fix(llmobs): respect enabled setting when forking (#11026)

The code was assuming that the LLMObs service is enabled when forking.
If a user had explicitly disabled it via the disable() method or
DD_LLMOBS_ENABLED=0 it would be re-enabled in the child process.

Potentially closes #10859

ddtrace/llmobs/_llmobs.py

@@ -124,26 +124,14 @@ def _child_after_fork(self):
         self._evaluator_runner = self._evaluator_runner.recreate()
         self._trace_processor._span_writer = self._llmobs_span_writer
         self._trace_processor._evaluator_runner = self._evaluator_runner
-        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 writers after fork")
-
-        try:
-            self._evaluator_runner.start()
-        except ServiceStatusError:
-            log.debug("Error starting evaluator runner after fork")
+        if self.enabled:
+            self._start_service()
 
     def _start_service(self) -> None:
         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})
+            self.tracer.configure(settings={"FILTERS": tracer_filters})
         try:
             self._llmobs_span_writer.start()
             self._llmobs_eval_metric_writer.start()
@@ -245,6 +233,7 @@ def enable(
 
         if integrations_enabled:
             cls._patch_integrations()
+
         # override the default _instance with a new tracer
         cls._instance = cls(tracer=_tracer)
         cls.enabled = True

releasenotes/notes/llm-obs-fork-disabled-f710f0ccde71c36b.yaml

@@ -0,0 +1,4 @@
+---
+fixes:
+  - |
+    LLM Observability: Resolves errors where the disabled setting was being ignored when forking.

tests/llmobs/test_llmobs_service.py

@@ -1574,7 +1574,7 @@ def test_llmobs_fork_recreates_and_restarts_evaluator_runner(mock_ragas_evaluato
 
 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)
+    monkeypatch.setenv("_DD_LLMOBS_WRITER_INTERVAL", "5.0")
     with mock.patch("ddtrace.internal.writer.HTTPWriter._send_payload"):
         llmobs_service.enable(_tracer=DummyTracer(), ml_app="test_app")
         pid = os.fork()
@@ -1684,6 +1684,49 @@ def process_trace(self, trace):
         llmobs_service.disable()
 
 
+def test_llmobs_fork_disabled(monkeypatch):
+    """Test that after being disabled the service remains disabled when forking"""
+    monkeypatch.setenv("DD_LLMOBS_ENABLED", "0")
+    svc = llmobs_service(tracer=DummyTracer())
+    pid = os.fork()
+    assert not svc.enabled, "both the parent and child should be disabled"
+    assert svc._llmobs_span_writer.status == ServiceStatus.STOPPED
+    assert svc._llmobs_eval_metric_writer.status == ServiceStatus.STOPPED
+    if not pid:
+        svc.disable()
+        os._exit(12)
+
+    _, status = os.waitpid(pid, 0)
+    exit_code = os.WEXITSTATUS(status)
+    assert exit_code == 12
+    svc.disable()
+
+
+def test_llmobs_fork_disabled_then_enabled(monkeypatch):
+    """Test that after being initially disabled, the service can be enabled in a fork"""
+    monkeypatch.setenv("DD_LLMOBS_ENABLED", "0")
+    svc = llmobs_service._instance
+    pid = os.fork()
+    assert not svc.enabled, "both the parent and child should be disabled"
+    assert svc._llmobs_span_writer.status == ServiceStatus.STOPPED
+    assert svc._llmobs_eval_metric_writer.status == ServiceStatus.STOPPED
+    if not pid:
+        # Enable the service in the child
+        with override_global_config(dict(_dd_api_key="<not-a-real-api-key>", _llmobs_ml_app="<ml-app-name>")):
+            monkeypatch.setenv("DD_LLMOBS_ENABLED", "1")
+            llmobs_service.enable(_tracer=DummyTracer())
+        svc = llmobs_service._instance
+        assert svc._llmobs_span_writer.status == ServiceStatus.RUNNING
+        assert svc._llmobs_eval_metric_writer.status == ServiceStatus.RUNNING
+        svc.disable()
+        os._exit(12)
+
+    _, status = os.waitpid(pid, 0)
+    exit_code = os.WEXITSTATUS(status)
+    assert exit_code == 12
+    svc.disable()
+
+
 def test_annotation_context_modifies_span_tags(LLMObs):
     with LLMObs.annotation_context(tags={"foo": "bar"}):
         with LLMObs.agent(name="test_agent") as span: