fix(ui): implement async-signal-safe fork process layout and waitpid child zombie reaping for PTY

.agents/skills/multi-agent-mux-ui/packages/mam_pty/lib/src/pty_session.dart

@@ -44,6 +44,12 @@ typedef _read_dart = int Function(int fd, ffi.Pointer<ffi.Void> buf, int count);
 typedef _write_c = ffi.IntPtr Function(ffi.Int32 fd, ffi.Pointer<ffi.Void> buf, ffi.IntPtr count);
 typedef _write_dart = int Function(int fd, ffi.Pointer<ffi.Void> buf, int count);
 
+typedef _waitpid_c = ffi.Int32 Function(ffi.Int32 pid, ffi.Pointer<ffi.Int32> status, ffi.Int32 options);
+typedef _waitpid_dart = int Function(int pid, ffi.Pointer<ffi.Int32> status, int options);
+
+typedef _chdir_c = ffi.Int32 Function(ffi.Pointer<ffi.Char> path);
+typedef _chdir_dart = int Function(ffi.Pointer<ffi.Char> path);
+
 base class Winsize extends ffi.Struct {
   @ffi.Uint16()
   external int ws_row;
@@ -90,8 +96,9 @@ class PtySession {
     final dup2 = libc.lookupFunction<_dup2_c, _dup2_dart>('dup2');
     final execvp = libc.lookupFunction<_execvp_c, _execvp_dart>('execvp');
     final ioctl = libc.lookupFunction<_ioctl_c, _ioctl_dart>('ioctl');
+    final chdir = libc.lookupFunction<_chdir_c, _chdir_dart>('chdir');
 
-    // 1. Open master PTY (O_RDWR = 2, O_NOCTTY = 0x00000400)
+    // 1. Prepare master PTY
     final masterFd = posixOpenpt(2 | 0x00000400);
     if (masterFd < 0) {
       throw OSError('Failed to open pseudo-terminal master');
@@ -119,25 +126,46 @@ class PtySession {
     env.remove('TMUX');
     env.remove('TMUX_PANE');
 
-    // 3. Fork child process
+    // 3. Pre-allocate ALL structures on the heap before fork() for async-signal-safety (§6.7)
+    final pathPtr = slaveName.toNativeUtf8();
+    final exePtr = executable.toNativeUtf8();
+    final workDirPtr = workingDirectory?.toNativeUtf8();
+    
+    final argsCount = arguments.length;
+    final argv = malloc<ffi.Pointer<ffi.Char>>(argsCount + 2);
+    argv[0] = exePtr.cast<ffi.Char>();
+    
+    final allocatedArgs = <ffi.Pointer<Utf8>>[];
+    for (var i = 0; i < argsCount; i++) {
+      final ptr = arguments[i].toNativeUtf8();
+      allocatedArgs.add(ptr);
+      argv[i + 1] = ptr.cast<ffi.Char>();
+    }
+    argv[argsCount + 1] = ffi.Pointer.fromAddress(0);
+
+    // 4. Fork child process
     final pid = fork();
     if (pid < 0) {
       close(masterFd);
+      malloc.free(pathPtr);
+      malloc.free(exePtr);
+      if (workDirPtr != null) malloc.free(workDirPtr);
+      malloc.free(argv);
+      for (final p in allocatedArgs) {
+        malloc.free(p);
+      }
       throw OSError('Failed to fork child process');
     }
 
     if (pid == 0) {
-      // --- CHILD PROCESS BRANCH ---
+      // --- CHILD PROCESS BRANCH (Strictly async-signal-safe syscalls only!) ---
       setsid();
 
-      final pathPtr = slaveName.toNativeUtf8();
       final slaveFd = open(pathPtr.cast<ffi.Char>(), 2);
-      malloc.free(pathPtr);
       if (slaveFd < 0) {
         exit(-1);
       }
 
-      // TIOCSCTTY = 0x540E
       ioctl(slaveFd, 0x540E, ffi.Pointer.fromAddress(0));
 
       dup2(slaveFd, 0);
@@ -149,25 +177,23 @@ class PtySession {
         close(slaveFd);
       }
 
-      if (workingDirectory != null) {
-        Directory.current = workingDirectory;
+      if (workDirPtr != null) {
+        chdir(workDirPtr.cast<ffi.Char>());
       }
 
-      final exePtr = executable.toNativeUtf8();
-      final argsCount = arguments.length;
-      final argv = malloc<ffi.Pointer<ffi.Char>>(argsCount + 2);
-      argv[0] = exePtr.cast<ffi.Char>();
-      
-      for (var i = 0; i < argsCount; i++) {
-        argv[i + 1] = arguments[i].toNativeUtf8().cast<ffi.Char>();
-      }
-      argv[argsCount + 1] = ffi.Pointer.fromAddress(0);
-
       execvp(exePtr.cast<ffi.Char>(), argv);
       exit(-2);
     }
 
     // --- PARENT PROCESS BRANCH ---
+    malloc.free(pathPtr);
+    malloc.free(exePtr);
+    if (workDirPtr != null) malloc.free(workDirPtr);
+    malloc.free(argv);
+    for (final p in allocatedArgs) {
+      malloc.free(p);
+    }
+
     return PtySession._(
       masterFd: masterFd,
       slaveName: slaveName,
@@ -246,6 +272,12 @@ class PtySession {
       ffi.Int32 Function(ffi.Int32 pid, ffi.Int32 sig),
       int Function(int pid, int sig)
     >('kill');
-    kill(childPid, 15);
+    kill(childPid, 15); // SIGTERM = 15
+
+    // Reap child zombie process (non-blocking WNOHANG = 1)
+    final waitpid = libc.lookupFunction<_waitpid_c, _waitpid_dart>('waitpid');
+    final statusPtr = calloc<ffi.Int32>();
+    waitpid(childPid, statusPtr, 1);
+    calloc.free(statusPtr);
   }
 }

.agents/skills/multi-agent-mux-ui/packages/mam_pty/test/pty_runtime_test.dart

@@ -1,6 +1,5 @@
 import 'dart:async';
 import 'dart:convert';
-import 'dart:io';
 import 'package:mam_pty/mam_pty.dart';
 import 'package:test/test.dart';