From d69d95e2aa3cbedaa8955679af8f5d584e723a1d Mon Sep 17 00:00:00 2001
From: Henry Su <henrysu4707@gmail.com>
Date: Mon, 20 Jul 2026 14:51:37 -0500
Subject: [PATCH] fix(structlog): preserve existing processors

Assisted-by: OpenAI Codex
---
 .../instrumentation/structlog/__init__.py     | 20 +++++----
 .../tests/test_structlog.py                   | 42 +++++++++++++++++++
 2 files changed, 54 insertions(+), 8 deletions(-)

diff --git a/instrumentation/opentelemetry-instrumentation-structlog/src/opentelemetry/instrumentation/structlog/__init__.py b/instrumentation/opentelemetry-instrumentation-structlog/src/opentelemetry/instrumentation/structlog/__init__.py
index c280938ec3..50117b3fdd 100644
--- a/instrumentation/opentelemetry-instrumentation-structlog/src/opentelemetry/instrumentation/structlog/__init__.py
+++ b/instrumentation/opentelemetry-instrumentation-structlog/src/opentelemetry/instrumentation/structlog/__init__.py
@@ -345,13 +345,17 @@ def _instrument(self, **kwargs):
         config = structlog.get_config()
         current_processors = list(config.get("processors", []))
 
-        # Insert before the last processor, assumed to be the renderer.
-        if current_processors:
-            insert_position = len(current_processors) - 1
-        else:
-            insert_position = 0
+        # Insert before the last processor, assumed to be the renderer, unless
+        # the app has already configured an OTel processor.
+        if not any(
+            isinstance(p, StructlogProcessor) for p in current_processors
+        ):
+            if current_processors:
+                insert_position = len(current_processors) - 1
+            else:
+                insert_position = 0
 
-        current_processors.insert(insert_position, processor)
+            current_processors.insert(insert_position, processor)
 
         # Reconfigure structlog with the new processor chain
         StructlogInstrumentor._original_configure(
@@ -418,11 +422,11 @@ def _uninstrument(self, **kwargs):
         config = structlog.get_config()
         current_processors = list(config.get("processors", []))
 
-        # Remove all StructlogProcessor instances
+        # Remove only the processor added by this instrumentor.
         new_processors = [
             p
             for p in current_processors
-            if not isinstance(p, StructlogProcessor)
+            if p is not StructlogInstrumentor._processor
         ]
         configured_by_app = StructlogInstrumentor._is_configured_by_app
 
diff --git a/instrumentation/opentelemetry-instrumentation-structlog/tests/test_structlog.py b/instrumentation/opentelemetry-instrumentation-structlog/tests/test_structlog.py
index d42e3f97b0..5bdda99d03 100644
--- a/instrumentation/opentelemetry-instrumentation-structlog/tests/test_structlog.py
+++ b/instrumentation/opentelemetry-instrumentation-structlog/tests/test_structlog.py
@@ -470,6 +470,48 @@ def test_instrument_adds_processor(self):
         )
         self.assertTrue(has_otel_processor)
 
+    def test_instrument_does_not_duplicate_existing_processor(self):
+        """Test instrument() reuses an existing OTel processor."""
+        existing_processor = StructlogProcessor(
+            logger_provider=self.logger_provider
+        )
+        structlog.configure(
+            processors=[
+                existing_processor,
+                structlog.dev.ConsoleRenderer(),
+            ]
+        )
+
+        StructlogInstrumentor().instrument(
+            logger_provider=self.logger_provider
+        )
+
+        processors = structlog.get_config()["processors"]
+        otel_processors = [
+            p for p in processors if isinstance(p, StructlogProcessor)
+        ]
+        self.assertEqual(otel_processors, [existing_processor])
+
+    def test_uninstrument_preserves_existing_processor(self):
+        """Test uninstrument() preserves an app-configured OTel processor."""
+        existing_processor = StructlogProcessor(
+            logger_provider=self.logger_provider
+        )
+        structlog.configure(
+            processors=[
+                existing_processor,
+                structlog.dev.ConsoleRenderer(),
+            ]
+        )
+
+        StructlogInstrumentor().instrument(
+            logger_provider=self.logger_provider
+        )
+        StructlogInstrumentor().uninstrument()
+
+        processors = structlog.get_config()["processors"]
+        self.assertIn(existing_processor, processors)
+
     def test_instrument_without_prior_structlog_configuration_emits_logs(self):
         """Test instrument() emits logs before the app configures structlog."""
         structlog.reset_defaults()