Make ConcurrentMultiSpanProcessor fork safe

Author: gregoiredx

CHANGELOG.md

@@ -24,6 +24,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   ([#4847](https://github.com/open-telemetry/opentelemetry-python/pull/4847))
 - Prevent possible endless recursion from happening in `SimpleLogRecordProcessor.on_emit`,
   ([#4799](https://github.com/open-telemetry/opentelemetry-python/pull/4799)).
+- Make ConcurrentMultiSpanProcessor fork safe
+  ([#4862](https://github.com/open-telemetry/opentelemetry-python/pull/4862))
 
 ## Version 1.39.0/0.60b0 (2025-12-03)
 

opentelemetry-sdk/src/opentelemetry/sdk/trace/__init__.py

@@ -18,9 +18,11 @@
 import concurrent.futures
 import json
 import logging
+import os
 import threading
 import traceback
 import typing
+import weakref
 from os import environ
 from time import time_ns
 from types import MappingProxyType, TracebackType
@@ -238,6 +240,16 @@ def __init__(self, num_threads: int = 2):
         # iterating through it on "on_start" and "on_end".
         self._span_processors = ()  # type: Tuple[SpanProcessor, ...]
         self._lock = threading.Lock()
+        self._init_executor(num_threads)
+        if hasattr(os, "register_at_fork"):
+            # Only the main thread is kept in forked processed, the executor
+            # needs to be re-instantiated to get a fresh pool of threads:
+            weak_reinit = weakref.WeakMethod(self._init_executor)
+            os.register_at_fork(
+                after_in_child=lambda: weak_reinit()(num_threads)
+            )
+
+    def _init_executor(self, num_threads: int) -> None:
         self._executor = concurrent.futures.ThreadPoolExecutor(
             max_workers=num_threads
         )

opentelemetry-sdk/tests/trace/test_span_processor.py

@@ -13,9 +13,13 @@
 # limitations under the License.
 
 import abc
+import gc
+import multiprocessing
+import os
 import time
 import typing
 import unittest
+import weakref
 from platform import python_implementation, system
 from threading import Event
 from typing import Optional
@@ -26,6 +30,10 @@
 from opentelemetry import trace as trace_api
 from opentelemetry.context import Context
 from opentelemetry.sdk import trace
+from opentelemetry.sdk.trace.export import SimpleSpanProcessor
+from opentelemetry.sdk.trace.export.in_memory_span_exporter import (
+    InMemorySpanExporter,
+)
 
 
 def span_event_start_fmt(span_processor_name, span_name):
@@ -486,3 +494,66 @@ def test_force_flush_late_by_span_processor(self):
         for mock_processor in mocks:
             self.assertEqual(1, mock_processor.force_flush.call_count)
         multi_processor.shutdown()
+
+    def test_processor_gc(self):
+        multi_processor = trace.ConcurrentMultiSpanProcessor(5)
+        weak_ref = weakref.ref(multi_processor)
+        multi_processor.shutdown()
+
+        # When the processor is garbage collected
+        del multi_processor
+        gc.collect()
+
+        # Then the reference to the processor should no longer exist
+        self.assertIsNone(
+            weak_ref(),
+            "The ConcurrentMultiSpanProcessor object created by this test wasn't garbage collected",
+        )
+
+    @unittest.skipUnless(hasattr(os, "fork"), "needs *nix")
+    def test_batch_span_processor_fork(self):
+        multiprocessing_context = multiprocessing.get_context("fork")
+        tracer_provider = trace.TracerProvider()
+        tracer = tracer_provider.get_tracer(__name__)
+        exporter = InMemorySpanExporter()
+        multi_processor = trace.ConcurrentMultiSpanProcessor(2)
+        multi_processor.add_span_processor(SimpleSpanProcessor(exporter))
+        tracer_provider.add_span_processor(multi_processor)
+
+        # Use the ConcurrentMultiSpanProcessor in the main process.
+        # This is necessary in this test to start using the underlying ThreadPoolExecutor and avoid false positive:
+        with tracer.start_as_current_span("main process before fork span"):
+            pass
+        assert (
+            exporter.get_finished_spans()[-1].name
+            == "main process before fork span"
+        )
+
+        # The forked ConcurrentMultiSpanProcessor is usable in the child process:
+        def child(conn):
+            with tracer.start_as_current_span("child process span"):
+                pass
+            conn.send(exporter.get_finished_spans()[-1].name)
+            conn.close()
+
+        parent_conn, child_conn = multiprocessing_context.Pipe()
+        process = multiprocessing_context.Process(
+            target=child, args=(child_conn,)
+        )
+        process.start()
+        has_response = parent_conn.poll(timeout=5)
+        if not has_response:
+            process.kill()
+            self.fail(
+                "The child process did not send any message after 5 seconds, it's very probably locked"
+            )
+        process.join(timeout=5)
+        assert parent_conn.recv() == "child process span"
+
+        # The ConcurrentMultiSpanProcessor is still usable in the main process after the child process termination:
+        with tracer.start_as_current_span("main process after fork span"):
+            pass
+        assert (
+            exporter.get_finished_spans()[-1].name
+            == "main process after fork span"
+        )