Add a function for checking keyboard enhancement support (#732)

* Add a function for checking keyboard enhancement support

This follows the Kitty documentation's recommended way to check for
progressive keyboard enhancement: query the flags and then query the
primary device attributes (which is broadly supported). If we receive
only the device attributes, the protocol is not supported.

* Check keyboard enhancement in the event-read example

examples/event-read.rs

@@ -13,7 +13,7 @@ use crossterm::{
         read, DisableBracketedPaste, DisableFocusChange, DisableMouseCapture, EnableBracketedPaste,
         EnableFocusChange, EnableMouseCapture, Event, KeyCode,
     },
-    execute,
+    execute, queue,
     terminal::{disable_raw_mode, enable_raw_mode},
     Result,
 };
@@ -69,22 +69,38 @@ fn main() -> Result<()> {
     enable_raw_mode()?;
 
     let mut stdout = stdout();
+
+    let supports_keyboard_enhancement = matches!(
+        crossterm::terminal::supports_keyboard_enhancement(),
+        Ok(true)
+    );
+
+    if supports_keyboard_enhancement {
+        queue!(
+            stdout,
+            PushKeyboardEnhancementFlags(
+                KeyboardEnhancementFlags::DISAMBIGUATE_ESCAPE_CODES
+                    | KeyboardEnhancementFlags::REPORT_ALL_KEYS_AS_ESCAPE_CODES
+                    | KeyboardEnhancementFlags::REPORT_EVENT_TYPES
+            )
+        )?;
+    }
+
     execute!(
         stdout,
         EnableBracketedPaste,
         EnableFocusChange,
         EnableMouseCapture,
-        PushKeyboardEnhancementFlags(
-            KeyboardEnhancementFlags::DISAMBIGUATE_ESCAPE_CODES
-                | KeyboardEnhancementFlags::REPORT_ALL_KEYS_AS_ESCAPE_CODES
-                | KeyboardEnhancementFlags::REPORT_EVENT_TYPES
-        )
     )?;
 
     if let Err(e) = print_events() {
         println!("Error: {:?}\r", e);
     }
 
+    if supports_keyboard_enhancement {
+        queue!(stdout, PopKeyboardEnhancementFlags)?;
+    }
+
     execute!(
         stdout,
         DisableBracketedPaste,

src/event.rs

@@ -934,6 +934,12 @@ pub(crate) enum InternalEvent {
     /// A cursor position (`col`, `row`).
     #[cfg(unix)]
     CursorPosition(u16, u16),
+    /// The progressive keyboard enhancement flags enabled by the terminal.
+    #[cfg(unix)]
+    KeyboardEnhancementFlags(KeyboardEnhancementFlags),
+    /// Attributes and architectural class of the terminal.
+    #[cfg(unix)]
+    PrimaryDeviceAttributes,
 }
 
 #[cfg(test)]

src/event/filter.rs

@@ -17,6 +17,35 @@ impl Filter for CursorPositionFilter {
     }
 }
 
+#[cfg(unix)]
+#[derive(Debug, Clone)]
+pub(crate) struct KeyboardEnhancementFlagsFilter;
+
+#[cfg(unix)]
+impl Filter for KeyboardEnhancementFlagsFilter {
+    fn eval(&self, event: &InternalEvent) -> bool {
+        // This filter checks for either a KeyboardEnhancementFlags response or
+        // a PrimaryDeviceAttributes response. If we receive the PrimaryDeviceAttributes
+        // response but not KeyboardEnhancementFlags, the terminal does not support
+        // progressive keyboard enhancement.
+        matches!(
+            *event,
+            InternalEvent::KeyboardEnhancementFlags(_) | InternalEvent::PrimaryDeviceAttributes
+        )
+    }
+}
+
+#[cfg(unix)]
+#[derive(Debug, Clone)]
+pub(crate) struct PrimaryDeviceAttributesFilter;
+
+#[cfg(unix)]
+impl Filter for PrimaryDeviceAttributesFilter {
+    fn eval(&self, event: &InternalEvent) -> bool {
+        matches!(*event, InternalEvent::PrimaryDeviceAttributes)
+    }
+}
+
 #[derive(Debug, Clone)]
 pub(crate) struct EventFilter;
 
@@ -45,7 +74,8 @@ impl Filter for InternalEventFilter {
 #[cfg(unix)]
 mod tests {
     use super::{
-        super::Event, CursorPositionFilter, EventFilter, Filter, InternalEvent, InternalEventFilter,
+        super::Event, CursorPositionFilter, EventFilter, Filter, InternalEvent,
+        InternalEventFilter, KeyboardEnhancementFlagsFilter, PrimaryDeviceAttributesFilter,
     };
 
     #[test]
@@ -54,6 +84,23 @@ mod tests {
         assert!(CursorPositionFilter.eval(&InternalEvent::CursorPosition(0, 0)));
     }
 
