Fix debugger stepping actions in forked process (#1921)

* Fix debugger stepping actions in forked process

Fix the debugger stepping state when debugging a process that has been
forked from the main process. The new sys.monitoring mechanism didn't
fully clear the thread local storage after a fork leading to a state
where the forked child process tracked the wrong thread information and
was never updated on the latest continue action.

* Add stepping test for forked process

* Add line ending back in for cleaner diff

* More formatting reversions

CONTRIBUTING.md

@@ -115,7 +115,7 @@ In order to update the source, you would:
 You might need to regenerate the Cython modules after any changes. This can be done by:
 
 - Install Python latest (3.12 as of this writing)
-- pip install cython, django>=1.9, setuptools>=0.9, wheel>0.21, twine
+- pip install cython 'django>=1.9' 'setuptools>=0.9' 'wheel>0.21' twine
 - On a windows machine:
   - set FORCE_PYDEVD_VC_VARS=C:\Program Files (x86)\Microsoft Visual Studio\2017\BuildTools\VC\Auxiliary\Build\vcvars64.bat
   - in the pydevd folder: python .\build_tools\build.py

src/debugpy/_vendored/pydevd/_pydevd_sys_monitoring/_pydevd_sys_monitoring.py

@@ -755,6 +755,16 @@ def enable_code_tracing(thread_ident: Optional[int], code, frame) -> bool:
 
     return _enable_code_tracing(py_db, additional_info, func_code_info, code, frame, False)
 
+# fmt: off
+# IFDEF CYTHON
+# cpdef reset_thread_local_info():
+# ELSE
+def reset_thread_local_info():
+# ENDIF
+# fmt: on
+    """Resets the thread local info TLS store for use after a fork()."""
+    global _thread_local_info
+    _thread_local_info = threading.local()
 
 # fmt: off
 # IFDEF CYTHON

src/debugpy/_vendored/pydevd/_pydevd_sys_monitoring/_pydevd_sys_monitoring_cython.pyx

@@ -761,6 +761,16 @@ cpdef enable_code_tracing(unsigned long thread_ident, code, frame):
 
     return _enable_code_tracing(py_db, additional_info, func_code_info, code, frame, False)
 
+# fmt: off
+# IFDEF CYTHON -- DONT EDIT THIS FILE (it is automatically generated)
+cpdef reset_thread_local_info():
+# ELSE
+# def reset_thread_local_info():
+# ENDIF
+# fmt: on
+    """Resets the thread local info TLS store for use after a fork()."""
+    global _thread_local_info
+    _thread_local_info = threading.local()
 
 # fmt: off
 # IFDEF CYTHON -- DONT EDIT THIS FILE (it is automatically generated)

src/debugpy/_vendored/pydevd/pydevd.py

@@ -3354,6 +3354,9 @@ def settrace_forked(setup_tracing=True):
             if clear_thread_local_info is not None:
                 clear_thread_local_info()
 
+            if PYDEVD_USE_SYS_MONITORING:
+                pydevd_sys_monitoring.reset_thread_local_info()
+
             settrace(
                 host,
                 port=port,

tests/debugpy/test_multiproc.py

@@ -87,7 +87,7 @@ def test_multiprocessing(pyfile, target, run, start_method):
             p.join()
 
         def child(q, a):
-            print("entering child")
+            print("entering child")  # @bp
             assert q.get() == "foo?"
             a.put(Foo())
 
@@ -136,7 +136,20 @@ def test_multiprocessing(pyfile, target, run, start_method):
 
         with debug.Session(child_config) as child_session:
             with child_session.start():
-                pass
+                child_session.set_breakpoints(code_to_debug, all)
+
+            expected_frame = some.dap.frame(code_to_debug, line="bp")
+            stop = child_session.wait_for_stop(
+                "breakpoint",
+                expected_frames=[expected_frame],
+            )
+            child_session.request('stepIn', {"threadId": stop.thread_id})
+
+            stop = child_session.wait_for_stop(
+                "step",
+                expected_frames=[some.dap.frame(code_to_debug, line=expected_frame.items['line'] + 1)],
+            )
+            child_session.request_continue()
 
             expected_grandchild_config = expected_subprocess_config(child_session)
             grandchild_config = child_session.wait_for_next_event("debugpyAttach")