Make debugging's pytest_configure re-entrant

This is relevant when using runpytest in-process.

Fixes:

E             def test_1(testdir):
E                 testdir.runpytest()
E         >       __import__('pdb').set_trace()
E
E         ../../test_trace_after_runpytest.py:3:
E         …/Vcs/pytest/src/_pytest/debugging.py:81: in set_trace
E             tw = _pytest.config.create_terminal_writer(cls._config)
E
E         config = None, args = (), kwargs = {}, tw = <py._io.terminalwriter.TerminalWriter object at 0x7f1097088160>
E
E             def create_terminal_writer(config, *args, **kwargs):
E                 """Create a TerminalWriter instance configured according to the options
E                 in the config object. Every code which requires a TerminalWriter object
E                 and has access to a config object should use this function.
E                 """
E                 tw = py.io.TerminalWriter(*args, **kwargs)
E         >       if config.option.color == "yes":
E         E       AttributeError: 'NoneType' object has no attribute 'option'

1 files changed, 27 insertions, 0 deletions

diff --git a/testing/test_pdb.py b/testing/test_pdb.py
index 56fe5fc7a..ec1af5b56 100644
--- a/testing/test_pdb.py
+++ b/testing/test_pdb.py
@@ -784,3 +784,30 @@ class TestTraceOption:
         assert "1 passed" in rest
         assert "reading from stdin while output" not in rest
         TestPDB.flush(child)
+
+
+def test_trace_after_runpytest(testdir):
+    """Test that debugging's pytest_configure is re-entrant."""
+    p1 = testdir.makepyfile(
+        """
+        from _pytest.debugging import pytestPDB
+
+        def test_outer(testdir):
+            from _pytest.debugging import pytestPDB
+
+            assert len(pytestPDB._saved) == 1
+
+            testdir.runpytest("-k test_inner")
+
+            __import__('pdb').set_trace()
+
+        def test_inner(testdir):
+            assert len(pytestPDB._saved) == 2
+    """
+    )
+    child = testdir.spawn_pytest("-p pytester %s -k test_outer" % p1)
+    child.expect(r"\(Pdb")
+    child.sendline("c")
+    rest = child.read().decode("utf8")
+    TestPDB.flush(child)
+    assert child.exitstatus == 0, rest