fix(internal): handle missing lib_config entry (#9022)

This PR fixes an issue regarding the remote config module where a
payload missing the ``lib_config`` entry would crash the application due
to an unhandled ``KeyError`` exception.

```
KeyError: 'lib_config'
lib_config = config["lib_config"]
File "/usr/local/lib/python3.10/site-packages/ddtrace/settings/config.py", line 747, in _handle_remoteconfig
self._callback(data, test_tracer=test_tracer)
File "/usr/local/lib/python3.10/site-packages/ddtrace/internal/remoteconfig/_subscribers.py", line 36, in _exec_callback
self._exec_callback(data, test_tracer=test_tracer)
File "/usr/local/lib/python3.10/site-packages/ddtrace/internal/remoteconfig/_subscribers.py", line 41, in _get_data_from_connector_and_exec
self._get_data_from_connector_and_exec()
File "/usr/local/lib/python3.10/site-packages/ddtrace/internal/remoteconfig/_subscribers.py", line 46, in periodic
Traceback (most recent call last):
```

## 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] Risks are described (performance impact, potential for breakage,
maintainability)
- [x] Change is maintainable (easy to change, telemetry, documentation)
- [x] [Library release note
guidelines](https://ddtrace.readthedocs.io/en/stable/releasenotes.html)
are followed or label `changelog/no-changelog` is set
- [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))
- [x] If this PR changes the public interface, I've notified
`@DataDog/apm-tees`.

## Reviewer Checklist

- [x] Title is accurate
- [x] All changes are related to the pull request's stated goal
- [x] Description motivates each change
- [x] Avoids breaking
[API](https://ddtrace.readthedocs.io/en/stable/versioning.html#interfaces)
changes
- [x] Testing strategy adequately addresses listed risks
- [x] Change is maintainable (easy to change, telemetry, documentation)
- [x] Release note makes sense to a user of the library
- [x] Author has 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)

---------

Co-authored-by: kyle <kyle@verhoog.ca>

ddtrace/settings/config.py

@@ -748,7 +748,7 @@ def _handle_remoteconfig(self, data, test_tracer=None):
         config = data["config"][0]
         base_rc_config = {n: None for n in self._config}
 
-        if config:
+        if config and "lib_config" in config:
             lib_config = config["lib_config"]
             if "tracing_sampling_rate" in lib_config:
                 base_rc_config["_trace_sample_rate"] = lib_config["tracing_sampling_rate"]

releasenotes/notes/fix-missing-lib_config-f2c0a6a40f525bff.yaml

@@ -0,0 +1,4 @@
+---
+fixes:
+  - |
+    internal: This fix resolves an error regarding the remote config module with payloads missing a ``lib_config`` entry

tests/internal/test_settings.py

@@ -164,6 +164,36 @@ def test_settings_parametrized(testcase, config, monkeypatch):
         assert config._get_source(expected_name) == expected_source
 
 
+def test_settings_missing_lib_config(config, monkeypatch):
+    testcase = {
+        "env": {"DD_TRACE_ENABLED": "true"},
+        "code": {"_tracing_enabled": True},
+        "rc": {},
+        "expected": {"_tracing_enabled": True},
+        "expected_source": {"_tracing_enabled": "code"},
+    }
+    for env_name, env_value in testcase.get("env", {}).items():
+        monkeypatch.setenv(env_name, env_value)
+        config._reset()
+
+    for code_name, code_value in testcase.get("code", {}).items():
+        setattr(config, code_name, code_value)
+
+    base_rc_config = _base_rc_config({})
+
+    # Delete "lib_config" from the remote config
+    del base_rc_config["config"][0]["lib_config"]
+    assert "lib_config" not in base_rc_config["config"][0]
+
+    config._handle_remoteconfig(base_rc_config, None)
+
+    for expected_name, expected_value in testcase["expected"].items():
+        assert getattr(config, expected_name) == expected_value
+
+    for expected_name, expected_source in testcase.get("expected_source", {}).items():
+        assert config._get_source(expected_name) == expected_source
+
+
 def test_config_subscription(config):
     for s in ("_trace_sample_rate", "logs_injection", "trace_http_header_tags"):
         _handler = mock.MagicMock()