+    #[test]
+    fn test_keyboard_enhancement_status_filter_filters_keyboard_enhancement_status() {
+        assert!(!KeyboardEnhancementFlagsFilter.eval(&InternalEvent::Event(Event::Resize(10, 10))));
+        assert!(
+            KeyboardEnhancementFlagsFilter.eval(&InternalEvent::KeyboardEnhancementFlags(
+                crate::event::KeyboardEnhancementFlags::DISAMBIGUATE_ESCAPE_CODES
+            ))
+        );
+        assert!(KeyboardEnhancementFlagsFilter.eval(&InternalEvent::PrimaryDeviceAttributes));
+    }
+
+    #[test]
+    fn test_primary_device_attributes_filter_filters_primary_device_attributes() {
+        assert!(!PrimaryDeviceAttributesFilter.eval(&InternalEvent::Event(Event::Resize(10, 10))));
+        assert!(PrimaryDeviceAttributesFilter.eval(&InternalEvent::PrimaryDeviceAttributes));
+    }
+
     #[test]
     fn test_event_filter_filters_events() {
         assert!(EventFilter.eval(&InternalEvent::Event(Event::Resize(10, 10))));

src/event/sys/unix/parse.rs

@@ -2,8 +2,9 @@ use std::io;
 
 use crate::{
     event::{
-        Event, KeyCode, KeyEvent, KeyEventKind, KeyEventState, KeyModifiers, MediaKeyCode,
+        Event, KeyCode, KeyEvent, KeyEventKind, KeyEventState, KeyModifiers,
-        ModifierKeyCode, MouseButton, MouseEvent, MouseEventKind,
+        KeyboardEnhancementFlags, MediaKeyCode, ModifierKeyCode, MouseButton, MouseEvent,
+        MouseEventKind,
     },
     ErrorKind, Result,
 };
@@ -177,6 +178,11 @@ pub(crate) fn parse_csi(buffer: &[u8]) -> Result<Option<InternalEvent>> {
         b'P' => Some(Event::Key(KeyCode::F(1).into())),
         b'Q' => Some(Event::Key(KeyCode::F(2).into())),
         b'S' => Some(Event::Key(KeyCode::F(4).into())),
+        b'?' => match buffer[buffer.len() - 1] {
+            b'u' => return parse_csi_keyboard_enhancement_flags(buffer),
+            b'c' => return parse_csi_primary_device_attributes(buffer),
+            _ => None,
+        },
         b'0'..=b'9' => {
             // Numbered escape code.
             if buffer.len() == 3 {
@@ -251,6 +257,51 @@ pub(crate) fn parse_csi_cursor_position(buffer: &[u8]) -> Result<Option<Internal
     Ok(Some(InternalEvent::CursorPosition(x, y)))
 }
 
+fn parse_csi_keyboard_enhancement_flags(buffer: &[u8]) -> Result<Option<InternalEvent>> {
+    // ESC [ ? flags u
+    assert!(buffer.starts_with(&[b'\x1B', b'[', b'?'])); // ESC [ ?
+    assert!(buffer.ends_with(&[b'u']));
+
+    if buffer.len() < 5 {
+        return Ok(None);
+    }
+
+    let bits = buffer[3];
+    let mut flags = KeyboardEnhancementFlags::empty();
+
+    if bits & 1 != 0 {
+        flags |= KeyboardEnhancementFlags::DISAMBIGUATE_ESCAPE_CODES;
+    }
+    if bits & 2 != 0 {
+        flags |= KeyboardEnhancementFlags::REPORT_EVENT_TYPES;
+    }
+    // *Note*: this is not yet supported by crossterm.
+    // if bits & 4 != 0 {
+    //     flags |= KeyboardEnhancementFlags::REPORT_ALTERNATE_KEYS;
+    // }
+    if bits & 8 != 0 {
+        flags |= KeyboardEnhancementFlags::REPORT_ALL_KEYS_AS_ESCAPE_CODES;
+    }
+    // *Note*: this is not yet supported by crossterm.
+    // if bits & 16 != 0 {
+    //     flags |= KeyboardEnhancementFlags::REPORT_ASSOCIATED_TEXT;
+    // }
+
+    Ok(Some(InternalEvent::KeyboardEnhancementFlags(flags)))
+}
+
+fn parse_csi_primary_device_attributes(buffer: &[u8]) -> Result<Option<InternalEvent>> {
+    // ESC [ 64 ; attr1 ; attr2 ; ... ; attrn ; c
+    assert!(buffer.starts_with(&[b'\x1B', b'[', b'?']));
+    assert!(buffer.ends_with(&[b'c']));
+
+    // This is a stub for parsing the primary device attributes. This response is not
+    // exposed in the crossterm API so we don't need to parse the individual attributes yet.
+    // See <https://vt100.net/docs/vt510-rm/DA1.html>
+
+    Ok(Some(InternalEvent::PrimaryDeviceAttributes))
+}
+
 fn parse_modifiers(mask: u8) -> KeyModifiers {
     let modifier_mask = mask.saturating_sub(1);
     let mut modifiers = KeyModifiers::empty();

src/terminal.rs

@@ -98,6 +98,8 @@ use crate::{csi, impl_display, Result};
 
 pub(crate) mod sys;
 
+pub use sys::supports_keyboard_enhancement;
+
 /// Tells whether the raw mode is enabled.
 ///
 /// Please have a look at the [raw mode](./index.html#raw-mode) section.

src/terminal/sys.rs

@@ -1,8 +1,12 @@
 //! This module provides platform related functions.
 
