This patch comes from upstream: http://bugs.python.org/issue26228
It has not yet been committed, but seems on track to be, and we need
it to fix a bug.
--- Python-3.9.0/Lib/pty.py
+++ Python-3.9.0/Lib/pty.py
@@ -138,7 +138,7 @@ def _copy(master_fd, master_read=_read,
         if master_fd in rfds:
             data = master_read(master_fd)
             if not data:  # Reached EOF.
-                fds.remove(master_fd)
+                return
             else:
                 os.write(STDOUT_FILENO, data)
         if STDIN_FILENO in rfds:
@@ -155,7 +155,15 @@ def spawn(argv, master_read=_read, stdin
     sys.audit('pty.spawn', argv)
     pid, master_fd = fork()
     if pid == CHILD:
-        os.execlp(argv[0], *argv)
+        try:
+            os.execlp(argv[0], *argv)
+        except:
+            # If we wanted to be really clever, we would use
+            # the same method as subprocess() to pass the error
+            # back to the parent.  For now just dump stack trace.
+            traceback.print_exc()
+        finally:
+            os._exit(1)
     try:
         mode = tty.tcgetattr(STDIN_FILENO)
         tty.setraw(STDIN_FILENO)
@@ -165,6 +173,10 @@ def spawn(argv, master_read=_read, stdin
     try:
         _copy(master_fd, master_read, stdin_read)
     except OSError:
+        # Some OSes never return an EOF on pty, just raise
+        # an error instead.
+        pass
+    finally:
         if restore:
             tty.tcsetattr(STDIN_FILENO, tty.TCSAFLUSH, mode)
 
--- Python-3.9.0/Lib/test/test_pty.py
+++ Python-3.9.0/Lib/test/test_pty.py
@@ -306,7 +306,7 @@ class SmallPtyTests(unittest.TestCase):
         socketpair[1].close()
         os.close(write_to_stdin_fd)
 
-        # Expect two select calls, the last one will cause IndexError
+        # Expect two select calls, then a normal return on master EOF
         pty.select = self._mock_select
         self.select_rfds_lengths.append(2)
         self.select_rfds_results.append([mock_stdin_fd, masters[0]])
@@ -314,8 +314,7 @@ 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])
+        pty._copy(masters[0])
 
 
 def tearDownModule():