| 1 | //! Job-control suspend/resume handshake for the TUI (#6169). |
| 2 | //! |
| 3 | //! A full-screen TUI that never handles job control poisons the shell it was |
| 4 | //! started from: once the process group reads a controlling tty it does not |
| 5 | //! own, the kernel stops it (SIGTTIN — or the user stops it with SIGTSTP) with |
| 6 | //! every mode it enabled still active — raw mode, mouse reporting, bracketed |
| 7 | //! paste, the alternate screen — and whatever shell is in the foreground is |
| 8 | //! then fed raw SGR/CUP escape fragments. The startup check |
| 9 | //! `terminal::require_foreground_terminal_owner` guards exactly one moment; |
| 10 | //! this module is the runtime half of the same contract: restore on stop, |
| 11 | //! rebuild on continue, like vim/less/htop. |
| 12 | //! |
| 13 | //! Shape (mirrors `fatal_signal_guard`, deliberately narrow): |
| 14 | //! |
| 15 | //! - **One handler serves both SIGTSTP and SIGTTIN.** The SIGTTIN case is the |
| 16 | //! one that bites in #6169: a handler that *returns* turns the background |
| 17 | //! read into `EIO`, which the input pump reports as a dead tty (the forbidden |
| 18 | //! EIO spin). A handler that never returns cannot: it always finishes with |
| 19 | //! `raise(SIGSTOP)`, which is uncatchable and unblockable. |
| 20 | //! - **The handler does exactly three things**, all async-signal-safe: write |
| 21 | //! the fixed restore bytes from `fatal_signal_guard::FATAL_RESTORE_BYTES` |
| 22 | //! (one byte table serves both process death and suspension — no second |
| 23 | //! table), `tcsetattr(TCSANOW)` from the cooked snapshot taken at install |
| 24 | //! time, and stop. **No crossterm call**: crossterm's raw-mode state lives |
| 25 | //! behind a mutex that the stopped input-pump thread may be holding, so |
| 26 | //! `disable_raw_mode()` here can deadlock inside a handler (and the process |
| 27 | //! is unstoppable while it does). No `tracing`, no allocation, no lock. |
| 28 | //! - **The SIGCONT handler is a single atomic store.** Re-entering modes and |
| 29 | //! repainting happen on the event-loop thread in normal context, where |
| 30 | //! crossterm is safe to call. |
| 31 | //! - **The resume action is skipped** while another owner has the terminal (the |
| 32 | //! child-handoff pause) or while this process group is still in the |
| 33 | //! background (a plain `bg`): re-entering raw mode and the alternate screen |
| 34 | //! then would steal the shell's tty. |
| 35 | //! |
| 36 | //! Out of scope by design (see the issue's maintainer discussion): a |
| 37 | //! `tcgetpgrp` pre-poll guard and the `restart_detached` liveness lie. Both are |
| 38 | //! check-then-act, and neither can close the race — the background `read(2)` |
| 39 | //! itself is the atomic foreground test. |
| 40 | //! |
| 41 | //! Not recoverable: SIGKILL while stopped (no handler runs), exactly as with |
| 42 | //! the fatal guard. Kill-switch: `CODEWHALE_DISABLE_JOB_CONTROL_GUARD=1`. |
| 43 | |
| 44 | #[cfg(unix)] |
| 45 | use std::sync::OnceLock; |
| 46 | use std::sync::atomic::{AtomicBool, AtomicU8, Ordering}; |
| 47 | |
| 48 | #[cfg(unix)] |
| 49 | use super::fatal_signal_guard::FATAL_RESTORE_BYTES; |
| 50 | |
| 51 | /// The teardown the stop handler writes. Deliberately the fatal guard's byte |
| 52 | /// string: one table, so a mode added to one restore path cannot be missing |
