fix(profiling): remove unnecessary access to frame.f_locals (#15434)

We've been seeing a low number of crashes that have the following stack
traces.

```
PyFrame_FastToLocalsWithError
_PyObject_GenericGetAttrWithDict
__pyx_f_7ddtrace_9profiling_9collector_10_traceback__extract_class_name.constprop.0
__pyx_pw_7ddtrace_9profiling_9collector_10_traceback_5pyframe_to_frames
```

which corresponds to `value = frame.f_locals[argname]` in
`extract_class_name()` function.

Fore more details see notebook,
https://app.datadoghq.com/notebook/13515661/pyframe-to-frames-crashes

The reason this could lead to a crash is that the `frame` could be in an
invalid state, partially deallocated, or corrupted. And this doesn't
happen on our stack sampler (Echion) as it doesn't use it, and just use
`co_qualname` (Python 3.11+) or `co_name` (Python < 3.11)

This code is no longer needed as we don't use `class_name` field from
`DDFrame` named tuple. Memory profiler has been putting an empty string.
Lock profiler populated it but then it didn't export it in the sample.

More appropriate way to populate the class name for Python versions
3.11+ would be using `co_qualname` as Echion does. I'll follow up with
this in a separate PR. And this is why we see fully qualified name on
our CPU time view, but function name only on memory/lock views.

CPU time view
<img width="206" height="94" alt="Screenshot 2025-11-26 at 2 02 50 PM"
src="https://github.com/user-attachments/assets/a56efc8a-0dd2-41c2-8fab-5cc786d3a737"
/>

Memory view
<img width="141" height="94" alt="Screenshot 2025-11-26 at 2 02 35 PM"
src="https://github.com/user-attachments/assets/efa917f8-40c8-4818-88e7-9c157bdfd5cf"
/>

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-->

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(cherry picked from commit 124b19d)

Author: taegyunkim

ddtrace/profiling/collector/_memalloc_tb.cpp

@@ -13,8 +13,6 @@
 /* A string containing "<unknown>" just in case we can't store the real function
  * or file name. */
 static PyObject* unknown_name = NULL;
-/* A string containing "" */
-static PyObject* empty_string = NULL;
 
 #define TRACEBACK_SIZE(NFRAME) (sizeof(traceback_t) + sizeof(frame_t) * (NFRAME - 1))
 
@@ -135,12 +133,6 @@ memalloc_tb_init(uint16_t max_nframe)
         PyUnicode_InternInPlace(&unknown_name);
     }
 
-    if (empty_string == NULL) {
-        empty_string = PyUnicode_FromString("");
-        if (empty_string == NULL)
-            return -1;
-        PyUnicode_InternInPlace(&empty_string);
-    }
     return 0;
 }
 
@@ -297,7 +289,7 @@ traceback_to_tuple(traceback_t* tb)
     PyObject* stack = PyTuple_New(tb->nframe);
 
     for (uint16_t nframe = 0; nframe < tb->nframe; nframe++) {
-        PyObject* frame_tuple = PyTuple_New(4);
+        PyObject* frame_tuple = PyTuple_New(3);
 
         frame_t* frame = &tb->frames[nframe];
 
@@ -306,9 +298,6 @@ traceback_to_tuple(traceback_t* tb)
         PyTuple_SET_ITEM(frame_tuple, 1, PyLong_FromUnsignedLong(frame->lineno));
         Py_INCREF(frame->name);
         PyTuple_SET_ITEM(frame_tuple, 2, frame->name);
-        /* Class name */
-        Py_INCREF(empty_string);
-        PyTuple_SET_ITEM(frame_tuple, 3, empty_string);
 
         // Try to set the class.  If we cannot (e.g., if the sofile is reloaded
         // without module initialization), then this will result in an error if

ddtrace/profiling/collector/_traceback.pyx

@@ -8,31 +8,6 @@ from ddtrace.profiling.event import DDFrame
 log = get_logger(__name__)
 
