Defer registering SIGCONT handler until UnixConsole is actually used

Related: gh-161254

Author: meshulam

Lib/_pyrepl/unix_console.py

@@ -267,8 +267,6 @@ def _my_getstr(cap: str, optional: bool = False) -> bytes | None:
         )
         self.cursor_visible = True
 
-        signal.signal(signal.SIGCONT, self._sigcont_handler)
-
     def _sigcont_handler(self, signum, frame):
         self.restore()
         self.prepare()
@@ -505,6 +503,7 @@ def prepare(self) -> None:
 
         try:
             self.old_sigwinch = signal.signal(signal.SIGWINCH, self.__sigwinch)
+            signal.signal(signal.SIGCONT, self._sigcont_handler)
         except ValueError:
             pass
 
@@ -515,6 +514,9 @@ def restore(self) -> None:
         Restore the console to the default state
         """
         trace("unix.restore")
+        if not hasattr(self, "_UnixConsole__svtermstate"):
+            # prepare() never ran, so there is nothing to restore
+            return
         self.__write(ANSIColors.RESET)
         self.__disable_bracketed_paste()
         self.__maybe_write_code(self._rmkx)
@@ -533,6 +535,11 @@ def restore(self) -> None:
                     raise e
             del self.old_sigwinch
 
+        try:
+            signal.signal(signal.SIGCONT, signal.SIG_DFL)
+        except ValueError:
+            pass
+
     def push_char(self, char: int | bytes) -> None:
         """
         Push a character to the console event queue.

Lib/test/test_pyrepl/test_unix_console.py

@@ -401,6 +401,26 @@ def test_restore_in_thread(self, _os_write):
         thread.start()
         thread.join()  # this should not raise
 
+    @unittest.skipIf(not hasattr(signal, "SIGCONT"), "needs signal.SIGCONT")
+    def test_sigcont_is_armed_only_by_prepare(self, _os_write):
+        # gh-161254: A console must install its SIGCONT handler only while it
+        # owns the terminal, i.e. between prepare() and restore().
+        console = UnixConsole(term="xterm")
+        self.assertEqual(signal.getsignal(signal.SIGCONT), signal.SIG_DFL)
+        console.prepare()
+        self.assertEqual(
+            signal.getsignal(signal.SIGCONT), console._sigcont_handler
+        )
+        console.restore()
+        self.assertEqual(signal.getsignal(signal.SIGCONT), signal.SIG_DFL)
+
+    @unittest.skipIf(not hasattr(signal, "SIGCONT"), "needs signal.SIGCONT")
+    def test_restore_without_prepare(self, _os_write):
+        # gh-161254
+        console = UnixConsole(term="xterm")
+        self.assertFalse(hasattr(console, "_UnixConsole__svtermstate"))
+        console.restore()  # this should not raise
+
 
 @unittest.skipIf(sys.platform == "win32", "No Unix console on Windows")
 class TestUnixConsoleEIOHandling(TestCase):