fix: implement dynamic PTY resize (ResizePty) (#57)

The shim's ResizePty was a no-op, causing interactive terminals to be
stuck at 80x24. Implement file-based IPC between shim and runtime daemon:

- Shim writes "width height" to /run/reaper/<id>/resize (or exec variant)
- Runtime daemon polls the resize file every 100ms via a watcher thread
- On change, daemon applies TIOCSWINSZ ioctl to the PTY master fd

Supports both do_start() and exec_with_pty() PTY sessions.

Closes #56

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: miguelgila

CLAUDE.md

@@ -366,7 +366,7 @@ cargo clippy --target x86_64-unknown-linux-gnu --all-targets
 
 ### 🔄 Known Limitations
 - Multi-container pods not fully tested
-- ResizePty returns OK but is no-op (no dynamic PTY resize)
+- ResizePty polling interval is 100ms (resize may not feel instant)
 - No cgroup resource limits (by design)
 - No namespace isolation (by design)
 - Volume mounts are shared across all workloads (no per-container isolation)

docs/CURRENT_STATE.md

@@ -262,7 +262,7 @@ See [DEVELOPMENT.md](DEVELOPMENT.md) for development setup and [TESTING.md](TEST
 ### ⏳ Not Started
 - [ ] User/group ID management (currently disabled)
 - [ ] Signal handling robustness
-- [ ] Dynamic PTY resize (ResizePty)
+- [x] Dynamic PTY resize (ResizePty)
 - [ ] Resource monitoring (stats)
 - [ ] Performance optimization
 

docs/SHIMV2_DESIGN.md

@@ -110,7 +110,7 @@ service Task {
 | Pause/Resume | ⚠️ | Returns OK but no-op (no cgroup freezer) |
 | Checkpoint | ⚠️ | Not implemented (no CRIU) |
 | Exec | ✅ | Implemented with PTY support |
-| ResizePty | ⚠️ | Returns OK but no-op (no dynamic resize) |
+| ResizePty | ✅ | Shim writes dimensions to resize file, runtime daemon applies via TIOCSWINSZ |
 | CloseIO | ⚠️ | Not implemented |
 | Update | ⚠️ | Not implemented (no cgroups) |
 

src/bin/containerd-shim-reaper-v2/main.rs

@@ -1486,8 +1486,32 @@ impl Task for ReaperTask {
             "resize_pty() called - container_id={}, exec_id={}, width={}, height={}",
             req.id, req.exec_id, req.width, req.height
         );
-        // TODO: Propagate window size to PTY master (requires IPC with runtime daemon)
-        // For now, return success - terminal works but won't resize dynamically
+
+        if req.width == 0 && req.height == 0 {
+            return Ok(api::Empty::new());
+        }
+
+        // Write resize dimensions to a file the runtime daemon polls.
+        // For exec processes, use exec-specific resize file.
+        let resize_file = if req.exec_id.is_empty() {
+            format!("{}/{}/resize", runtime_state_dir(), req.id)
+        } else {
+            format!(
+                "{}/{}/exec-{}-resize",
+                runtime_state_dir(),
+                req.id,
+                req.exec_id
+            )
+        };
+
+        let content = format!("{} {}\n", req.width, req.height);
+        if let Err(e) = std::fs::write(&resize_file, content) {
+            warn!(
+                "resize_pty() - failed to write resize file {}: {}",
+                resize_file, e
+            );
+        }
+
         Ok(api::Empty::new())
     }
 

src/bin/reaper-runtime/main.rs

@@ -13,6 +13,8 @@ use state::{
     delete as delete_state, load_exec_state, load_pid, load_state, save_exec_state, save_pid,
     save_state, ContainerState, OciUser,
 };
+#[cfg(target_os = "linux")]
+use state::{exec_resize_path, resize_path};
 
 #[cfg(target_os = "linux")]
 mod overlay;
@@ -243,6 +245,58 @@ fn exit_code_from_status(status: std::process::ExitStatus) -> i32 {
     }
 }
 
+/// Spawn a thread that polls a resize file and applies `TIOCSWINSZ` to the PTY master.
+///
+/// The shim writes `"width height\n"` to `resize_file`. This thread polls every 100ms,
+/// reads the dimensions, applies them via ioctl, then deletes the file. The thread exits
+/// when `stop` is set to `true` (signaled by the caller after the child process exits).
+#[cfg(target_os = "linux")]
+fn spawn_resize_watcher(
+    master_raw_fd: i32,
+    resize_file: PathBuf,
+    stop: std::sync::Arc<std::sync::atomic::AtomicBool>,
+) {
+    std::thread::spawn(move || {
+        use std::sync::atomic::Ordering;
+        while !stop.load(Ordering::Relaxed) {
+            if resize_file.exists() {
+                if let Ok(content) = fs::read_to_string(&resize_file) {
+                    let _ = fs::remove_file(&resize_file);
+                    let parts: Vec<&str> = content.split_whitespace().collect();
+                    if parts.len() == 2 {
+                        if let (Ok(width), Ok(height)) =
+                            (parts[0].parse::<u16>(), parts[1].parse::<u16>())
+                        {
+                            let ws = nix::libc::winsize {
+                                ws_row: height,
+                                ws_col: width,
+                                ws_xpixel: 0,
+                                ws_ypixel: 0,
+                            };
+                            let ret = unsafe {
+                                nix::libc::ioctl(
+                                    master_raw_fd,
+                                    nix::libc::TIOCSWINSZ as _,
+                                    &ws as *const nix::libc::winsize,
+                                )
+                            };
+                            if ret < 0 {
+                                tracing::warn!(
+                                    "TIOCSWINSZ failed: {}",
+                                    std::io::Error::last_os_error()
+                                );
+                            } else {
+                                tracing::debug!("PTY resized to {}x{}", width, height);
+                            }
+                        }
+                    }
+                }
+            }
+            std::thread::sleep(std::time::Duration::from_millis(100));
+        }
+    });
+}
+
 #[allow(clippy::too_many_arguments)]
 fn do_create(
     id: &str,
@@ -658,13 +712,31 @@ fn do_start(id: &str, bundle: &Path) -> Result<()> {
                         // Close slave in parent - child has it via dup2
                         drop(pty.slave);
 
