pdb: fix usage in child thread after main thread exited

Fixes pytest-dev#5228.

changelog/5228.bugfix.rst

@@ -0,0 +1 @@
+Fix ``pdb.set_trace`` wrapper when used in child threads after main thread exited.

src/_pytest/debugging.py

@@ -77,9 +77,7 @@ def pytest_configure(config):
     if config.getvalue("usepdb"):
         config.pluginmanager.register(PdbInvoke(), "pdbinvoke")
 
-    pytestPDB._saved.append(
-        (pdb.set_trace, pytestPDB._pluginmanager, pytestPDB._config, pytestPDB._pdb_cls)
-    )
+    pytestPDB._saved.append(pdb.set_trace)
     pdb.set_trace = pytestPDB.set_trace
     pytestPDB._pluginmanager = config.pluginmanager
     pytestPDB._config = config
@@ -88,12 +86,7 @@ def pytest_configure(config):
     # NOTE: not using pytest_unconfigure, since it might get called although
     #       pytest_configure was not (if another plugin raises UsageError).
     def fin():
-        (
-            pdb.set_trace,
-            pytestPDB._pluginmanager,
-            pytestPDB._config,
-            pytestPDB._pdb_cls,
-        ) = pytestPDB._saved.pop()
+        pdb.set_trace = pytestPDB._saved.pop()
 
     config._cleanup.append(fin)
 

testing/test_pdb.py

@@ -1191,3 +1191,47 @@ def test(monkeypatch):
     result = testdir.runpytest(str(p1))
     result.stdout.fnmatch_lines(["E *BdbQuit", "*= 1 failed in*"])
     assert result.ret == 1
+
+
+def test_pdb_in_thread_after_exit(testdir):
+    """Ensure that pdb.set_trace works after main thread exited already.
+
+    This tests both continuation after the main thread exited, and a new
+    set_trace afterwards.
+    """
+    p1 = testdir.makepyfile(
+        """
+        import threading
+
+        main_thread = threading.main_thread()
+
+        def test():
+            evt = threading.Event()
+
+            def target():
+                print("target_" + "start")
+                evt.set()
+                assert main_thread.is_alive()
+                __import__('pdb').set_trace()
+                assert not main_thread.is_alive()
+                __import__('pdb').set_trace()
+                print("target_" + "end")
+
+            thread = threading.Thread(target=target)
+            thread.start()
+
+            evt.wait()
+        """
+    )
+    child = testdir.spawn_pytest(str(p1) + " -s")
+    child.expect("target_start")
+    child.expect("= 1 passed in")  # main thread exited
+    child.expect(r"\(Pdb")
+    child.sendline("c")
+    child.expect(r"\(Pdb")
+    child.sendline("c")
+    child.expect("target_end")
+    child.wait()
+    rest = child.read().decode("utf8")
+    assert "Exception in thread" not in rest
+    assert child.exitstatus == 0