pdb: trigger pytest_enter_pdb hook with post-mortem

This is required for pytest-pdb to be called with `--pdb`.

TODO:

- [ ] test
- [ ] pass mode to hook, e.g. "post_mortem" in this case?

changelog/4908.bugfix.rst

@@ -0,0 +1 @@
+The ``pytest_enter_pdb`` hook gets called with post-mortem (``--pdb``).

src/_pytest/debugging.py

@@ -212,6 +212,17 @@ class pytestPDB(object):
                             self._pytest_capman.suspend_global_capture(in_=True)
                     return ret
 
+                def get_stack(self, f, t):
+                    stack, i = super(PytestPdbWrapper, self).get_stack(f, t)
+                    if f is None:
+                        # Find last non-hidden frame.
+                        i = max(0, len(stack) - 1)
+                        while i and stack[i][0].f_locals.get(
+                            "__tracebackhide__", False
+                        ):
+                            i -= 1
+                    return stack, i
+
             _pdb = PytestPdbWrapper(**kwargs)
             cls._pluginmanager.hook.pytest_enter_pdb(config=cls._config, pdb=_pdb)
         else:
@@ -298,22 +309,8 @@ def _postmortem_traceback(excinfo):
         return excinfo._excinfo[2]
 
 
-def _find_last_non_hidden_frame(stack):
-    i = max(0, len(stack) - 1)
-    while i and stack[i][0].f_locals.get("__tracebackhide__", False):
-        i -= 1
-    return i
-
-
 def post_mortem(t):
-    class Pdb(pytestPDB._pdb_cls, object):
-        def get_stack(self, f, t):
-            stack, i = super(Pdb, self).get_stack(f, t)
-            if f is None:
-                i = _find_last_non_hidden_frame(stack)
-            return stack, i
-
-    p = Pdb()
+    p = pytestPDB._init_pdb()
     p.reset()
     p.interaction(None, t)
     if p.quitting:

testing/test_pdb.py

@@ -744,7 +744,8 @@ class TestPDB(object):
             ["E   NameError: *xxx*", "*! *Exit: Quitting debugger !*"]  # due to EOF
         )
 
-    def test_enter_leave_pdb_hooks_are_called(self, testdir):
+    @pytest.mark.parametrize("post_mortem", (False, True))
+    def test_enter_leave_pdb_hooks_are_called(self, post_mortem, testdir):
         testdir.makeconftest(
             """
             mypdb = None
@@ -773,16 +774,25 @@ class TestPDB(object):
             """
             import pytest
 
-            def test_foo():
+            def test_set_trace():
                 pytest.set_trace()
                 assert 0
+
+            def test_post_mortem():
+                assert 0
         """
         )
-        child = testdir.spawn_pytest(str(p1))
+        if post_mortem:
+            child = testdir.spawn_pytest(str(p1) + " --pdb -s -k test_post_mortem")
+        else:
+            child = testdir.spawn_pytest(str(p1) + " -k test_set_trace")
         child.expect("enter_pdb_hook")
         child.sendline("c")
-        child.expect(r"PDB continue \(IO-capturing resumed\)")
-        child.expect("Captured stdout call")
+        if post_mortem:
+            child.expect(r"PDB continue")
+        else:
+            child.expect(r"PDB continue \(IO-capturing resumed\)")
+            child.expect("Captured stdout call")
         rest = child.read().decode("utf8")
         assert "leave_pdb_hook" in rest
         assert "1 failed" in rest