(Arc::new(ctxt))
-        } else {
+        #[inline]
+        fn x_backend() -> Result<UnixBackend, XNotSupported> {
             match XConnection::new(Some(x_error_callback)) {
-                Ok(x) => UnixBackend::X(Arc::new(x)),
-                Err(e) => UnixBackend::Error(e),
+                Ok(x) => Ok(UnixBackend::X(Arc::new(x))),
+                Err(e) => Err(e),
             }
         }
+        #[inline]
+        fn wayland_backend() -> Result<UnixBackend, ()> {
+             wayland::WaylandContext::init()
+                .map(|ctx| UnixBackend::Wayland(Arc::new(ctx)))
+                .ok_or(())
+        }
+        match env::var(BACKEND_PREFERENCE_ENV_VAR) {
+            Ok(s) => match s.as_str() {
+                "x11" => x_backend().unwrap_or_else(|e| UnixBackend::Error(Some(e), None)),
+                "wayland" => wayland_backend().unwrap_or_else(|_| {
+                    UnixBackend::Error(None, Some("Wayland not available".into()))
+                }),
+                _ => panic!("Unknown environment variable value for {}, try one of `x11`,`wayland`",
+                            BACKEND_PREFERENCE_ENV_VAR),
+            },
+            Err(_) => {
+                // Try wayland, fallback to X11
+                wayland_backend().unwrap_or_else(|_| {
+                    x_backend().unwrap_or_else(|x_err| {
+                        UnixBackend::Error(Some(x_err), Some("Wayland not available".into()))
+                    })
+                })
+            },
+        }
     };
 );
 
@@ -85,7 +118,7 @@ pub fn get_available_monitors() -> VecDeque<MonitorId> {
                                     .into_iter()
                                     .map(MonitorId::X)
                                     .collect(),
-        UnixBackend::Error(_) => { let mut d = VecDeque::new(); d.push_back(MonitorId::None); d},
+        UnixBackend::Error(..) => { let mut d = VecDeque::new(); d.push_back(MonitorId::None); d},
     }
 }
 
@@ -94,7 +127,7 @@ pub fn get_primary_monitor() -> MonitorId {
     match *UNIX_BACKEND {
         UnixBackend::Wayland(ref ctxt) => MonitorId::Wayland(wayland::get_primary_monitor(ctxt)),
         UnixBackend::X(ref connec) => MonitorId::X(x11::get_primary_monitor(connec)),
-        UnixBackend::Error(_) => MonitorId::None,
+        UnixBackend::Error(..) => MonitorId::None,
     }
 }
 
@@ -147,7 +180,7 @@ impl Window2 {
             UnixBackend::X(_) => {
                 x11::Window2::new(events_loop, window, pl_attribs).map(Window2::X)
             },
-            UnixBackend::Error(_) => {
+            UnixBackend::Error(..) => {
                 // If the Backend is Error(), it is not possible to instanciate an EventsLoop at all,
                 // thus this function cannot be called!
                 unreachable!()
@@ -324,7 +357,7 @@ impl EventsLoop {
                 EventsLoop::X(x11::EventsLoop::new(ctxt.clone()))
             },
 
-            UnixBackend::Error(_) => {
+            UnixBackend::Error(..) => {
                 panic!("Attempted to create an EventsLoop while no backend was available.")
             }
         }

src/window.rs

@@ -320,6 +320,14 @@ impl Iterator for AvailableMonitorsIter {
 }
 
 /// Returns the list of all available monitors.
+///
+/// Usage will result in display backend initialisation, this can be controlled on linux
+/// using an environment variable `WINIT_UNIX_BACKEND`.
+/// > Legal values are `x11` and `wayland`. If this variable is set only the named backend
+/// > will be tried by winit. If it is not set, winit will try to connect to a wayland connection,
+/// > and if it fails will fallback on x11.
+/// >
+/// > If this variable is set with any other value, winit will panic.
 #[inline]
 pub fn get_available_monitors() -> AvailableMonitorsIter {
     let data = platform::get_available_monitors();
@@ -327,6 +335,14 @@ pub fn get_available_monitors() -> AvailableMonitorsIter {
 }
 
 /// Returns the primary monitor of the system.
+///
+/// Usage will result in display backend initialisation, this can be controlled on linux
+/// using an environment variable `WINIT_UNIX_BACKEND`.
+/// > Legal values are `x11` and `wayland`. If this variable is set only the named backend
+/// > will be tried by winit. If it is not set, winit will try to connect to a wayland connection,
+/// > and if it fails will fallback on x11.
+/// >
+/// > If this variable is set with any other value, winit will panic.
 #[inline]
 pub fn get_primary_monitor() -> MonitorId {
     MonitorId(platform::get_primary_monitor())