 
-cpdef _extract_class_name(frame):
-    # type: (...) -> str
-    """Extract class name from a frame, if possible.
-
-    :param frame: The frame object.
-    """
-    code = frame.f_code
-    if code.co_argcount > 0:
-        # Retrieve the name of the first argument, if the code object has any
-        argname = code.co_varnames[0]
-        try:
-            value = frame.f_locals[argname]
-        except Exception:
-            log.debug("Unable to extract class name from frame %r", frame, exc_info=True)
-            return ""
-        try:
-            if argname == "self":
-                return object.__getattribute__(type(value), "__name__")  # use type() and object.__getattribute__ to avoid side-effects
-            if argname == "cls":
-                return object.__getattribute__(value, "__name__")
-        except AttributeError:
-            return ""
-    return ""
-
-
 cpdef traceback_to_frames(traceback, max_nframes):
     """Serialize a Python traceback object into a list of tuple of (filename, lineno, function_name).
 
@@ -48,7 +23,7 @@ cpdef traceback_to_frames(traceback, max_nframes):
             frame = tb.tb_frame
             code = frame.f_code
             lineno = 0 if frame.f_lineno is None else frame.f_lineno
-            frames.insert(0, DDFrame(code.co_filename, lineno, code.co_name, _extract_class_name(frame)))
+            frames.insert(0, DDFrame(code.co_filename, lineno, code.co_name))
         nframes += 1
         tb = tb.tb_next
     return frames, nframes
@@ -92,7 +67,7 @@ cpdef pyframe_to_frames(frame, max_nframes):
                     return [], 0
 
             lineno = 0 if frame.f_lineno is None else frame.f_lineno
-            frames.append(DDFrame(code.co_filename, lineno, code.co_name, _extract_class_name(frame)))
+            frames.append(DDFrame(code.co_filename, lineno, code.co_name))
         nframes += 1
         frame = frame.f_back
     return frames, nframes

ddtrace/profiling/collector/stack.pyx

@@ -317,7 +317,6 @@ cdef stack_collect(ignore_profiler, thread_time, max_nframes, interval, wall_tim
                 handle.push_threadinfo(thread_id, thread_native_id, thread_name)
                 handle.push_task_id(task_id)
                 handle.push_task_name(task_name)
-                handle.push_class_name(frames[0].class_name)
                 for frame in frames:
                     handle.push_frame(frame.function_name, frame.file_name, 0, frame.lineno)
                 handle.flush_sample()
@@ -330,7 +329,6 @@ cdef stack_collect(ignore_profiler, thread_time, max_nframes, interval, wall_tim
             handle.push_cputime( cpu_time, 1)
             handle.push_walltime( wall_time, 1)
             handle.push_threadinfo(thread_id, thread_native_id, thread_name)
-            handle.push_class_name(frames[0].class_name)
             for frame in frames:
                 handle.push_frame(frame.function_name, frame.file_name, 0, frame.lineno)
             handle.push_span(span)
@@ -346,7 +344,6 @@ cdef stack_collect(ignore_profiler, thread_time, max_nframes, interval, wall_tim
                 handle.push_monotonic_ns(now_ns)
                 handle.push_threadinfo(thread_id, thread_native_id, thread_name)
                 handle.push_exceptioninfo(exc_type, 1)
-                handle.push_class_name(frames[0].class_name)
                 for frame in frames:
                     handle.push_frame(frame.function_name, frame.file_name, 0, frame.lineno)
                 handle.push_span(span)

ddtrace/profiling/event.py

@@ -2,5 +2,5 @@
 import typing
 
 
-DDFrame = namedtuple("DDFrame", ["file_name", "lineno", "function_name", "class_name"])
+DDFrame = namedtuple("DDFrame", ["file_name", "lineno", "function_name"])
 StackTraceType = typing.List[DDFrame]

releasenotes/notes/profiling-f_locals-c345ca501ca9b74f.yaml

@@ -0,0 +1,5 @@
+---
+fixes:
+  - |
+    profiling: Fixes a segmentation fault caused by accessing ``frame.f_locals``
+    while trying to retrieve class name of a ``PyFrameObject``.