| 53 | /// from the other. |
| 54 | #[cfg(unix)] |
| 55 | pub(crate) const SUSPEND_RESTORE_BYTES: &[u8] = FATAL_RESTORE_BYTES; |
| 56 | |
| 57 | /// A stop handler has run (and the process has been stopped by SIGSTOP). |
| 58 | const STOPPED_UNDER_HANDLER: u8 = 0b01; |
| 59 | /// SIGCONT arrived after such a stop: the terminal must be rebuilt. |
| 60 | const CONT_SEEN: u8 = 0b10; |
| 61 | /// Both bits: a handler-driven suspend is waiting for its resume repaint. |
| 62 | const PENDING_RESUME: u8 = STOPPED_UNDER_HANDLER | CONT_SEEN; |
| 63 | |
| 64 | /// Suspend handshake state. Only ever touched by a single atomic op per site, |
| 65 | /// so the signal handlers stay async-signal-safe. |
| 66 | static SUSPEND_STATE: AtomicU8 = AtomicU8::new(0); |
| 67 | |
| 68 | /// True once the restore bytes have been written for the current suspend |
| 69 | /// cycle. Cleared by [`mark_resumed`], so each new suspend restores again. |
| 70 | /// Guards both the byte write and the `tcsetattr` — repeated stops inside one |
| 71 | /// suspend cycle (SIGTTIN, SIGCONT, SIGTTIN again while still backgrounded) |
| 72 | /// must not repeat either. |
| 73 | static RESTORED: AtomicBool = AtomicBool::new(false); |
| 74 | |
| 75 | /// Cooked termios snapshot taken at install time, before `enable_raw_mode`. |
| 76 | /// |
| 77 | /// A `OnceLock` read from a signal handler is safe here for the same reason it |
| 78 | /// is in `fatal_signal_guard`: the write happens once, on the main thread, |
| 79 | /// before any worker exists, and is never written again. |
| 80 | #[cfg(unix)] |
| 81 | static ORIGINAL_TERMIOS: OnceLock<libc::termios> = OnceLock::new(); |
| 82 | |
| 83 | /// The cooked snapshot above, for the fatal-signal guard: one snapshot, taken |
| 84 | /// once before raw mode, serves every signal-time restore. |
| 85 | #[cfg(unix)] |
| 86 | pub(crate) fn original_termios() -> Option<&'static libc::termios> { |
| 87 | ORIGINAL_TERMIOS.get() |
| 88 | } |
| 89 | |
| 90 | /// Install the job-control guard. POSIX only; no-op elsewhere. |
| 91 | /// |
| 92 | /// Call once on the main thread, after the foreground-ownership check (the |
| 93 | /// termios snapshot below needs the still-cooked tty) and **before** |
| 94 | /// `enable_raw_mode()` — so every mode the TUI goes on to enable has a handler |
| 95 | /// that can undo it. |
| 96 | pub(crate) fn install_job_control_guard() { |
| 97 | #[cfg(unix)] |
| 98 | { |
| 99 | if std::env::var("CODEWHALE_DISABLE_JOB_CONTROL_GUARD") |
| 100 | .is_ok_and(|value| value == "1" || value == "true") |
| 101 | { |
| 102 | tracing::debug!("Job-control guard disabled by CODEWHALE_DISABLE_JOB_CONTROL_GUARD"); |
| 103 | return; |
| 104 | } |
| 105 | // Piped/embedded surfaces must never receive escape bytes, and have no |
| 106 | // job control to participate in. |
| 107 | if unsafe { libc::isatty(libc::STDOUT_FILENO) } == 0 { |
| 108 | tracing::debug!("Job-control guard skipped: stdout is not a TTY"); |
| 109 | return; |
| 110 | } |
| 111 | // Snapshot the cooked attributes now. In the handler this is the only |
| 112 | // way back: crossterm's own raw-mode teardown is behind a lock. |
| 113 | let mut original: libc::termios = unsafe { std::mem::zeroed() }; |
| 114 | // SAFETY: `original` is a fully owned, properly sized `termios`. |
| 115 | if unsafe { libc::tcgetattr(libc::STDIN_FILENO, &mut original) } != 0 { |
