From dafde1357221635a851edf27b9f6f331765ad778 Mon Sep 17 00:00:00 2001
From: Tom You <email.tomyou@gmail.com>
Date: Wed, 8 Jul 2026 10:41:53 -0500
Subject: [PATCH] feat(runtime): add PTY session boundary

---
 rabby-runtime/src/lib.rs |   1 +
 rabby-runtime/src/pty.rs | 131 +++++++++++++++++++++++++++++++++++++++
 2 files changed, 132 insertions(+)
 create mode 100644 rabby-runtime/src/pty.rs

diff --git a/rabby-runtime/src/lib.rs b/rabby-runtime/src/lib.rs
index 279c93a..b62786f 100644
--- a/rabby-runtime/src/lib.rs
+++ b/rabby-runtime/src/lib.rs
@@ -1 +1,2 @@
 pub mod config_store;
+pub mod pty;
diff --git a/rabby-runtime/src/pty.rs b/rabby-runtime/src/pty.rs
new file mode 100644
index 0000000..ae953ae
--- /dev/null
+++ b/rabby-runtime/src/pty.rs
@@ -0,0 +1,131 @@
+use std::fmt;
+
+pub trait PtySession {
+    fn write(&mut self, bytes: &[u8]) -> Result<(), PtyError>;
+    fn resize(&mut self, cols: u16, rows: u16) -> Result<(), PtyError>;
+    fn try_read(&mut self) -> Result<Vec<u8>, PtyError>;
+}
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub enum PtyError {
+    InvalidSize,
+    Closed,
+    Io(String),
+}
+
+impl fmt::Display for PtyError {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            Self::InvalidSize => write!(f, "invalid PTY size"),
+            Self::Closed => write!(f, "PTY session is closed"),
+            Self::Io(message) => write!(f, "PTY I/O error: {message}"),
+        }
+    }
+}
+
+impl std::error::Error for PtyError {}
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct PtySize {
+    pub cols: u16,
+    pub rows: u16,
+}
+
+impl PtySize {
+    pub fn new(cols: u16, rows: u16) -> Result<Self, PtyError> {
+        if cols == 0 || rows == 0 {
+            return Err(PtyError::InvalidSize);
+        }
+        Ok(Self { cols, rows })
+    }
+}
+
+#[derive(Debug, Clone)]
+pub struct FakePtySession {
+    written: Vec<u8>,
+    output: Vec<u8>,
+    size: PtySize,
+    closed: bool,
+}
+
+impl FakePtySession {
+    pub fn new(cols: u16, rows: u16) -> Self {
+        Self {
+            written: Vec::new(),
+            output: Vec::new(),
+            size: PtySize::new(cols, rows).expect("fake PTY size is valid"),
+            closed: false,
+        }
+    }
+
+    pub fn queue_output(&mut self, bytes: impl AsRef<[u8]>) {
+        self.output.extend_from_slice(bytes.as_ref());
+    }
+
+    pub fn written(&self) -> &[u8] {
+        &self.written
+    }
+
+    pub fn size(&self) -> &PtySize {
+        &self.size
+    }
+}
+
+impl PtySession for FakePtySession {
+    fn write(&mut self, bytes: &[u8]) -> Result<(), PtyError> {
+        if self.closed {
+            return Err(PtyError::Closed);
+        }
+        self.written.extend_from_slice(bytes);
+        Ok(())
+    }
+
+    fn resize(&mut self, cols: u16, rows: u16) -> Result<(), PtyError> {
+        self.size = PtySize::new(cols, rows)?;
+        Ok(())
+    }
+
+    fn try_read(&mut self) -> Result<Vec<u8>, PtyError> {
+        if self.closed {
+            return Err(PtyError::Closed);
+        }
+        Ok(std::mem::take(&mut self.output))
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn fake_pty_records_writes_and_resizes() {
+        let mut pty = FakePtySession::new(80, 24);
+
+        pty.write(b"echo ok\n").unwrap();
+        pty.resize(120, 40).unwrap();
+
+        assert_eq!(pty.written(), b"echo ok\n");
+        assert_eq!(
+            pty.size(),
+            &PtySize {
+                cols: 120,
+                rows: 40
+            }
+        );
+    }
+
+    #[test]
+    fn fake_pty_reads_queued_output_once() {
+        let mut pty = FakePtySession::new(80, 24);
+        pty.queue_output(b"ok");
+
+        assert_eq!(pty.try_read().unwrap(), b"ok");
+        assert!(pty.try_read().unwrap().is_empty());
+    }
+
+    #[test]
+    fn rejects_zero_sized_pty() {
+        assert_eq!(PtySize::new(0, 24).unwrap_err(), PtyError::InvalidSize);
+        assert_eq!(PtySize::new(80, 0).unwrap_err(), PtyError::InvalidSize);
+    }
+}