Fix --trace for parametrized tests

Without this, the second time it tries to stop in a parametrized
function it raises instead:

`ValueError: --trace can't be used with a fixture named func!`

Implementation idea, test (and changelog tweaks) thanks to @blueyed

Co-Authored-By: Ronny Pfannschmidt <opensource@ronnypfannschmidt.de>

changelog/6099.bugfix.rst

@@ -0,0 +1 @@
+Fix ``--trace`` when used with parametrized functions.

src/_pytest/debugging.py

@@ -1,5 +1,7 @@
 """ interactive debugging with PDB, the Python Debugger. """
 import argparse
+import bdb
+import functools
 import pdb
 import sys
 from doctest import UnexpectedException
@@ -274,13 +276,24 @@ def pytest_pyfunc_call(self, pyfuncitem):
 def _test_pytest_function(pyfuncitem):
     _pdb = pytestPDB._init_pdb("runcall")
     testfunction = pyfuncitem.obj
-    pyfuncitem.obj = _pdb.runcall
-    if "func" in pyfuncitem._fixtureinfo.argnames:  # pragma: no branch
-        raise ValueError("--trace can't be used with a fixture named func!")
-    pyfuncitem.funcargs["func"] = testfunction
-    new_list = list(pyfuncitem._fixtureinfo.argnames)
-    new_list.append("func")
-    pyfuncitem._fixtureinfo.argnames = tuple(new_list)
+
+    # we need a copy of bdb.runcall because it takes `func` as a parameter
+    # which means we can't have a kwarg called func
+    @functools.wraps(testfunction)
+    def runcall(*args, **kwds):
+        _pdb.reset()
+        sys.settrace(_pdb.trace_dispatch)
+        res = None
+        try:
+            res = testfunction(*args, **kwds)
+        except bdb.BdbQuit:
+            pass
+        finally:
+            _pdb.quitting = True
+            sys.settrace(None)
+        return res
+
+    pyfuncitem.obj = runcall
 
 
 def _enter_pdb(node, excinfo, rep):

testing/test_pdb.py

@@ -1025,6 +1025,51 @@ def test_3():
         assert "Exit: Quitting debugger" not in child.before.decode("utf8")
         TestPDB.flush(child)
 
+    def test_trace_with_parametrize_handles_shared_fixtureinfo(self, testdir):
+        p1 = testdir.makepyfile(
+            """
+            import pytest
+            @pytest.mark.parametrize('myparam', [1,2])
+            def test_1(myparam, request):
+                assert myparam in (1, 2)
+                assert request.function.__name__ == "test_1"
+            @pytest.mark.parametrize('func', [1,2])
+            def test_func(func, request):
+                assert func in (1, 2)
+                assert request.function.__name__ == "test_func"
+            @pytest.mark.parametrize('myparam', [1,2])
+            def test_func_kw(myparam, request, func="func_kw"):
+                assert myparam in (1, 2)
+                assert func == "func_kw"
+                assert request.function.__name__ == "test_func_kw"
+            """
+        )
+        child = testdir.spawn_pytest("--trace " + str(p1))
+        for func, argname in [
+            ("test_1", "myparam"),
+            ("test_func", "func"),
+            ("test_func_kw", "myparam"),
+        ]:
+            child.expect_exact("> PDB runcall (IO-capturing turned off) >")
+            child.expect_exact(func)
+            child.expect_exact("Pdb")
+            child.sendline("args")
+            child.expect_exact("{} = 1\r\n".format(argname))
+            child.expect_exact("Pdb")
+            child.sendline("c")
+            child.expect_exact("Pdb")
+            child.sendline("args")
+            child.expect_exact("{} = 2\r\n".format(argname))
+            child.expect_exact("Pdb")
+            child.sendline("c")
+            child.expect_exact("> PDB continue (IO-capturing resumed) >")
+        rest = child.read().decode("utf8")
+        assert "6 passed in" in rest
+        assert "reading from stdin while output" not in rest
+        # Only printed once - not on stderr.
+        assert "Exit: Quitting debugger" not in child.before.decode("utf8")
+        TestPDB.flush(child)
+
 
 def test_trace_after_runpytest(testdir):
     """Test that debugging's pytest_configure is re-entrant."""
@@ -1143,14 +1188,14 @@ def test():
             __import__("pdb").set_trace()
         """,
         mypdb="""
+        import pdb
         class Wrapped:
-            class MyPdb:
+            class MyPdb(pdb.Pdb):
                 def set_trace(self, *args):
                     print("set_trace_called", args)
 
-                def runcall(self, *args, **kwds):
-                    print("runcall_called", args, kwds)
-                    assert "func" in kwds
+                def trace_dispatch(self, frame, event, arg):
+                    print("trace_dispatch called", frame, event, arg)
         """,
     )
     result = testdir.runpytest(
@@ -1175,7 +1220,7 @@ def runcall(self, *args, **kwds):
         str(p1), "--pdbcls=mypdb:Wrapped.MyPdb", "--trace", syspathinsert=True
     )
     assert result.ret == 0
-    result.stdout.fnmatch_lines(["*runcall_called*", "* 1 passed in *"])
+    result.stdout.fnmatch_lines(["*trace_dispatch called*", "* 1 passed in *"])
 
 
 def test_raises_bdbquit_with_eoferror(testdir):