| 116 | tracing::warn!( |
| 117 | "Job-control guard: tcgetattr(stdin) failed; terminal attributes will not be restored on suspend" |
| 118 | ); |
| 119 | } else { |
| 120 | let _ = ORIGINAL_TERMIOS.set(original); |
| 121 | } |
| 122 | for signal in [libc::SIGTSTP, libc::SIGTTIN] { |
| 123 | // SAFETY: `signal` is a stop-class signal whose default action is |
| 124 | // "stop"; the handler is async-signal-safe by construction. |
| 125 | unsafe { install_stop_handler(signal) }; |
| 126 | } |
| 127 | // SAFETY: SIGCONT's default action is to continue, which we replace. |
| 128 | unsafe { install_continue_handler() }; |
| 129 | // The SIGTTIN path runs our stop handler while the group is in the |
| 130 | // BACKGROUND, and `tcsetattr` from a background group raises SIGTTOU |
| 131 | // (default action: stop) — the process would stop inside the handler |
| 132 | // before `raise(SIGSTOP)`, and the first `fg` would resume the rest of |
| 133 | // the handler and immediately stop again. Ignoring SIGTTOU is the |
| 134 | // standard full-screen-program disposition (vim does the same) and is |
| 135 | // the only way the background restore can complete. This is set in the |
| 136 | // installer (normal context), never in a handler. |
| 137 | unsafe { libc::signal(libc::SIGTTOU, libc::SIG_IGN) }; |
| 138 | tracing::debug!( |
| 139 | "Job-control guard installed (TSTP/TTIN -> restore + SIGSTOP, CONT -> resume)" |
| 140 | ); |
| 141 | } |
| 142 | #[cfg(not(unix))] |
| 143 | { |
| 144 | // Windows consoles have no job control; the console-mode cleanup path |
| 145 | // in `terminal.rs` covers the equivalent "mode leak" surface. |
| 146 | } |
| 147 | } |
| 148 | |
| 149 | /// True while a handler-driven suspend is waiting for its resume repaint. |
| 150 | /// |
| 151 | /// This is the event loop's first check each iteration. Deliberately a *peek*, |
| 152 | /// not a take: the state is only cleared by [`mark_resumed`] once the terminal |
| 153 | /// has actually been rebuilt. Consuming it here would lose the resume when the |
| 154 | /// action has to be deferred (a child owns the tty, or we are still a |
| 155 | /// background process group) — and a resume that is lost is a clobbered screen. |
| 156 | pub(crate) fn take_resume() -> bool { |
| 157 | SUSPEND_STATE.load(Ordering::Acquire) & PENDING_RESUME == PENDING_RESUME |
| 158 | } |
| 159 | |
| 160 | /// Acknowledge a completed resume: the next suspend restores from scratch. |
| 161 | /// |
| 162 | /// Narrow `fetch_and` rather than a plain store so bits set by a handler racing |
| 163 | /// with this call are not erased outright. A suspend landing inside the |
| 164 | /// load/acknowledge window can at worst lose one repaint; it can never lose the |
| 165 | /// stop, nor leak a mode, because the handler restores *before* stopping. |
| 166 | pub(crate) fn mark_resumed() { |
| 167 | let _ = SUSPEND_STATE.fetch_and(!PENDING_RESUME, Ordering::AcqRel); |
| 168 | RESTORED.store(false, Ordering::Release); |
| 169 | } |
| 170 | |
| 171 | /// Write the restore bytes, then `tcsetattr`, then stop — nothing else. |
| 172 | /// |
| 173 | /// # Safety |
| 174 | /// |
| 175 | /// Async-signal-safe by construction: `write(2)`, `tcsetattr(3)`, `raise(2)` |
| 176 | /// and two atomic flag updates on plain `static`s. No crossterm, no `tracing`, |
| 177 | /// no allocation, no lock, no `OnceLock` *initialization* (only a read of one |
