fix(profiling): ensure correct thread link [backport 1.18] (#6815)

Backport 827f553 from #6798 to 1.18.

We use the C API to retrieve the thread ID of the current thread to
create a link between threads and objects. This way we protect ourselves
from frameworks such as gevent that might turn thread IDs into task IDs.

## Testing strategy

Tested with a sample application that the endpoint information is
attached to profiles with this change.

## Checklist

- [x] Change(s) are motivated and described in the PR description.
- [x] Testing strategy is described if automated tests are not included
in the PR.
- [x] Risk is outlined (performance impact, potential for breakage,
maintainability, etc).
- [x] Change is maintainable (easy to change, telemetry, documentation).
- [x] [Library release note
guidelines](https://ddtrace.readthedocs.io/en/stable/releasenotes.html)
are followed. If no release note is required, add label
`changelog/no-changelog`.
- [x] Documentation is included (in-code, generated user docs, [public
corp docs](https://github.com/DataDog/documentation/)).
- [x] Backport labels are set (if
[applicable](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting))

## Reviewer Checklist

- [x] Title is accurate.
- [x] No unnecessary changes are introduced.
- [x] Description motivates each change.
- [x] Avoids breaking
[API](https://ddtrace.readthedocs.io/en/stable/versioning.html#interfaces)
changes unless absolutely necessary.
- [x] Testing strategy adequately addresses listed risk(s).
- [x] Change is maintainable (easy to change, telemetry, documentation).
- [x] Release note makes sense to a user of the library.
- [x] Reviewer has explicitly acknowledged and discussed the performance
implications of this PR as reported in the benchmarks PR comment.
- [x] Backport labels are set in a manner that is consistent with the
[release branch maintenance
policy](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting)
- [x] If this PR touches code that signs or publishes builds or
packages, or handles credentials of any kind, I've requested a review
from `@DataDog/security-design-and-guidance`.
- [x] This PR doesn't touch any of that.

Co-authored-by: Gabriele N. Tornetta <[email protected]>

ddtrace/profiling/_threading.pyx

@@ -10,9 +10,20 @@ from six.moves import _thread
 from ddtrace import _threading as ddtrace_threading
 
 
-IF UNAME_SYSNAME == "Linux":
-    from cpython cimport PyLong_FromLong
+from cpython cimport PyLong_FromLong
+
+
+cdef extern from "<Python.h>":
+    # This one is provided as an opaque struct from Cython's
+    # cpython/pystate.pxd, but we need to access some of its fields so we
+    # redefine it here.
+    ctypedef struct PyThreadState:
+        unsigned long thread_id
 
+    PyThreadState* PyThreadState_Get()
+
+
+IF UNAME_SYSNAME == "Linux":
     from ddtrace.internal.module import ModuleWatchdog
     from ddtrace.internal.wrapping import wrap
 
@@ -108,28 +119,22 @@ class _ThreadLink(_thread_link_base):
     ):
         # type: (...) -> None
         """Link an object to the current running thread."""
-        self._thread_id_to_object[_thread.get_ident()] = weakref.ref(obj)
+        # Because threads might become tasks with some frameworks (e.g. gevent),
+        # we retrieve the thread ID using the C API instead of the Python API.
+        self._thread_id_to_object[<object>PyLong_FromLong(PyThreadState_Get().thread_id)] = weakref.ref(obj)
 
     def clear_threads(self,
                       existing_thread_ids,  # type: typing.Set[int]
                       ):
-        """Clear the stored list of threads based on the list of existing thread ids.
+        """Clean up the thread linking map.
 
-        If any thread that is part of this list was stored, its data will be deleted.
+        We remove all threads that are not in the existing thread IDs.
 
         :param existing_thread_ids: A set of thread ids to keep.
         """
-        # This code clears the thread/object mapping by clearing a copy and swapping it in an atomic operation This is
-        # needed to be able to have this whole class lock-free and avoid concurrency issues.
-        # The fact that it is lock free means we might lose some accuracy, but it's worth the trade-off for speed and simplicity.
-        new_thread_id_to_object_mapping = self._thread_id_to_object.copy()
-        # Iterate over a copy of the list of keys since it's mutated during our iteration.
-        for thread_id in list(new_thread_id_to_object_mapping):
-            if thread_id not in existing_thread_ids:
-                del new_thread_id_to_object_mapping[thread_id]
-
-        # Swap with the new list
-        self._thread_id_to_object = new_thread_id_to_object_mapping
+        self._thread_id_to_object = {
+            k: v for k, v in self._thread_id_to_object.items() if k in existing_thread_ids
+        }
 
     def get_object(
             self,

docs/spelling_wordlist.txt

@@ -251,4 +251,5 @@ programmatically
 DES
 Blowfish
 Gitlab
-CMake
+hotspot
+CMake

releasenotes/notes/fix-profiler-thread-link-cbe14bf53e6071db.yaml

@@ -0,0 +1,6 @@
+---
+fixes:
+  - |
+    profiling: fixed a bug that prevented profiles from being correctly
+    correlated to traces in gevent-based applications, thus causing code hotspot
+    and end point data to be missing from the UI.