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/input_method.rs

@@ -0,0 +1,277 @@
+use std::env;
+use std::fmt;
+use std::ptr;
+use std::sync::Arc;
+use std::os::raw::c_char;
+use std::ffi::{CStr, CString, IntoStringError};
+
+use super::{ffi, util, XConnection, XError};
+
+unsafe fn open_im(
+    xconn: &Arc<XConnection>,
+    locale_modifiers: &CStr,
+) -> Option<ffi::XIM> {
+    // XSetLocaleModifiers returns...
+    // * The current locale modifiers if it's given a NULL pointer.
+    // * The new locale modifiers if we succeeded in setting them.
+    // * NULL if the locale modifiers string is malformed.
+    (xconn.xlib.XSetLocaleModifiers)(locale_modifiers.as_ptr());
+
+    let im = (xconn.xlib.XOpenIM)(
+        xconn.display,
+        ptr::null_mut(),
+        ptr::null_mut(),
+        ptr::null_mut(),
+    );
+
+    if im.is_null() {
+        None
+    } else {
+        Some(im)
+    }
+}
+
+#[derive(Debug)]
+pub struct InputMethod {
+    pub im: ffi::XIM,
+    name: String,
+}
+
+impl InputMethod {
+    fn new(im: ffi::XIM, name: String) -> Self {
+        InputMethod { im, name }
+    }
+}
+
+#[derive(Debug)]
+pub enum InputMethodResult {
+    /// Input method used locale modifier from `XMODIFIERS` environment variable.
+    XModifiers(InputMethod),
+    /// Input method used internal fallback locale modifier.
+    Fallback(InputMethod),
+    /// Input method could not be opened using any locale modifier tried.
+    Failure,
+}
+
+impl InputMethodResult {
+    pub fn is_fallback(&self) -> bool {
+        if let &InputMethodResult::Fallback(_) = self {
+            true
+        } else {
+            false
+        }
+    }
+
+    pub fn ok(self) -> Option<InputMethod> {
+        use self::InputMethodResult::*;
+        match self {
+            XModifiers(im) | Fallback(im) => Some(im),
+            Failure => None,
+        }
+    }
+}
+
+#[derive(Debug, Clone)]
+enum GetXimServersError {
+    XError(XError),
+    GetPropertyError(util::GetPropertyError),
+    InvalidUtf8(IntoStringError),
+}
+
+// The root window has a property named XIM_SERVERS, which contains a list of atoms represeting
+// the availabile XIM servers. For instance, if you're using ibus, it would contain an atom named
+// "@server=ibus". It's possible for this property to contain multiple atoms, though presumably
+// rare. Note that we replace "@server=" with "@im=" in order to match the format of locale
+// modifiers, since we don't want a user who's looking at logs to ask "am I supposed to set
+// XMODIFIERS to `@server=ibus`?!?"
+unsafe fn get_xim_servers(xconn: &Arc<XConnection>) -> Result<Vec<String>, GetXimServersError> {
+    let servers_atom = util::get_atom(&xconn, b"XIM_SERVERS\0")
+        .map_err(GetXimServersError::XError)?;
+
+    let root = (xconn.xlib.XDefaultRootWindow)(xconn.display);
+
+    let mut atoms: Vec<ffi::Atom> = util::get_property(
+        &xconn,
+        root,
+        servers_atom,
+        ffi::XA_ATOM,
+    ).map_err(GetXimServersError::GetPropertyError)?;
+
+    let mut names: Vec<*const c_char> = Vec::with_capacity(atoms.len());
+    (xconn.xlib.XGetAtomNames)(
+        xconn.display,
+        atoms.as_mut_ptr(),
+        atoms.len() as _,
+        names.as_mut_ptr() as _,
+    );
+    names.set_len(atoms.len());
+
+    let mut formatted_names = Vec::with_capacity(names.len());
+    for name in names {
+        let string = CStr::from_ptr(name)
+            .to_owned()
+            .into_string()
+            .map_err(GetXimServersError::InvalidUtf8)?;
+        (xconn.xlib.XFree)(name as _);
+        formatted_names.push(string.replace("@server=", "@im="));
+    }
+    xconn.check_errors().map_err(GetXimServersError::XError)?;
+    Ok(formatted_names)
+}
+
+#[derive(Clone)]
+struct InputMethodName {
+    c_string: CString,
+    string: String,
+}
+
+impl InputMethodName {
+    pub fn from_string(string: String) -> Self {
+        let c_string = CString::new(string.clone())
+            .expect("String used to construct CString contained null byte");
+        InputMethodName {
+            c_string,
+            string,
+        }
+    }
+
+    pub fn from_str(string: &str) -> Self {
+        let c_string = CString::new(string)
+            .expect("String used to construct CString contained null byte");
+        InputMethodName {
+            c_string,
+            string: string.to_owned(),
+        }
+    }
+}
+
+impl fmt::Debug for InputMethodName {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        self.string.fmt(f)
+    }
+}
+
+#[derive(Debug, Clone)]
+struct PotentialInputMethod {
+    name: InputMethodName,
+    successful: Option<bool>,
+}
+
+impl PotentialInputMethod {
+    pub fn from_string(string: String) -> Self {
+        PotentialInputMethod {
+            name: InputMethodName::from_string(string),
+            successful: None,
+        }
+    }
+
+    pub fn from_str(string: &str) -> Self {
+        PotentialInputMethod {
+            name: InputMethodName::from_str(string),
+            successful: None,
+        }
+    }
+
+    pub fn reset(&mut self) {
+        self.successful = None;
+    }
+
+    pub fn open_im(&mut self, xconn: &Arc<XConnection>) -> Option<InputMethod> {
+        let im = unsafe { open_im(xconn, &self.name.c_string) };
+        self.successful = Some(im.is_some());
+        im.map(|im| InputMethod::new(im, self.name.string.clone()))
+    }
+}
+
+// By logging this struct, you get a sequential listing of every locale modifier tried, where it
+// came from, and if it succceeded.
