fix(llmobs): recreate eval metric writer on fork (#10710)

This PR ensures that the `LLMObsEvalMetricWriter` is correctly recreated
and restarted on a forked process.

Previously, on a process fork we were not recreating/restarting the eval
metric writer worker.

Mirrors #10249 but for eval
metric writer

## Checklist
- [x] PR author has checked that all the criteria below are met
- The PR description includes an overview of the change
- The PR description articulates the motivation for the change
- The change includes tests OR the PR description describes a testing
strategy
- The PR description notes risks associated with the change, if any
- Newly-added code is easy to change
- The change follows the [library release note
guidelines](https://ddtrace.readthedocs.io/en/stable/releasenotes.html)
- The change includes or references documentation updates if necessary
- Backport labels are set (if
[applicable](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting))

## Reviewer Checklist
- [x] Reviewer has checked that all the criteria below are met
- Title is accurate
- All changes are related to the pull request's stated goal
- Avoids breaking
[API](https://ddtrace.readthedocs.io/en/stable/versioning.html#interfaces)
changes
- Testing strategy adequately addresses listed risks
- Newly-added code is easy to change
- Release note makes sense to a user of the library
- If necessary, author has acknowledged and discussed the performance
implications of this PR as reported in the benchmarks PR comment
- 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)

---------

Co-authored-by: lievan <evan.li@datadoqhq.com>
Co-authored-by: Yun Kim <35776586+Yun-Kim@users.noreply.github.com>
(cherry picked from commit 5dbd7ef)

ddtrace/llmobs/_llmobs.py

@@ -90,15 +90,17 @@ def __init__(self, tracer=None):
 
     def _child_after_fork(self):
         self._llmobs_span_writer = self._llmobs_span_writer.recreate()
+        self._llmobs_eval_metric_writer = self._llmobs_eval_metric_writer.recreate()
         self._trace_processor._span_writer = self._llmobs_span_writer
         tracer_filters = self.tracer._filters
         if not any(isinstance(tracer_filter, LLMObsTraceProcessor) for tracer_filter in tracer_filters):
             tracer_filters += [self._trace_processor]
         self.tracer.configure(settings={"FILTERS": tracer_filters})
         try:
             self._llmobs_span_writer.start()
+            self._llmobs_eval_metric_writer.start()
         except ServiceStatusError:
-            log.debug("Error starting LLMObs span writer after fork")
+            log.debug("Error starting LLMObs writers after fork")
 
     def _start_service(self) -> None:
         tracer_filters = self.tracer._filters

ddtrace/llmobs/_writer.py

@@ -141,6 +141,14 @@ def _url(self) -> str:
     def _data(self, events: List[Any]) -> Dict[str, Any]:
         raise NotImplementedError
 
+    def recreate(self) -> "BaseLLMObsWriter":
+        return self.__class__(
+            site=self._site,
+            api_key=self._api_key,
+            interval=self._interval,
+            timeout=self._timeout,
+        )
+
 
 class LLMObsEvalMetricWriter(BaseLLMObsWriter):
     """Writer to the Datadog LLMObs Custom Eval Metrics Endpoint."""

releasenotes/notes/fix-llmobs-forked-eval-metric-writer-55d50d1ed8fc10ea.yaml

@@ -0,0 +1,6 @@
+---
+fixes:
+  - |
+    LLM Observability: This fix resolves an issue where LLM Observability evaluation metrics were not being submitted 
+    in forked processes. The evaluation metric writer thread now automatically restarts when a forked 
+    process is detected.

tests/llmobs/test_llmobs_service.py

@@ -1362,20 +1362,7 @@ def test_activate_distributed_headers_activates_context(LLMObs, mock_logs):
             mock_activate.assert_called_once_with(dummy_context)
 