+#[cfg(unix)]
+pub use self::unix::supports_keyboard_enhancement;
 #[cfg(unix)]
 pub(crate) use self::unix::{disable_raw_mode, enable_raw_mode, is_raw_mode_enabled, size};
 #[cfg(windows)]
+pub use self::windows::supports_keyboard_enhancement;
+#[cfg(windows)]
 pub(crate) use self::windows::{
     clear, disable_raw_mode, enable_raw_mode, is_raw_mode_enabled, scroll_down, scroll_up,
     set_size, set_window_title, size,

src/terminal/sys/unix.rs

@@ -1,7 +1,9 @@
 //! UNIX related logic for terminal manipulation.
 
 use std::fs::File;
+use std::io::Write;
 use std::os::unix::io::{IntoRawFd, RawFd};
+use std::time::Duration;
 use std::{io, mem, process};
 
 use libc::{
@@ -11,7 +13,9 @@ use libc::{
 use parking_lot::Mutex;
 
 use crate::error::Result;
+use crate::event::filter::{KeyboardEnhancementFlagsFilter, PrimaryDeviceAttributesFilter};
 use crate::event::sys::unix::file_descriptor::{tty_fd, FileDesc};
+use crate::event::{poll_internal, read_internal, InternalEvent};
 
 // Some(Termios) -> we're in the raw mode and this is the previous mode
 // None -> we're not in the raw mode
@@ -88,6 +92,72 @@ pub(crate) fn disable_raw_mode() -> Result<()> {
     Ok(())
 }
 
+/// Queries the terminal's support for progressive keyboard enhancement.
+///
+/// On unix systems, this function will block and possibly time out while
+/// [`crossterm::event::read`](crate::event::read) or [`crossterm::event::poll`](crate::event::poll) are being called.
+pub fn supports_keyboard_enhancement() -> Result<bool> {
+    if is_raw_mode_enabled() {
+        read_supports_keyboard_enhancement_raw()
+    } else {
+        read_supports_keyboard_enhancement_flags()
+    }
+}
+
+fn read_supports_keyboard_enhancement_flags() -> Result<bool> {
+    enable_raw_mode()?;
+    let flags = read_supports_keyboard_enhancement_raw();
+    disable_raw_mode()?;
+    flags
+}
+
+fn read_supports_keyboard_enhancement_raw() -> Result<bool> {
+    // This is the recommended method for testing support for the keyboard enhancement protocol.
+    // We send a query for the flags supported by the terminal and then the primary device attributes
+    // query. If we receive the primary device attributes response but not the keyboard enhancement
+    // flags, none of the flags are supported.
+    //
+    // See <https://sw.kovidgoyal.net/kitty/keyboard-protocol/#detection-of-support-for-this-protocol>
+
+    // ESC [ ? u        Query progressive keyboard enhancement flags (kitty protocol).
+    // ESC [ c          Query primary device attributes.
+    const QUERY: &[u8] = b"\x1B[?u\x1B[c";
+
+    if let Err(_) = File::open("/dev/tty").and_then(|mut file| {
+        file.write_all(QUERY)?;
+        file.flush()
+    }) {
+        let mut stdout = io::stdout();
+        stdout.write_all(QUERY)?;
+        stdout.flush()?;
+    }
+
+    loop {
+        match poll_internal(
+            Some(Duration::from_millis(2000)),
+            &KeyboardEnhancementFlagsFilter,
+        ) {
+            Ok(true) => {
+                match read_internal(&KeyboardEnhancementFlagsFilter) {
+                    Ok(InternalEvent::KeyboardEnhancementFlags(_current_flags)) => {
+                        // Flush the PrimaryDeviceAttributes out of the event queue.
+                        read_internal(&PrimaryDeviceAttributesFilter).ok();
+                        return Ok(true);
+                    }
+                    _ => return Ok(false),
+                }
+            }
+            Ok(false) => {
+                return Err(io::Error::new(
+                    io::ErrorKind::Other,
+                    "The keyboard enhancement status could not be read within a normal duration",
+                ));
+            }
+            Err(_) => {}
+        }
+    }
+}
+
 /// execute tput with the given argument and parse
 /// the output as a u16.
 ///

src/terminal/sys/windows.rs

@@ -58,6 +58,13 @@ pub(crate) fn size() -> Result<(u16, u16)> {
     ))
 }
 
+/// Queries the terminal's support for progressive keyboard enhancement.
+///
+/// This always returns `Ok(false)` on Windows.
+pub fn supports_keyboard_enhancement() -> Result<bool> {
+    Ok(false)
+}
+
 pub(crate) fn clear(clear_type: ClearType) -> Result<()> {
     let screen_buffer = ScreenBuffer::current()?;
     let csbi = screen_buffer.info()?;