bpo-26228: Fix pty EOF handling (GH-12049)

On non-Linux POSIX platforms, like FreeBSD or macOS,
the FD used to read a forked PTY may signal its exit not
by raising an error but by sending empty data to the read
syscall. This case wasn't handled, leading to hanging
`pty.spawn` calls.

Co-authored-by: Reilly Tucker Siemens <reilly@tuckersiemens.com>
Co-authored-by: Łukasz Langa <lukasz@langa.pl>

Lib/test/test_pty.py

@@ -5,8 +5,9 @@ from test.support.import_helper import import_module
 import_module('termios')
 
 import errno
-import pty
 import os
+import pty
+import tty
 import sys
 import select
 import signal
@@ -123,12 +124,6 @@ class PtyTest(unittest.TestCase):
 
     @staticmethod
     def handle_sighup(signum, frame):
-        # bpo-38547: if the process is the session leader, os.close(master_fd)
-        # of "master_fd, slave_name = pty.master_open()" raises SIGHUP
-        # signal: just ignore the signal.
-        #
-        # NOTE: the above comment is from an older version of the test;
-        # master_open() is not being used anymore.
         pass
 
     @expectedFailureIfStdinIsTTY
@@ -190,13 +185,6 @@ class PtyTest(unittest.TestCase):
             self.assertEqual(_get_term_winsz(slave_fd), new_stdin_winsz,
                              "openpty() failed to set slave window size")
 
-        # Solaris requires reading the fd before anything is returned.
-        # My guess is that since we open and close the slave fd
-        # in master_open(), we need to read the EOF.
-        #
-        # NOTE: the above comment is from an older version of the test;
-        # master_open() is not being used anymore.
-
         # Ensure the fd is non-blocking in case there's nothing to read.
         blocking = os.get_blocking(master_fd)
         try:
@@ -324,22 +312,40 @@ class PtyTest(unittest.TestCase):
 
         self.assertEqual(data, b"")
 
+    def test_spawn_doesnt_hang(self):
+        pty.spawn([sys.executable, '-c', 'print("hi there")'])
+
 class SmallPtyTests(unittest.TestCase):
     """These tests don't spawn children or hang."""
 
     def setUp(self):
         self.orig_stdin_fileno = pty.STDIN_FILENO
         self.orig_stdout_fileno = pty.STDOUT_FILENO
+        self.orig_pty_close = pty.close
+        self.orig_pty__copy = pty._copy
+        self.orig_pty_fork = pty.fork
         self.orig_pty_select = pty.select
+        self.orig_pty_setraw = pty.setraw
+        self.orig_pty_tcgetattr = pty.tcgetattr
+        self.orig_pty_tcsetattr = pty.tcsetattr
+        self.orig_pty_waitpid = pty.waitpid
         self.fds = []  # A list of file descriptors to close.
         self.files = []
         self.select_rfds_lengths = []
         self.select_rfds_results = []
+        self.tcsetattr_mode_setting = None
 
     def tearDown(self):
         pty.STDIN_FILENO = self.orig_stdin_fileno
         pty.STDOUT_FILENO = self.orig_stdout_fileno
+        pty.close = self.orig_pty_close
+        pty._copy = self.orig_pty__copy
+        pty.fork = self.orig_pty_fork
         pty.select = self.orig_pty_select
+        pty.setraw = self.orig_pty_setraw
+        pty.tcgetattr = self.orig_pty_tcgetattr
+        pty.tcsetattr = self.orig_pty_tcsetattr
+        pty.waitpid = self.orig_pty_waitpid
         for file in self.files:
             try:
                 file.close()
@@ -367,6 +373,14 @@ class SmallPtyTests(unittest.TestCase):
         self.assertEqual(self.select_rfds_lengths.pop(0), len(rfds))
         return self.select_rfds_results.pop(0), [], []
 
+    def _make_mock_fork(self, pid):
+        def mock_fork():
+            return (pid, 12)
+        return mock_fork
+
+    def _mock_tcsetattr(self, fileno, opt, mode):
+        self.tcsetattr_mode_setting = mode
+
     def test__copy_to_each(self):
         """Test the normal data case on both master_fd and stdin."""
         read_from_stdout_fd, mock_stdout_fd = self._pipe()
@@ -407,7 +421,6 @@ class SmallPtyTests(unittest.TestCase):
         socketpair[1].close()
         os.close(write_to_stdin_fd)
 
-        # Expect two select calls, the last one will cause IndexError
         pty.select = self._mock_select
         self.select_rfds_lengths.append(2)
         self.select_rfds_results.append([mock_stdin_fd, masters[0]])
@@ -415,12 +428,34 @@ class SmallPtyTests(unittest.TestCase):
         # both encountered an EOF before the second select call.
         self.select_rfds_lengths.append(0)
 
-        with self.assertRaises(IndexError):
-            pty._copy(masters[0])
+        # We expect the function to return without error.
+        self.assertEqual(pty._copy(masters[0]), None)
+
+    def test__restore_tty_mode_normal_return(self):
+        """Test that spawn resets the tty mode no when _copy returns normally."""
+
+        # PID 1 is returned from mocked fork to run the parent branch
+        # of code
+        pty.fork = self._make_mock_fork(1)
+
+        status_sentinel = object()
+        pty.waitpid = lambda _1, _2: [None, status_sentinel]
+        pty.close = lambda _: None
+
+        pty._copy = lambda _1, _2, _3: None
+
+        mode_sentinel = object()
+        pty.tcgetattr = lambda fd: mode_sentinel
+        pty.tcsetattr = self._mock_tcsetattr
+        pty.setraw = lambda _: None
+
+        self.assertEqual(pty.spawn([]), status_sentinel, "pty.waitpid process status not returned by pty.spawn")
+        self.assertEqual(self.tcsetattr_mode_setting, mode_sentinel, "pty.tcsetattr not called with original mode value")
 
 
 def tearDownModule():
     reap_children()
 
+
 if __name__ == "__main__":
     unittest.main()