Handle zero foreground process groups on non-Linux targets

src/backend/libc/termios/syscalls.rs

@@ -167,10 +167,9 @@ pub(crate) fn tcgetpgrp(fd: BorrowedFd<'_>) -> io::Result<Pid> {
     unsafe {
         let pid = ret_pid_t(c::tcgetpgrp(borrowed_fd(fd)))?;
 
-        // This doesn't appear to be documented, but on Linux, it appears
-        // `tcsetpgrp` can succeed and set the pid to 0 if we pass it a
-        // pseudo-terminal device fd. For now, translate it into `OPNOTSUPP`.
-        #[cfg(linux_kernel)]
+        // On some platforms, a pseudo-terminal without a foreground process
+        // group returns 0, which `Pid` cannot represent. Translate this into
+        // `OPNOTSUPP`, as in the linux_raw backend.
         if pid == 0 {
             return Err(io::Errno::OPNOTSUPP);
         }

src/termios/tc.rs

@@ -55,7 +55,7 @@ pub fn tcgetwinsize<Fd: AsFd>(fd: Fd) -> io::Result<Winsize> {
 ///
 /// Also known as the `TIOCGPGRP` operation with `ioctl`.
 ///
-/// On Linux, if `fd` is a pseudo-terminal, the underlying system call here can
+/// If `fd` is a pseudo-terminal, the underlying system call here can
 /// return a pid of 0, which rustix's `Pid` type doesn't support. So rustix
 /// instead handles this case by failing with [`io::Errno::OPNOTSUPP`] if the
 /// pid is 0.

tests/termios/pgrp.rs

@@ -33,15 +33,15 @@ fn pgrp_pseudoterminal() {
         Err(err) => panic!("{:?}", err),
     };
 
-    // Linux's `tcgetpgrp` returns 0 here, which is not documented, so rustix
-    // translates it into `OPNOTSUPP`.
-    #[cfg(linux_kernel)]
+    // Linux's and Apple's `tcgetpgrp` return 0 here, which is not documented,
+    // so rustix translates it into `OPNOTSUPP`.
+    #[cfg(any(linux_kernel, apple))]
     assert_eq!(tcgetpgrp(&pty), Err(rustix::io::Errno::OPNOTSUPP));
 
     // FreeBSD's `tcgetpgrp` returns 100000 here, or presumably some other
     // number if that number is already taken, which is documented behavior,
     // but impossible to test for reliably.
-    #[cfg(not(linux_kernel))]
+    #[cfg(not(any(linux_kernel, apple)))]
     assert!(matches!(tcgetpgrp(&pty), Ok(_)));
 
     // We shouldn't be able to set the process group to [`Pid::INIT`].