fix: hold PTY lock for entire Terminal lifetime on musl

The previous SPAWN_LOCK only serialized the openpty+fork/exec call, but
concurrent PTY I/O operations after spawn also trigger SIGSEGV/SIGBUS in
musl internals. Store the MutexGuard in the Terminal struct so the lock
is held for the Terminal's entire lifetime, ensuring only one PTY is
active at a time on musl.

https://claude.ai/code/session_011H8UR3gS6hoyQAf2x7Dfw8

Author: claude

crates/pty_terminal/src/terminal.rs

@@ -51,6 +51,12 @@ pub struct Terminal {
     pub pty_reader: PtyReader,
     pub pty_writer: PtyWriter,
     pub child_handle: ChildHandle,
+
+    /// On musl libc, concurrent PTY operations (openpty, fork/exec, FD I/O)
+    /// trigger SIGSEGV/SIGBUS in musl internals. This guard serializes the
+    /// entire PTY lifecycle so only one Terminal is active at a time.
+    #[cfg(target_env = "musl")]
+    _pty_guard: std::sync::MutexGuard<'static, ()>,
 }
 
 struct Vt100Callbacks {
@@ -256,14 +262,15 @@ impl Terminal {
     ///
     /// Panics if the writer lock is poisoned when the background thread closes it.
     pub fn spawn(size: ScreenSize, cmd: CommandBuilder) -> anyhow::Result<Self> {
-        // On musl libc (Alpine Linux), concurrent openpty + fork/exec operations
-        // trigger SIGSEGV/SIGBUS in musl internals (observed in sysconf, fcntl).
-        // Serialize PTY creation and child spawning to avoid the race.
+        // On musl libc (Alpine Linux), concurrent PTY operations (openpty,
+        // fork/exec, FD I/O) trigger SIGSEGV/SIGBUS in musl internals.
+        // Hold the lock for the Terminal's entire lifetime so only one PTY
+        // is active at a time.
         // See: https://github.com/voidzero-dev/vite-task/pull/278
         #[cfg(target_env = "musl")]
-        static SPAWN_LOCK: std::sync::Mutex<()> = std::sync::Mutex::new(());
+        static PTY_LOCK: std::sync::Mutex<()> = std::sync::Mutex::new(());
         #[cfg(target_env = "musl")]
-        let _spawn_guard = SPAWN_LOCK.lock().unwrap_or_else(|e| e.into_inner());
+        let pty_guard = PTY_LOCK.lock().unwrap_or_else(|e| e.into_inner());
 
         let pty_pair = portable_pty::native_pty_system().openpty(portable_pty::PtySize {
             rows: size.rows,
@@ -315,6 +322,8 @@ impl Terminal {
             pty_reader: PtyReader { reader, parser: Arc::clone(&parser) },
             pty_writer: PtyWriter { writer, parser, master },
             child_handle: ChildHandle { child_killer, exit_status },
+            #[cfg(target_env = "musl")]
+            _pty_guard: pty_guard,
         })
     }
 }