+                        // Capture master raw fd for resize ioctl before converting to File
+                        #[cfg(target_os = "linux")]
+                        let master_raw_fd = {
+                            use std::os::unix::io::AsRawFd;
+                            pty.master.as_raw_fd()
+                        };
+
                         // Convert PTY master OwnedFd to File for I/O
                         let master_file: std::fs::File = pty.master.into();
                         let master_clone = master_file.try_clone().unwrap_or_else(|e| {
                             tracing::error!("failed to clone master fd: {}", e);
                             std::process::exit(1);
                         });
 
+                        // Start PTY resize watcher thread
+                        #[cfg(target_os = "linux")]
+                        let resize_stop =
+                            std::sync::Arc::new(std::sync::atomic::AtomicBool::new(false));
+                        #[cfg(target_os = "linux")]
+                        spawn_resize_watcher(
+                            master_raw_fd,
+                            resize_path(&container_id),
+                            resize_stop.clone(),
+                        );
+
                         // Relay: stdin FIFO → PTY master (user input to process)
                         if let Some(ref state) = io_state {
                             if let Some(ref stdin_path) = state.stdin {
@@ -746,6 +818,8 @@ fn do_start(id: &str, bundle: &Path) -> Result<()> {
 
                         match child.wait() {
                             Ok(exit_status) => {
+                                #[cfg(target_os = "linux")]
+                                resize_stop.store(true, std::sync::atomic::Ordering::Relaxed);
                                 let exit_code = exit_code_from_status(exit_status);
                                 if let Ok(mut state) = load_state(&container_id) {
                                     state.status = "stopped".into();
@@ -754,6 +828,8 @@ fn do_start(id: &str, bundle: &Path) -> Result<()> {
                                 }
                             }
                             Err(_e) => {
+                                #[cfg(target_os = "linux")]
+                                resize_stop.store(true, std::sync::atomic::Ordering::Relaxed);
                                 if let Ok(mut state) = load_state(&container_id) {
                                     state.status = "stopped".into();
                                     state.exit_code = Some(1);
@@ -1167,13 +1243,30 @@ fn exec_with_pty(
     // Close slave in parent - child has it via dup2
     drop(pty.slave);
 
+    // Capture master raw fd for resize ioctl before converting to File
+    #[cfg(target_os = "linux")]
+    let master_raw_fd = {
+        use std::os::unix::io::AsRawFd;
+        pty.master.as_raw_fd()
+    };
+
     // Convert PTY master OwnedFd to File for I/O
     let master_file: std::fs::File = pty.master.into();
     let master_clone = master_file.try_clone().unwrap_or_else(|e| {
         tracing::error!("failed to clone master fd: {}", e);
         std::process::exit(1);
     });
 
+    // Start PTY resize watcher thread
+    #[cfg(target_os = "linux")]
+    let resize_stop = std::sync::Arc::new(std::sync::atomic::AtomicBool::new(false));
+    #[cfg(target_os = "linux")]
+    spawn_resize_watcher(
+        master_raw_fd,
+        exec_resize_path(container_id, exec_id),
+        resize_stop.clone(),
+    );
+
     // Start relay threads
     // stdin FIFO → PTY master (user input to process)
     if let Some(ref stdin_p) = stdin_path {
@@ -1226,10 +1319,13 @@ fn exec_with_pty(
     }
 
     // Wait for child
-    match child.wait() {
+    let exit = match child.wait() {
         Ok(status) => exit_code_from_status(status),
         Err(_) => 1,
-    }
+    };
+    #[cfg(target_os = "linux")]
+    resize_stop.store(true, std::sync::atomic::Ordering::Relaxed);
+    exit
 }
 
 #[allow(clippy::too_many_arguments)]

src/bin/reaper-runtime/state.rs

@@ -102,6 +102,18 @@ pub fn pid_path(id: &str) -> PathBuf {
     container_dir(id).join("pid")
 }
 
+/// Path for PTY resize signaling (shim writes width/height, runtime reads)
+#[cfg(target_os = "linux")]
+pub fn resize_path(id: &str) -> PathBuf {
+    container_dir(id).join("resize")
+}
+
+/// Path for exec PTY resize signaling
+#[cfg(target_os = "linux")]
+pub fn exec_resize_path(container_id: &str, exec_id: &str) -> PathBuf {
+    container_dir(container_id).join(format!("exec-{}-resize", exec_id))
+}
+
 pub fn save_state(state: &ContainerState) -> anyhow::Result<()> {
     validate_id(&state.id)?;
     let dir = container_dir(&state.id);