| 178 | /// that was filled before any thread existed). |
| 179 | #[cfg(unix)] |
| 180 | unsafe extern "C" fn stop_handler(_signal: libc::c_int) { |
| 181 | unsafe { |
| 182 | // One restore per suspend cycle, whatever order the stops arrive in. |
| 183 | if !RESTORED.swap(true, Ordering::AcqRel) { |
| 184 | let mut written: usize = 0; |
| 185 | while written < SUSPEND_RESTORE_BYTES.len() { |
| 186 | let n = libc::write( |
| 187 | libc::STDOUT_FILENO, |
| 188 | SUSPEND_RESTORE_BYTES.as_ptr().add(written) as *const libc::c_void, |
| 189 | SUSPEND_RESTORE_BYTES.len() - written, |
| 190 | ); |
| 191 | if n <= 0 { |
| 192 | break; |
| 193 | } |
| 194 | written += n as usize; |
| 195 | } |
| 196 | if written == 0 { |
| 197 | let _ = libc::write( |
| 198 | libc::STDERR_FILENO, |
| 199 | SUSPEND_RESTORE_BYTES.as_ptr() as *const libc::c_void, |
| 200 | SUSPEND_RESTORE_BYTES.len(), |
| 201 | ); |
| 202 | } |
| 203 | // TCSANOW: never TCSADRAIN/TCSAFLUSH, which can block on output a |
| 204 | // stopped peer will never drain. |
| 205 | if let Some(original) = ORIGINAL_TERMIOS.get() { |
| 206 | let _ = libc::tcsetattr(libc::STDIN_FILENO, libc::TCSANOW, original); |
| 207 | } |
| 208 | } |
| 209 | |
| 210 | // Record that the stop is ours, then stop for real. SIGSTOP cannot be |
| 211 | // caught or blocked, so the handler always ends here: SIGTTIN is never |
| 212 | // allowed to return into the pump as `EIO`. |
| 213 | SUSPEND_STATE.fetch_or(STOPPED_UNDER_HANDLER, Ordering::Release); |
| 214 | libc::raise(libc::SIGSTOP); |
| 215 | } |
| 216 | } |
| 217 | |
| 218 | /// One atomic store. Nothing else — every mode change happens on the event loop |
| 219 | /// thread, in normal context. |
| 220 | #[cfg(unix)] |
| 221 | unsafe extern "C" fn continue_handler(_signal: libc::c_int) { |
| 222 | SUSPEND_STATE.fetch_or(CONT_SEEN, Ordering::Release); |
| 223 | } |
| 224 | |
| 225 | /// Install [`stop_handler`] for one stop-class signal via `sigaction`. |
| 226 | /// |
| 227 | /// # Safety |
| 228 | /// |
| 229 | /// `signal` must be a signal whose default action is "stop". |
| 230 | #[cfg(unix)] |
| 231 | unsafe fn install_stop_handler(signal: libc::c_int) { |
| 232 | unsafe { |
| 233 | // Zero the whole struct then set our two fields; the remaining members |
| 234 | // (empty signal mask, per-OS plumbing) are exactly what a zeroed |
| 235 | // default means, and the wrapper fills in what it owns. SA_RESTART |
| 236 | // keeps the input pump's interrupted read restarted after resume. |
| 237 | let mut action: libc::sigaction = std::mem::zeroed(); |
| 238 | action.sa_sigaction = stop_handler as *const () as libc::sighandler_t; |
| 239 | action.sa_flags = libc::SA_RESTART; |
| 240 | if libc::sigaction(signal, &action, std::ptr::null_mut()) != 0 { |
| 241 | tracing::warn!(signal, "job-control guard install failed"); |
| 242 | } |
| 243 | } |
| 244 | } |
| 245 | |
| 246 | /// # Safety |
| 247 | /// |
| 248 | /// Installed only from [`install_job_control_guard`], on the main thread. |
| 249 | #[cfg(unix)] |
| 250 | unsafe fn install_continue_handler() { |
| 251 | unsafe { |
| 252 | let mut action: libc::sigaction = std::mem::zeroed(); |
| 253 | action.sa_sigaction = continue_handler as *const () as libc::sighandler_t; |
| 254 | action.sa_flags = libc::SA_RESTART; |
| 255 | if libc::sigaction(libc::SIGCONT, &action, std::ptr::null_mut()) != 0 { |
