fix(llmobs): llmobs can be re-enabled on fork with agentless enabled [backport 3.3] (#13037)

Backport 269b3d8 from #12889 to 3.3.

Due to a regression in
#11961, agentless writers
fail to init in LLMObs on fork, as the `agentless_url` is not passed
down in `recreate`, resulting in:

```
File "/app/.venv/lib/python3.11/site-packages/ddtrace/llmobs/_writer.py", line 255, in __init__ 	
	raise ValueError("agentless_url is required for agentless mode")
	
ValueError: agentless_url is required for agentless mode
```

The fix is passing the value down appropriately.

## 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: Sam Brenner <[email protected]>

Author: github-actions[bot]

ddtrace/llmobs/_writer.py

@@ -274,6 +274,8 @@ def __init__(
             intake_url = agent.get_trace_url()
             headers[EVP_SUBDOMAIN_HEADER_NAME] = EVP_SUBDOMAIN_HEADER_VALUE
 
+        self.agentless_url = agentless_url
+
         super(LLMObsSpanWriter, self).__init__(
             intake_url=intake_url,
             clients=clients,
@@ -326,6 +328,7 @@ def recreate(self):
             interval=self._interval,
             timeout=self._timeout,
             is_agentless=config._llmobs_agentless_enabled,
+            agentless_url=self.agentless_url,
         )
 
 

releasenotes/notes/llmobs-agentless-writer-on-fork-fix-d43c50cbf3d86297.yaml

@@ -0,0 +1,4 @@
+---
+fixes:
+  - |
+    LLM Observability: fixes an issue where LLMObs could not be enabled in a forked process when setting ``agentless_enabled=True`` or ``DD_LLMOBS_AGENTLESS_ENABLED=true``.

tests/llmobs/test_llmobs_service.py

@@ -1359,6 +1359,28 @@ def test_llmobs_fork_recreates_and_restarts_span_writer():
         llmobs_service.disable()
 
 
+def test_llmobs_fork_recreates_and_restarts_agentless_span_writer():
+    """Test that forking a process correctly recreates and restarts the LLMObsSpanWriter."""
+    with override_global_config(dict(_dd_api_key="<not-a-real-key>")):
+        with mock.patch("ddtrace.internal.writer.HTTPWriter._send_payload"):
+            llmobs_service.enable(_tracer=DummyTracer(), ml_app="test_app", agentless_enabled=True)
+            original_span_writer = llmobs_service._instance._llmobs_span_writer
+            pid = os.fork()
+            if pid:  # parent
+                assert llmobs_service._instance._llmobs_span_writer == original_span_writer
+                assert llmobs_service._instance._llmobs_span_writer.status == ServiceStatus.RUNNING
+            else:  # child
+                assert llmobs_service._instance._llmobs_span_writer != original_span_writer
+                assert llmobs_service._instance._llmobs_span_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_recreates_and_restarts_eval_metric_writer():
     """Test that forking a process correctly recreates and restarts the LLMObsEvalMetricWriter."""
     with mock.patch("ddtrace.llmobs._writer.BaseLLMObsWriter.periodic"):