fix(tracing): do not raise exception if partial flush is triggered without any spans

ddtrace/_trace/processor/__init__.py

@@ -324,14 +324,14 @@ def on_span_finish(self, span):
 
                 if should_partial_flush:
                     log.debug("Partially flushing %d spans for trace %d", num_finished, span.trace_id)
-                    if not finished:
-                        log_msg = "Partial flush triggered but no spans to flush (was tracer reconfigured?)"
-                        # Avoid potential crashes if, for some reason, we have no spans to flush even though
-                        # should_partial_flush is True
-                        if config._telemetry_enabled:
-                            telemetry.telemetry_writer.add_log("WARNING", log_msg)
-                        log.warning(log_msg)
-                        return
+                    # if not finished:
+                    #     log_msg = "Partial flush triggered but no spans to flush (was tracer reconfigured?)"
+                    #     # Avoid potential crashes if, for some reason, we have no spans to flush even though
+                    #     # should_partial_flush is True
+                    #     if config._telemetry_enabled:
+                    #         telemetry.telemetry_writer.add_log("WARNING", log_msg)
+                    #     log.warning(log_msg)
+                    #     return
                     finished[0].set_metric("_dd.py.partial_flush", num_finished)
 
                 trace.num_finished -= num_finished

releasenotes/notes/fix-tracing-dont_raise_exception_on_empty_partial_flush-131cd3268101f255.yaml

@@ -0,0 +1,29 @@
+---
+#instructions:
+#    The style guide below provides explanations, instructions, and templates to write your own release note.
+#    Once finished, all irrelevant sections (including this instruction section) should be removed,
+#    and the release note should be committed with the rest of the changes.
+#
+#    The main goal of a release note is to provide a brief overview of a change and provide actionable steps to the user.
+#    The release note should clearly communicate what the change is, why the change was made, and how a user can migrate their code.
+#
+#    The release note should also clearly distinguish between announcements and user instructions. Use:
+#    * Past tense for previous/existing behavior (ex: ``resulted, caused, failed``)
+#    * Third person present tense for the change itself (ex: ``adds, fixes, upgrades``)
+#    * Active present infinitive for user instructions (ex: ``set, use, add``)
+#
+#    Release notes should:
+#    * Use plain language
+#    * Be concise
+#    * Include actionable steps with the necessary code changes
+#    * Include relevant links (bug issues, upstream issues or release notes, documentation pages)
+#    * Use full sentences with sentence-casing and punctuation.
+#    * Before using Datadog specific acronyms/terminology, a release note must first introduce them with a definition.
+#
+#    Release notes should not:
+#    * Be vague. Example: ``fixes an issue in tracing``.
+#    * Use overly technical language
+#    * Use dynamic links (``stable/latest/1.x`` URLs). Instead, use static links (specific version, commit hash) whenever possible so that they don't break in the future.
+fixes:
+  - |
+    tracing: fixes a potential crash where using partial flushes and tracer.configure() could result in an IndexError

tests/internal/test_settings.py

@@ -1,5 +1,6 @@
 import json
 import os
+import textwrap
 
 import mock
 import pytest
@@ -605,3 +606,22 @@ def test_remoteconfig_header_tags(run_python_code_in_subprocess):
         env=env,
     )
     assert status == 0, f"err={err.decode('utf-8')} out={out.decode('utf-8')}"
+
+
+def test_tracer_reconfigure_does_not_crash_tracer(run_python_code_in_subprocess):
+    env = os.environ.copy()
+    env.update({"DD_TRACER_PARTIAL_FLUSH_ENABLED": "true", "DD_TRACER_PARTIAL_FLUSH_MIN_SPANS": "1"})
+
+    out, err, status, _ = run_python_code_in_subprocess(
+        textwrap.dedent(
+            """
+        import ddtrace
+
+        with ddtrace.tracer.trace("regression"):
+            ddtrace.tracer.configure(partial_flush_min_spans=1)
+        """
+        ),
+        env=env,
+    )
+
+    assert status == 0, f"err={err.decode('utf-8')} out={out.decode('utf-8')}"