 
-def _task(llmobs_service, errors, original_pid, original_span_writer_id):
-    """Task in test_llmobs_fork which asserts that LLMObs in a forked process correctly recreates the writer."""
-    try:
-        with llmobs_service.workflow():
-            with llmobs_service.task():
-                assert llmobs_service._instance.tracer._pid != original_pid
-                assert id(llmobs_service._instance._llmobs_span_writer) != original_span_writer_id
-        assert llmobs_service._instance._llmobs_span_writer.enqueue.call_count == 2
-        assert llmobs_service._instance._llmobs_span_writer._encoder.encode.call_count == 2
-    except AssertionError as e:
-        errors.put(e)
-
-
-def test_llmobs_fork_recreates_and_restarts_writer():
+def test_llmobs_fork_recreates_and_restarts_span_writer():
     """Test that forking a process correctly recreates and restarts the LLMObsSpanWriter."""
     with mock.patch("ddtrace.internal.writer.HTTPWriter._send_payload"):
         llmobs_service.enable(_tracer=DummyTracer(), ml_app="test_app")
@@ -1405,6 +1392,30 @@ def test_llmobs_fork_recreates_and_restarts_writer():
         llmobs_service.disable()
 
 
+def test_llmobs_fork_recreates_and_restarts_eval_metric_writer():
+    """Test that forking a process correctly recreates and restarts the LLMObsSpanWriter."""
+    with mock.patch("ddtrace.llmobs._writer.BaseLLMObsWriter.periodic"):
+        llmobs_service.enable(_tracer=DummyTracer(), ml_app="test_app")
+        original_pid = llmobs_service._instance.tracer._pid
+        original_eval_metric_writer = llmobs_service._instance._llmobs_eval_metric_writer
+        pid = os.fork()
+        if pid:  # parent
+            assert llmobs_service._instance.tracer._pid == original_pid
+            assert llmobs_service._instance._llmobs_eval_metric_writer == original_eval_metric_writer
+            assert llmobs_service._instance._llmobs_eval_metric_writer.status == ServiceStatus.RUNNING
+        else:  # child
+            assert llmobs_service._instance.tracer._pid != original_pid
+            assert llmobs_service._instance._llmobs_eval_metric_writer != original_eval_metric_writer
+            assert llmobs_service._instance._llmobs_eval_metric_writer.status == ServiceStatus.RUNNING
+            llmobs_service.disable()
+            os._exit(12)
+
+        _, status = os.waitpid(pid, 0)
+        exit_code = os.WEXITSTATUS(status)
+        assert exit_code == 12
+        llmobs_service.disable()
+
+
 def test_llmobs_fork_create_span(monkeypatch):
     """Test that forking a process correctly encodes new spans created in each process."""
     monkeypatch.setenv("_DD_LLMOBS_WRITER_INTERVAL", 5.0)
@@ -1429,6 +1440,37 @@ def test_llmobs_fork_create_span(monkeypatch):
         llmobs_service.disable()
 
 
+def test_llmobs_fork_submit_evaluation(monkeypatch):
+    """Test that forking a process correctly encodes new spans created in each process."""
+    monkeypatch.setenv("_DD_LLMOBS_WRITER_INTERVAL", 5.0)
+    with mock.patch("ddtrace.llmobs._writer.BaseLLMObsWriter.periodic"):
+        llmobs_service.enable(_tracer=DummyTracer(), ml_app="test_app", api_key="test_api_key")
+        pid = os.fork()
+        if pid:  # parent
+            llmobs_service.submit_evaluation(
+                span_context={"span_id": "123", "trace_id": "456"},
+                label="toxicity",
+                metric_type="categorical",
+                value="high",
+            )
+            assert len(llmobs_service._instance._llmobs_eval_metric_writer._buffer) == 1
+        else:  # child
+            llmobs_service.submit_evaluation(
+                span_context={"span_id": "123", "trace_id": "456"},
+                label="toxicity",
+                metric_type="categorical",
+                value="high",
+            )
+            assert len(llmobs_service._instance._llmobs_eval_metric_writer._buffer) == 1
+            llmobs_service.disable()
+            os._exit(12)
+
+        _, status = os.waitpid(pid, 0)
+        exit_code = os.WEXITSTATUS(status)
+        assert exit_code == 12
+        llmobs_service.disable()
+
+
 def test_llmobs_fork_custom_filter(monkeypatch):
     """Test that forking a process correctly keeps any custom filters."""