Lazy import rlcompleter in pdb to avoid deadlock in subprocess

Author: gaogaotiantian

Lib/pdb.py

@@ -100,7 +100,6 @@
 import _pyrepl.utils
 
 from contextlib import ExitStack, closing, contextmanager
-from rlcompleter import Completer
 from types import CodeType
 from warnings import deprecated
 
@@ -1187,6 +1186,9 @@ def completedefault(self, text, line, begidx, endidx):
             return [f"${name}" for name in conv_vars if name.startswith(text[1:])]
 
         # Use rlcompleter to do the completion
+        # GH-138860
+        # This needs to be lazy-imported to avoid deadlock
+        from rlcompleter import Completer
         state = 0
         matches = []
         completer = Completer(self.curframe.f_globals | self.curframe.f_locals)
@@ -1205,6 +1207,9 @@ def _enable_rlcompleter(self, ns):
 
         try:
             old_completer = readline.get_completer()
+            # GH-138860
+            # This needs to be lazy-imported to avoid deadlock
+            from rlcompleter import Completer
             completer = Completer(ns)
             readline.set_completer(completer.complete)
             yield

Lib/test/test_pdb.py

@@ -4688,6 +4688,28 @@ def foo():
         stdout, _ = self._run_script(script, commands)
         self.assertIn("42", stdout)
 
+    def test_readline_not_imported(self):
+        """GH-138860
+        Directly or indirectly importing readline might deadlock a subprocess
+        if it's launched with process_group=0 or preexec_fn=setpgrp
+        
+        It's also a pattern that readline is never imported with just import pdb.
+
+        This test is to ensure that readline is not imported for import pdb.
+        It's possible that we have a good reason to do that in the future.
+        """
+
+        script = textwrap.dedent("""
+            import sys
+            import pdb
+            if "readline" in sys.modules:
+                print("readline imported")
+        """)
+        commands = ""
+        stdout, stderr = self._run_script(script, commands)
+        self.assertNotIn("readline imported", stdout)
+        self.assertEqual(stderr, "")
+
 
 @support.force_colorized_test_class
 class PdbTestColorize(unittest.TestCase):