x11: Overhaul XIM code (#451)

Fixes #195
Fixes #277
Fixes #455

* Read `XMODIFIERS` explicitly/directly instead of calling `XSetLocaleModifiers` with an
empty string. This is useful for debugging purposes, and more clear to read and handle.
* Fallback to local input method if the one specified in `XMODIFIERS` is later closed on the
server end (i.e. if ibus/fcitx is terminated). Previously, that would cause the event loop
to freeze and usually also segfault.
* If using the fallback input method, respond to the `XMODIFIERS` input method later
becoming available. This means that the input method restarting is handled, and that even if
the program was started while ibus/fcitx/etc. was unavailable, it will start using it as
soon as it becomes available.
* Only one input method is opened for the whole event loop, with each window having its own
input context.
* IME works completely out of the box now, no longer requiring application developers to
call `setlocale` or `XSetLocaleModifiers`.
* Detailed error messages are provided if no input method could be opened. However, no
information is provided to the user if their intended `XMODIFIERS` input method failed to
open but the fallbacks (which will ostensibly always succeed) succeeded; in my opinion, this
is something that is best filled by adding a logging feature to winit.

src/platform/linux/x11/ime/context.rs

@@ -0,0 +1,134 @@
+use std::ptr;
+use std::sync::Arc;
+use std::os::raw::{c_short, c_void};
+
+use super::{ffi, util, XConnection, XError};
+
+#[derive(Debug)]
+pub enum ImeContextCreationError {
+    XError(XError),
+    Null,
+}
+
+unsafe fn create_pre_edit_attr<'a>(
+    xconn: &'a Arc<XConnection>,
+    ic_spot: &'a ffi::XPoint,
+) -> util::XSmartPointer<'a, c_void> {
+    util::XSmartPointer::new(
+        xconn,
+        (xconn.xlib.XVaCreateNestedList)(
+            0,
+            ffi::XNSpotLocation_0.as_ptr() as *const _,
+            ic_spot,
+            ptr::null_mut::<()>(),
+        ),
+    ).expect("XVaCreateNestedList returned NULL")
+}
+
+// WARNING: this struct doesn't destroy its XIC resource when dropped.
+// This is intentional, as it doesn't have enough information to know whether or not the context
+// still exists on the server. Since `ImeInner` has that awareness, destruction must be handled
+// through `ImeInner`.
+#[derive(Debug)]
+pub struct ImeContext {
+    pub ic: ffi::XIC,
+    pub ic_spot: ffi::XPoint,
+}
+
+impl ImeContext {
+    pub unsafe fn new(
+        xconn: &Arc<XConnection>,
+        im: ffi::XIM,
+        window: ffi::Window,
+        ic_spot: Option<ffi::XPoint>,
+    ) -> Result<Self, ImeContextCreationError> {
+        let ic = if let Some(ic_spot) = ic_spot {
+            ImeContext::create_ic_with_spot(xconn, im, window, ic_spot)
+        } else {
+            ImeContext::create_ic(xconn, im, window)
+        };
+
+        let ic = ic.ok_or(ImeContextCreationError::Null)?;
+        xconn.check_errors().map_err(ImeContextCreationError::XError)?;
+
+        Ok(ImeContext {
+            ic,
+            ic_spot: ic_spot.unwrap_or_else(|| ffi::XPoint { x: 0, y: 0 }),
+        })
+    }
+
+    unsafe fn create_ic(
+        xconn: &Arc<XConnection>,
+        im: ffi::XIM,
+        window: ffi::Window,
+    ) -> Option<ffi::XIC> {
+        let ic = (xconn.xlib.XCreateIC)(
+            im,
+            ffi::XNInputStyle_0.as_ptr() as *const _,
+            ffi::XIMPreeditNothing | ffi::XIMStatusNothing,
+            ffi::XNClientWindow_0.as_ptr() as *const _,
+            window,
+            ptr::null_mut::<()>(),
+        );
+        if ic.is_null() {
+            None
+        } else {
+            Some(ic)
+        }
+    }
+
+    unsafe fn create_ic_with_spot(
+        xconn: &Arc<XConnection>,
+        im: ffi::XIM,
+        window: ffi::Window,
+        ic_spot: ffi::XPoint,
+    ) -> Option<ffi::XIC> {
+        let pre_edit_attr = create_pre_edit_attr(xconn, &ic_spot);
+        let ic = (xconn.xlib.XCreateIC)(
+            im,
+            ffi::XNInputStyle_0.as_ptr() as *const _,
+            ffi::XIMPreeditNothing | ffi::XIMStatusNothing,
+            ffi::XNClientWindow_0.as_ptr() as *const _,
+            window,
+            ffi::XNPreeditAttributes_0.as_ptr() as *const _,
+            pre_edit_attr.ptr,
+            ptr::null_mut::<()>(),
+        );
+        if ic.is_null() {
+            None
+        } else {
+            Some(ic)
+        }
+    }
+
+    pub fn focus(&self, xconn: &Arc<XConnection>) -> Result<(), XError> {
+        unsafe {
+            (xconn.xlib.XSetICFocus)(self.ic);
+        }
+        xconn.check_errors()
+    }
+
+    pub fn unfocus(&self, xconn: &Arc<XConnection>) -> Result<(), XError> {
+        unsafe {
+            (xconn.xlib.XUnsetICFocus)(self.ic);
+        }
+        xconn.check_errors()
+    }
+
+    pub fn set_spot(&mut self, xconn: &Arc<XConnection>, x: c_short, y: c_short) {
+        if self.ic_spot.x == x && self.ic_spot.y == y {
+            return;
+        }
+        self.ic_spot = ffi::XPoint { x, y };
+
+        unsafe {
+            let pre_edit_attr = create_pre_edit_attr(xconn, &self.ic_spot);
+            (xconn.xlib.XSetICValues)(
+                self.ic,
+                ffi::XNPreeditAttributes_0.as_ptr() as *const _,
+                pre_edit_attr.ptr,
+                ptr::null_mut::<()>(),
+            );
+        }
+    }
+}