Pass through isatty if the host is also unix

src/shims/unix/foreign_items.rs

@@ -440,12 +440,8 @@ fn emulate_foreign_item_by_name(
             // Miscellaneous
             "isatty" => {
                 let [fd] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
-                this.read_scalar(fd)?.to_i32()?;
-                // "returns 1 if fd is an open file descriptor referring to a terminal; otherwise 0 is returned, and errno is set to indicate the error"
-                // FIXME: we just say nothing is a terminal.
-                let enotty = this.eval_libc("ENOTTY")?;
-                this.set_last_error(enotty)?;
-                this.write_null(dest)?;
+                let result = this.isatty(fd)?;
+                this.write_scalar(Scalar::from_i32(result), dest)?;
             }
             "pthread_atfork" => {
                 let [prepare, parent, child] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;

src/shims/unix/fs.rs

@@ -50,6 +50,9 @@ fn close<'tcx>(
     ) -> InterpResult<'tcx, io::Result<i32>>;
 
     fn dup(&mut self) -> io::Result<Box<dyn FileDescriptor>>;
+
+    #[cfg(unix)]
+    fn as_unix_host_fd(&self) -> Option<i32>;
 }
 
 impl FileDescriptor for FileHandle {
@@ -114,6 +117,12 @@ fn dup(&mut self) -> io::Result<Box<dyn FileDescriptor>> {
         let duplicated = self.file.try_clone()?;
         Ok(Box::new(FileHandle { file: duplicated, writable: self.writable }))
     }
+
+    #[cfg(unix)]
+    fn as_unix_host_fd(&self) -> Option<i32> {
+        use std::os::unix::io::AsRawFd;
+        Some(self.file.as_raw_fd())
+    }
 }
 
 impl FileDescriptor for io::Stdin {
@@ -159,6 +168,11 @@ fn close<'tcx>(
     fn dup(&mut self) -> io::Result<Box<dyn FileDescriptor>> {
         Ok(Box::new(io::stdin()))
     }
+
+    #[cfg(unix)]
+    fn as_unix_host_fd(&self) -> Option<i32> {
+        Some(libc::STDIN_FILENO)
+    }
 }
 
 impl FileDescriptor for io::Stdout {
@@ -209,6 +223,11 @@ fn close<'tcx>(
     fn dup(&mut self) -> io::Result<Box<dyn FileDescriptor>> {
         Ok(Box::new(io::stdout()))
     }
+
+    #[cfg(unix)]
+    fn as_unix_host_fd(&self) -> Option<i32> {
+        Some(libc::STDOUT_FILENO)
+    }
 }
 
 impl FileDescriptor for io::Stderr {
@@ -252,6 +271,11 @@ fn close<'tcx>(
     fn dup(&mut self) -> io::Result<Box<dyn FileDescriptor>> {
         Ok(Box::new(io::stderr()))
     }
+
+    #[cfg(unix)]
+    fn as_unix_host_fd(&self) -> Option<i32> {
+        Some(libc::STDERR_FILENO)
+    }
 }
 
 #[derive(Debug)]
@@ -297,6 +321,11 @@ fn close<'tcx>(
     fn dup<'tcx>(&mut self) -> io::Result<Box<dyn FileDescriptor>> {
         Ok(Box::new(DummyOutput))
     }
+
+    #[cfg(unix)]
+    fn as_unix_host_fd(&self) -> Option<i32> {
+        None
+    }
 }
 
 #[derive(Debug)]
@@ -1660,6 +1689,38 @@ fn readlink(
             }
         }
     }
+
+    #[cfg_attr(not(unix), allow(unused))]
+    fn isatty(&mut self, miri_fd: &OpTy<'tcx, Tag>) -> InterpResult<'tcx, i32> {
+        let this = self.eval_context_mut();
+        #[cfg(unix)]
+        {
+            let miri_fd = this.read_scalar(miri_fd)?.to_i32()?;
+            if let Some(host_fd) =
+                this.machine.file_handler.handles.get(&miri_fd).and_then(|fd| fd.as_unix_host_fd())
+            {
+                // "returns 1 if fd is an open file descriptor referring to a terminal;
+                // otherwise 0 is returned, and errno is set to indicate the error"
+                // SAFETY: isatty has no preconditions
+                let is_tty = unsafe { libc::isatty(host_fd) };
+                if is_tty == 0 {
+                    let errno = std::io::Error::last_os_error()
+                        .raw_os_error()
+                        .map(Scalar::from_i32)
+                        .unwrap();
+                    this.set_last_error(errno)?;
+                }
+                return Ok(is_tty);
+            }
+        }
+        // We are attemping to use a Unix interface on a non-Unix platform, or we are on a Unix
+        // platform and the passed file descriptor is not open.
+        // FIXME: It should be possible to emulate this at least on Windows by using
+        // GetConsoleMode.
+        let enotty = this.eval_libc("ENOTTY")?;
+        this.set_last_error(enotty)?;
+        Ok(0)
+    }
 }
 
 /// Extracts the number of seconds and nanoseconds elapsed between `time` and the unix epoch when