fix(metrics): prevent thread leak by ensuring singleton initialization (googleapis#1492)

****Summary:****
This PR fixes a critical memory and thread leak in the
google-cloud-spanner client when built-in metrics are enabled (default
behavior).
Previously, the Client constructor unconditionally initialized a new
OpenTelemetry MeterProvider and PeriodicExportingMetricReader on every
instantiation. Each reader spawned a new background thread for metric
exporting that was never cleaned up or reused. In environments where
Client objects are frequently created (e.g., Cloud Functions, web
servers, or data pipelines), this caused a linear accumulation of
threads, leading to RuntimeError: can't start new thread and OOM
crashes.

****Fix Implementation:****
***Refactored Metrics Initialization (Thread Safety & Memory Leak
Fix)***:
Implemented a Singleton pattern for the OpenTelemetry MeterProvider
using threading.Lock to prevent infinite background thread creation
(memory leak).
Moved metrics initialization logic to a cleaner helper function
_initialize_metrics in client.py.
Replaced global mutable state in SpannerMetricsTracerFactory with
contextvars.ContextVar to ensure thread-safe, isolated metric tracing
across concurrent requests.
Updated MetricsInterceptor and MetricsCapture to correctly use the
thread-local tracer.
***Fixed Batch.commit Idempotency (AlreadyExists Regression):***
Modified Batch.commit to initialize nth_request and the attempt counter
outside the retry loop.
This ensures that retries (e.g., on ABORTED) reuse the same Request ID,
allowing Cloud Spanner to correctly deduplicate requests and preventing
spurious AlreadyExists (409) errors.
***Verification:***
Added tests/unit/test_metrics_concurrency.py to verify tracer isolation
and thread safety.
Cleaned up tests/unit/test_metrics.py and consolidated mocks in
conftest.py.

Author: sinhasubham

.kokoro/presubmit/presubmit.cfg

@@ -3,5 +3,5 @@
 # Only run a subset of all nox sessions
 env_vars: {
     key: "NOX_SESSION"
-    value: "unit-3.9 unit-3.12 cover docs docfx"
+    value: "unit-3.10 unit-3.12 cover docs docfx"
 }

google/cloud/spanner_v1/batch.py

@@ -13,6 +13,7 @@
 # limitations under the License.
 
 """Context manager for Cloud Spanner batched writes."""
+
 import functools
 from typing import List, Optional
 

google/cloud/spanner_v1/client.py

@@ -23,10 +23,12 @@
 * a :class:`~google.cloud.spanner_v1.instance.Instance` owns a
   :class:`~google.cloud.spanner_v1.database.Database`
 """
+
 import grpc
 import os
 import logging
 import warnings
+import threading
 
 from google.api_core.gapic_v1 import client_info
 from google.auth.credentials import AnonymousCredentials
@@ -99,11 +101,50 @@ def _get_spanner_optimizer_statistics_package():
 
 log = logging.getLogger(__name__)
 
+_metrics_monitor_initialized = False
+_metrics_monitor_lock = threading.Lock()
+
 
 def _get_spanner_enable_builtin_metrics_env():
     return os.getenv(SPANNER_DISABLE_BUILTIN_METRICS_ENV_VAR) != "true"
 
 
+def _initialize_metrics(project, credentials):
+    """
+    Initializes the Spanner built-in metrics.
+
+    This function sets up the OpenTelemetry MeterProvider and the SpannerMetricsTracerFactory.
+    It uses a lock to ensure that initialization happens only once.
+    """
+    global _metrics_monitor_initialized
+    if not _metrics_monitor_initialized:
+        with _metrics_monitor_lock:
+            if not _metrics_monitor_initialized:
+                meter_provider = metrics.NoOpMeterProvider()
+                try:
+                    if not _get_spanner_emulator_host():
+                        meter_provider = MeterProvider(
+                            metric_readers=[
+                                PeriodicExportingMetricReader(
+                                    CloudMonitoringMetricsExporter(
+                                        project_id=project,
+                                        credentials=credentials,
+                                    ),
+                                    export_interval_millis=METRIC_EXPORT_INTERVAL_MS,
+                                ),
+                            ]
+                        )
+                    metrics.set_meter_provider(meter_provider)
+                    SpannerMetricsTracerFactory()
+                    _metrics_monitor_initialized = True
+                except Exception as e:
+                    # log is already defined at module level
+                    log.warning(
+                        "Failed to initialize Spanner built-in metrics. Error: %s",
+                        e,
+                    )
+
+
 class Client(ClientWithProject):
     """Client for interacting with Cloud Spanner API.
 