tests/profiling/collector/test_memalloc.py

@@ -90,11 +90,10 @@ def test_iter_events():
             __file__,
             _ALLOC_LINE_NUMBER,
             "<listcomp>" if sys.version_info < (3, 12) else "_allocate_1k",
-            "",
         ):
             assert thread_id == threading.main_thread().ident
             if sys.version_info < (3, 12) and len(stack) > 1:
-                assert stack[1] == DDFrame(__file__, _ALLOC_LINE_NUMBER, "_allocate_1k", "")
+                assert stack[1] == DDFrame(__file__, _ALLOC_LINE_NUMBER, "_allocate_1k")
             object_count += sample.count
 
     assert object_count >= 1000
@@ -156,12 +155,11 @@ def test_iter_events_multi_thread():
             __file__,
             _ALLOC_LINE_NUMBER,
             "<listcomp>" if sys.version_info < (3, 12) else "_allocate_1k",
-            "",
         ):
             if thread_id == threading.main_thread().ident:
                 count_object += sample.count
                 if sys.version_info < (3, 12) and len(stack) > 1:
-                    assert stack[1] == DDFrame(__file__, _ALLOC_LINE_NUMBER, "_allocate_1k", "")
+                    assert stack[1] == DDFrame(__file__, _ALLOC_LINE_NUMBER, "_allocate_1k")
             elif thread_id == t.ident:
                 count_thread += sample.count
                 entry = 2 if sys.version_info < (3, 12) else 1
@@ -205,7 +203,6 @@ def _test_heap_impl(collector, max_nframe):
             __file__,
             _ALLOC_LINE_NUMBER,
             "<listcomp>" if sys.version_info < (3, 12) else "_allocate_1k",
-            "",
         ):
             break
     else:
@@ -229,7 +226,6 @@ def _test_heap_impl(collector, max_nframe):
             __file__,
             _ALLOC_LINE_NUMBER,
             "<listcomp>" if sys.version_info < (3, 12) else "_allocate_1k",
-            "",
         ):
             break
     else:
@@ -258,7 +254,6 @@ def _test_heap_impl(collector, max_nframe):
                 __file__,
                 _ALLOC_LINE_NUMBER,
                 "<listcomp>" if sys.version_info < (3, 12) else "_allocate_1k",
-                "",
             )
             and stack[entry].function_name == "_test_heap_impl"
         ):

tests/profiling/collector/test_stack.py

@@ -114,11 +114,9 @@ def _find_sleep_event(events, class_name):
 
     for e in events:
         for frame in e.frames:
-            if frame[0] == __file__.replace(".pyc", ".py") and frame[2] == "sleep_class" and frame[3] == class_name:
+            if frame[0] == __file__.replace(".pyc", ".py") and frame[2] == "sleep_class":
                 class_method_found = True
-            elif (
-                frame[0] == __file__.replace(".pyc", ".py") and frame[2] == "sleep_instance" and frame[3] == class_name
-            ):
+            elif frame[0] == __file__.replace(".pyc", ".py") and frame[2] == "sleep_instance":
                 class_classmethod_found = True
 
         if class_method_found and class_classmethod_found:

tests/profiling/collector/test_traceback.py

@@ -17,6 +17,6 @@ def test_check_traceback_to_frames():
 
     this_file = __file__.replace(".pyc", ".py")
     assert frames == [
-        (this_file, 7, "_x", ""),
-        (this_file, 15, "test_check_traceback_to_frames", ""),
+        (this_file, 7, "_x"),
+        (this_file, 15, "test_check_traceback_to_frames"),
     ]