| 256 | tracing::warn!(signal = libc::SIGCONT, "job-control guard install failed"); |
| 257 | } |
| 258 | } |
| 259 | } |
| 260 | |
| 261 | #[cfg(all(test, unix))] |
| 262 | mod tests { |
| 263 | use super::*; |
| 264 | |
| 265 | /// Serializes the state-machine test against any other test that might poke |
| 266 | /// the same statics. There is exactly one such test today; the lock keeps |
| 267 | /// that true if a second one is added. |
| 268 | static STATE_LOCK: std::sync::Mutex<()> = std::sync::Mutex::new(()); |
| 269 | |
| 270 | #[test] |
| 271 | fn job_control_restore_bytes_are_the_fatal_guard_table() { |
| 272 | // One byte table for death and suspension: if this ever forks into a |
| 273 | // second table, a mode can be restored on one path and leaked on the |
| 274 | // other. |
| 275 | assert_eq!(SUSPEND_RESTORE_BYTES, FATAL_RESTORE_BYTES); |
| 276 | // The teardown the suspend path cannot do without. |
| 277 | let bytes = String::from_utf8_lossy(SUSPEND_RESTORE_BYTES).to_string(); |
| 278 | for mode in [ |
| 279 | "?1000l", // mouse tracking |
| 280 | "?1002l", // button-event mouse tracking |
| 281 | "?1003l", // any-motion mouse tracking |
| 282 | "?1006l", // SGR mouse encoding |
| 283 | "?2004l", // bracketed paste |
| 284 | "?1049l", // alternate screen |
| 285 | ] { |
| 286 | assert!( |
| 287 | bytes.contains(mode), |
| 288 | "suspend restore must reset {mode}; got: {bytes:?}" |
| 289 | ); |
| 290 | } |
| 291 | } |
| 292 | |
| 293 | #[test] |
| 294 | fn job_control_state_bits_are_disjoint() { |
| 295 | // The resume test is a mask compare; aliasing bits would make a stop |
| 296 | // with no SIGCONT look resumable. |
| 297 | assert_eq!(STOPPED_UNDER_HANDLER & CONT_SEEN, 0); |
| 298 | assert_eq!(PENDING_RESUME, STOPPED_UNDER_HANDLER | CONT_SEEN); |
| 299 | } |
| 300 | |
| 301 | #[test] |
| 302 | fn job_control_state_machine_is_ordered_and_idempotent() { |
| 303 | let _guard = STATE_LOCK.lock().unwrap_or_else(|err| err.into_inner()); |
| 304 | SUSPEND_STATE.store(0, Ordering::Release); |
| 305 | RESTORED.store(false, Ordering::Release); |
| 306 | |
| 307 | // Nothing suspended: no resume. |
| 308 | assert!(!take_resume()); |
| 309 | mark_resumed(); |
| 310 | assert!(!take_resume()); |
| 311 | |
| 312 | // A bare SIGCONT (somebody else's `kill -CONT`) is not a resume: no |
| 313 | // handler-driven stop is on record. |
| 314 | SUSPEND_STATE.fetch_or(CONT_SEEN, Ordering::Release); |
| 315 | assert!(!take_resume()); |
| 316 | mark_resumed(); |
| 317 | assert!(!take_resume()); |
| 318 | |
| 319 | // The real handshake: the stop handler records the stop, SIGCONT |
| 320 | // records the continue, the loop sees both. |
| 321 | SUSPEND_STATE.fetch_or(STOPPED_UNDER_HANDLER, Ordering::Release); |
| 322 | assert!(!take_resume(), "stopped without SIGCONT is not resumable"); |
| 323 | SUSPEND_STATE.fetch_or(CONT_SEEN, Ordering::Release); |
| 324 | assert!(take_resume()); |
| 325 | // Peeking is idempotent: the action may have to be deferred, so the |
| 326 | // state must survive until it actually runs. |
| 327 | assert!(take_resume()); |
| 328 | |
| 329 | // Acknowledging resumes the cycle: no stale resume remains. |
| 330 | mark_resumed(); |
| 331 | assert!(!take_resume()); |
| 332 | assert!(!RESTORED.load(Ordering::Acquire)); |
| 333 | mark_resumed(); |
| 334 | assert!(!take_resume()); |
| 335 | } |
| 336 | } |
| 337 |