@@ -251,31 +292,12 @@ def __init__(
             "http://" in self._emulator_host or "https://" in self._emulator_host
         ):
             warnings.warn(_EMULATOR_HOST_HTTP_SCHEME)
-        # Check flag to enable Spanner builtin metrics
         if (
             _get_spanner_enable_builtin_metrics_env()
             and not disable_builtin_metrics
             and HAS_GOOGLE_CLOUD_MONITORING_INSTALLED
         ):
-            meter_provider = metrics.NoOpMeterProvider()
-            try:
-                if not _get_spanner_emulator_host():
-                    meter_provider = MeterProvider(
-                        metric_readers=[
-                            PeriodicExportingMetricReader(
-                                CloudMonitoringMetricsExporter(
-                                    project_id=project, credentials=credentials
-                                ),
-                                export_interval_millis=METRIC_EXPORT_INTERVAL_MS,
-                            ),
-                        ]
-                    )
-                metrics.set_meter_provider(meter_provider)
-                SpannerMetricsTracerFactory()
-            except Exception as e:
-                log.warning(
-                    "Failed to initialize Spanner built-in metrics. Error: %s", e
-                )
+            _initialize_metrics(project, credentials)
         else:
             SpannerMetricsTracerFactory(enabled=False)
 

google/cloud/spanner_v1/metrics/metrics_capture.py

@@ -20,6 +20,8 @@
 performance monitoring.
 """
 
+from contextvars import Token
+
 from .spanner_metrics_tracer_factory import SpannerMetricsTracerFactory
 
 
@@ -30,6 +32,9 @@ class MetricsCapture:
     the start and completion of metrics tracing for a given operation.
     """
 
+    _token: Token
+    """Token to reset the context variable after the operation completes."""
+
     def __enter__(self):
         """Enter the runtime context related to this object.
 
@@ -45,11 +50,11 @@ def __enter__(self):
             return self
 
         # Define a new metrics tracer for the new operation
-        SpannerMetricsTracerFactory.current_metrics_tracer = (
-            factory.create_metrics_tracer()
-        )
-        if SpannerMetricsTracerFactory.current_metrics_tracer:
-            SpannerMetricsTracerFactory.current_metrics_tracer.record_operation_start()
+        # Set the context var and keep the token for reset
+        tracer = factory.create_metrics_tracer()
+        self._token = SpannerMetricsTracerFactory.set_current_tracer(tracer)
+        if tracer:
+            tracer.record_operation_start()
         return self
 
     def __exit__(self, exc_type, exc_value, traceback):
@@ -70,6 +75,11 @@ def __exit__(self, exc_type, exc_value, traceback):
         if not SpannerMetricsTracerFactory().enabled:
             return False
 
-        if SpannerMetricsTracerFactory.current_metrics_tracer:
-            SpannerMetricsTracerFactory.current_metrics_tracer.record_operation_completion()
+        tracer = SpannerMetricsTracerFactory.get_current_tracer()
+        if tracer:
+            tracer.record_operation_completion()
+
+        # Reset the context var using the token
+        if getattr(self, "_token", None):
+            SpannerMetricsTracerFactory.reset_current_tracer(self._token)
         return False  # Propagate the exception if any

google/cloud/spanner_v1/metrics/metrics_interceptor.py

@@ -97,22 +97,17 @@ def _set_metrics_tracer_attributes(self, resources: Dict[str, str]) -> None:
         Args:
             resources (Dict[str, str]): A dictionary containing project, instance, and database information.
         """
-        if SpannerMetricsTracerFactory.current_metrics_tracer is None:
+        tracer = SpannerMetricsTracerFactory.get_current_tracer()
+        if tracer is None:
             return
 
         if resources:
             if "project" in resources:
-                SpannerMetricsTracerFactory.current_metrics_tracer.set_project(
-                    resources["project"]
-                )
+                tracer.set_project(resources["project"])
             if "instance" in resources:
-                SpannerMetricsTracerFactory.current_metrics_tracer.set_instance(
-                    resources["instance"]
-                )
+                tracer.set_instance(resources["instance"])
             if "database" in resources:
-                SpannerMetricsTracerFactory.current_metrics_tracer.set_database(
-                    resources["database"]
-                )
+                tracer.set_database(resources["database"])
 
     def intercept(self, invoked_method, request_or_iterator, call_details):
         """Intercept gRPC calls to collect metrics.
@@ -126,31 +121,32 @@ def intercept(self, invoked_method, request_or_iterator, call_details):
             The RPC response
         """
         factory = SpannerMetricsTracerFactory()
-        if (
-            SpannerMetricsTracerFactory.current_metrics_tracer is None
-            or not factory.enabled
-        ):
+        tracer = SpannerMetricsTracerFactory.get_current_tracer()
+        if tracer is None or not factory.enabled:
             return invoked_method(request_or_iterator, call_details)
 
