fix(ui): enforce async-signal-safe exit, blocking waitpid reaping, and unsetenv env isolation inside child PTY process

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

@@ -53,6 +53,12 @@ typedef _chdir_dart = int Function(ffi.Pointer<ffi.Char> path);
 typedef _fcntl_c = ffi.Int32 Function(ffi.Int32 fd, ffi.Int32 cmd, ffi.Int32 arg);
 typedef _fcntl_dart = int Function(int fd, int cmd, int arg);
 
+typedef _unsetenv_c = ffi.Int32 Function(ffi.Pointer<ffi.Char> name);
+typedef _unsetenv_dart = int Function(ffi.Pointer<ffi.Char> name);
+
+typedef _exit_c = ffi.Void Function(ffi.Int32 status);
+typedef _exit_dart = void Function(int status);
+
 base class Winsize extends ffi.Struct {
   @ffi.Uint16()
   external int ws_row;
@@ -101,6 +107,8 @@ class PtySession {
     final ioctl = libc.lookupFunction<_ioctl_c, _ioctl_dart>('ioctl');
     final chdir = libc.lookupFunction<_chdir_c, _chdir_dart>('chdir');
     final fcntl = libc.lookupFunction<_fcntl_c, _fcntl_dart>('fcntl');
+    final unsetenv = libc.lookupFunction<_unsetenv_c, _unsetenv_dart>('unsetenv');
+    final cExit = libc.lookupFunction<_exit_c, _exit_dart>('exit');
 
     // 1. Prepare master PTY
     final masterFd = posixOpenpt(2 | 0x00000400);
@@ -132,16 +140,15 @@ class PtySession {
       fcntl(masterFd, 4, flags | 2048);
     }
 
-    // 2. Resolve environment, isolating nested TMUX contexts (§6.7)
+    // 2. Pre-allocate ALL structures on the heap before fork() for async-signal-safety (§6.7)
-    final env = Map<String, String>.from(environment ?? Platform.environment);
-    env.remove('TMUX');
-    env.remove('TMUX_PANE');
-
-    // 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();
     
+    // Environment isolation variables to be cleared in child context
+    final tmuxNamePtr = 'TMUX'.toNativeUtf8();
+    final tmuxPaneNamePtr = 'TMUX_PANE'.toNativeUtf8();
+
     final argsCount = arguments.length;
     final argv = malloc<ffi.Pointer<ffi.Char>>(argsCount + 2);
     argv[0] = exePtr.cast<ffi.Char>();
@@ -154,12 +161,14 @@ class PtySession {
     }
     argv[argsCount + 1] = ffi.Pointer.fromAddress(0);
 
-    // 4. Fork child process
+    // 3. Fork child process
     final pid = fork();
     if (pid < 0) {
       close(masterFd);
       malloc.free(pathPtr);
       malloc.free(exePtr);
+      malloc.free(tmuxNamePtr);
+      malloc.free(tmuxPaneNamePtr);
       if (workDirPtr != null) malloc.free(workDirPtr);
       malloc.free(argv);
       for (final p in allocatedArgs) {
@@ -169,12 +178,18 @@ class PtySession {
     }
 
     if (pid == 0) {
-      // --- CHILD PROCESS BRANCH (Strictly async-signal-safe syscalls only!) ---
+      // --- CHILD PROCESS BRANCH (Strictly async-signal-safe OS calls only!) ---
+      // A. Disassociate controlling terminal
       setsid();
 
+      // B. Isolate from nested TMUX environments (§6.7)
+      unsetenv(tmuxNamePtr.cast<ffi.Char>());
+      unsetenv(tmuxPaneNamePtr.cast<ffi.Char>());
+
+      // C. Open slave and redirect stdio
       final slaveFd = open(pathPtr.cast<ffi.Char>(), 2);
       if (slaveFd < 0) {
-        exit(-1);
+        cExit(-1); // direct libc _exit to prevent async-signal-unsafe Dart runtime exit
       }
 
       ioctl(slaveFd, 0x540E, ffi.Pointer.fromAddress(0));
@@ -193,12 +208,14 @@ class PtySession {
       }
 
       execvp(exePtr.cast<ffi.Char>(), argv);
-      exit(-2);
+      cExit(-2); // direct libc _exit on exec failure
     }
 
     // --- PARENT PROCESS BRANCH ---
     malloc.free(pathPtr);
     malloc.free(exePtr);
+    malloc.free(tmuxNamePtr);
+    malloc.free(tmuxPaneNamePtr);
     if (workDirPtr != null) malloc.free(workDirPtr);
     malloc.free(argv);
     for (final p in allocatedArgs) {
@@ -279,16 +296,17 @@ class PtySession {
     final close = libc.lookupFunction<_close_c, _close_dart>('close');
     close(masterFd);
 
+    // Send SIGTERM to terminate child process
     final kill = libc.lookupFunction<
       ffi.Int32 Function(ffi.Int32 pid, ffi.Int32 sig),
       int Function(int pid, int sig)
     >('kill');
     kill(childPid, 15); // SIGTERM = 15
 
-    // Reap child zombie process (non-blocking WNOHANG = 1)
+    // Reap child zombie process (blocking waitpid options = 0 to guarantee reaping)
     final waitpid = libc.lookupFunction<_waitpid_c, _waitpid_dart>('waitpid');
     final statusPtr = calloc<ffi.Int32>();
-    waitpid(childPid, statusPtr, 1);
+    waitpid(childPid, statusPtr, 0); // Blocking wait guarantees process resource removal
     calloc.free(statusPtr);
   }
 }