+#[derive(Debug, Clone)]
+pub struct PotentialInputMethods {
+    // On correctly configured systems, the XMODIFIERS environemnt variable tells us everything we
+    // need to know.
+    xmodifiers: Option<PotentialInputMethod>,
+    // We have some standard options at our disposal that should ostensibly always work. For users
+    // who only need compose sequences, this ensures that the program launches without a hitch
+    // For users who need more sophisticated IME features, this is more or less a silent failure.
+    // Logging features should be added in the future to allow both audiences to be effectively
+    // served.
+    fallbacks: [PotentialInputMethod; 2],
+    // For diagnostic purposes, we include the list of XIM servers that the server reports as
+    // being available.
+    _xim_servers: Result<Vec<String>, GetXimServersError>,
+}
+
+impl PotentialInputMethods {
+    pub fn new(xconn: &Arc<XConnection>) -> Self {
+        let xmodifiers = env::var("XMODIFIERS")
+            .ok()
+            .map(PotentialInputMethod::from_string);
+        PotentialInputMethods {
+            // Since passing "" to XSetLocaleModifiers results in it defaulting to the value of
+            // XMODIFIERS, it's worth noting what happens if XMODIFIERS is also "". If simply
+            // running the program with `XMODIFIERS="" cargo run`, then assuming XMODIFIERS is
+            // defined in the profile (or parent environment) then that parent XMODIFIERS is used.
+            // If that XMODIFIERS value is also "" (i.e. if you ran `export XMODIFIERS=""`), then
+            // XSetLocaleModifiers uses the default local input method. Note that defining
+            // XMODIFIERS as "" is different from XMODIFIERS not being defined at all, since in
+            // that case, we get `None` and end up skipping ahead to the next method.
+            xmodifiers,
+            fallbacks: [
+                // This is a standard input method that supports compose equences, which should
+                // always be available. `@im=none` appears to mean the same thing.
+                PotentialInputMethod::from_str("@im=local"),
+                // This explicitly specifies to use the implementation-dependent default, though
+                // that seems to be equivalent to just using the local input method.
+                PotentialInputMethod::from_str("@im="),
+            ],
+            // The XIM_SERVERS property can have surprising values. For instance, when I exited
+            // ibus to run fcitx, it retained the value denoting ibus. Even more surprising is
+            // that the fcitx input method could only be successfully opened using "@im=ibus".
+            // Presumably due to this quirk, it's actually possible to alternate between ibus and
+            // fcitx in a running application.
+            _xim_servers: unsafe { get_xim_servers(xconn) },
+        }
+    }
+
+    // This resets the `successful` field of every potential input method, ensuring we have
+    // accurate information when this struct is re-used by the destruction/instantiation callbacks.
+    fn reset(&mut self) {
+        if let Some(ref mut input_method) = self.xmodifiers {
+            input_method.reset();
+        }
+
+        for input_method in &mut self.fallbacks {
+            input_method.reset();
+        }
+    }
+
+    pub fn open_im(
+        &mut self,
+        xconn: &Arc<XConnection>,
+        callback: Option<&Fn() -> ()>,
+    ) -> InputMethodResult {
+        use self::InputMethodResult::*;
+
+        self.reset();
+
+        if let Some(ref mut input_method) = self.xmodifiers {
+            let im = input_method.open_im(xconn);
+            if let Some(im) = im {
+                return XModifiers(im);
+            } else {
+                if let Some(ref callback) = callback {
+                    callback();
+                }
+            }
+        }
+
+        for input_method in &mut self.fallbacks {
+            let im = input_method.open_im(xconn);
+            if let Some(im) = im {
+                return Fallback(im);
+            }
+        }
+
+        Failure
+    }
+}