         # Setup Metric Tracer attributes from call details
-        ## Extract  Project / Instance / Databse from header information
-        resources = self._extract_resource_from_path(call_details.metadata)
-        self._set_metrics_tracer_attributes(resources)
+        ## Extract Project / Instance / Database from header information if not already set
+        if not (
+            tracer.client_attributes.get("project_id")
+            and tracer.client_attributes.get("instance_id")
+            and tracer.client_attributes.get("database")
+        ):
+            resources = self._extract_resource_from_path(call_details.metadata)
+            self._set_metrics_tracer_attributes(resources)
 
         ## Format method to be be spanner.<method name>
         method_name = self._remove_prefix(
             call_details.method, SPANNER_METHOD_PREFIX
         ).replace("/", ".")
 
-        SpannerMetricsTracerFactory.current_metrics_tracer.set_method(method_name)
-        SpannerMetricsTracerFactory.current_metrics_tracer.record_attempt_start()
+        tracer.set_method(method_name)
+        tracer.record_attempt_start()
         response = invoked_method(request_or_iterator, call_details)
-        SpannerMetricsTracerFactory.current_metrics_tracer.record_attempt_completion()
+        tracer.record_attempt_completion()
 
         # Process and send GFE metrics if enabled
-        if SpannerMetricsTracerFactory.current_metrics_tracer.gfe_enabled:
+        if tracer.gfe_enabled:
             metadata = response.initial_metadata()
-            SpannerMetricsTracerFactory.current_metrics_trace.record_gfe_metrics(
-                metadata
-            )
+            tracer.record_gfe_metrics(metadata)
         return response

google/cloud/spanner_v1/metrics/spanner_metrics_tracer_factory.py

@@ -19,6 +19,7 @@
 import os
 import logging
 from .constants import SPANNER_SERVICE_NAME
+import contextvars
 
 try:
     import mmh3
@@ -43,7 +44,9 @@ class SpannerMetricsTracerFactory(MetricsTracerFactory):
     """A factory for creating SpannerMetricsTracer instances."""
 
     _metrics_tracer_factory: "SpannerMetricsTracerFactory" = None
-    current_metrics_tracer: MetricsTracer = None
+    _current_metrics_tracer_ctx = contextvars.ContextVar(
+        "current_metrics_tracer", default=None
+    )
 
     def __new__(
         cls, enabled: bool = True, gfe_enabled: bool = False
@@ -80,10 +83,22 @@ def __new__(
             cls._metrics_tracer_factory.gfe_enabled = gfe_enabled
 
             if cls._metrics_tracer_factory.enabled != enabled:
-                cls._metrics_tracer_factory.enabeld = enabled
+                cls._metrics_tracer_factory.enabled = enabled
 
         return cls._metrics_tracer_factory
 
+    @staticmethod
+    def get_current_tracer() -> MetricsTracer:
+        return SpannerMetricsTracerFactory._current_metrics_tracer_ctx.get()
+
+    @staticmethod
+    def set_current_tracer(tracer: MetricsTracer) -> contextvars.Token:
+        return SpannerMetricsTracerFactory._current_metrics_tracer_ctx.set(tracer)
+
+    @staticmethod
+    def reset_current_tracer(token: contextvars.Token):
+        SpannerMetricsTracerFactory._current_metrics_tracer_ctx.reset(token)
+
     @staticmethod
     def _generate_client_uid() -> str:
         """Generate a client UID in the form of uuidv4@pid@hostname.

tests/unit/conftest.py

@@ -0,0 +1,27 @@
+# Copyright 2025 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import pytest
+from unittest.mock import patch
+
+
+@pytest.fixture(autouse=True)
+def mock_periodic_exporting_metric_reader():
+    """Globally mock PeriodicExportingMetricReader to prevent real network calls."""
+    with patch(
+        "google.cloud.spanner_v1.client.PeriodicExportingMetricReader"
+    ) as mock_client_reader, patch(
+        "opentelemetry.sdk.metrics.export.PeriodicExportingMetricReader"
+    ):
+        yield mock_client_reader