Don't check the errno value from isatty. (#468)

We don't actually do anything different for different errno values,
other than panic on unknown ones, and that isn't that isn't adding
much value compared to the cost of being an extra surprise when
porting to new OS's.

Fixes #467.

src/backend/libc/termios/syscalls.rs

@@ -10,13 +10,13 @@ use crate::fd::BorrowedFd;
 #[cfg(feature = "procfs")]
 #[cfg(not(any(target_os = "fuchsia", target_os = "wasi")))]
 use crate::ffi::CStr;
+#[cfg(not(target_os = "wasi"))]
 use crate::io;
 #[cfg(not(target_os = "wasi"))]
 use crate::process::{Pid, RawNonZeroPid};
 #[cfg(not(target_os = "wasi"))]
 use crate::termios::{Action, OptionalActions, QueueSelector, Speed, Termios, Winsize};
 use core::mem::MaybeUninit;
-use libc_errno::errno;
 
 #[cfg(not(target_os = "wasi"))]
 pub(crate) fn tcgetattr(fd: BorrowedFd<'_>) -> io::Result<Termios> {
@@ -142,26 +142,12 @@ pub(crate) fn cfsetspeed(termios: &mut Termios, speed: Speed) -> io::Result<()>
 }
 
 pub(crate) fn isatty(fd: BorrowedFd<'_>) -> bool {
-    let res = unsafe { c::isatty(borrowed_fd(fd)) };
-    if res == 0 {
-        match errno().0 {
-            #[cfg(not(any(target_os = "android", target_os = "linux")))]
-            c::ENOTTY => false,
-
-            // Old Linux versions reportedly return `EINVAL`.
-            // <https://man7.org/linux/man-pages/man3/isatty.3.html#ERRORS>
-            #[cfg(any(target_os = "android", target_os = "linux"))]
-            c::ENOTTY | c::EINVAL => false,
-
-            // Darwin mysteriously returns `EOPNOTSUPP` sometimes.
-            #[cfg(any(target_os = "ios", target_os = "macos"))]
-            c::EOPNOTSUPP => false,
-
-            err => panic!("unexpected error from isatty: {:?}", err),
-        }
-    } else {
-        true
-    }
+    // Use the return value of `isatty` alone. We don't check `errno` because
+    // we return `bool` rather than `io::Result<bool>`, because we assume
+    // `BorrrowedFd` protects us from `EBADF`, and any other reasonably
+    // anticipated errno value would end up interpreted as "assume it's not a
+    // terminal" anyway.
+    unsafe { c::isatty(borrowed_fd(fd)) != 0 }
 }
 
 #[cfg(feature = "procfs")]