Fix handling of empty metric collection cycles

CHANGELOG.md

@@ -7,9 +7,12 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## Unreleased
 
+- Fix handling of empty metric collection cycles
+  ([#3335](https://github.com/open-telemetry/opentelemetry-python/pull/3335))
 - Fix error when no LoggerProvider configured for LoggingHandler
   ([#3423](https://github.com/open-telemetry/opentelemetry-python/pull/3423))
-
+
+
 ## Version 1.20.0/0.41b0 (2023-09-04)
 
 - Modify Prometheus exporter to translate non-monotonic Sums into Gauges

opentelemetry-sdk/src/opentelemetry/sdk/metrics/_internal/_view_instrument_match.py

@@ -16,7 +16,7 @@
 from logging import getLogger
 from threading import Lock
 from time import time_ns
-from typing import Dict, List, Sequence
+from typing import Dict, List, Optional, Sequence
 
 from opentelemetry.metrics import Instrument
 from opentelemetry.sdk.metrics._internal.aggregation import (
@@ -126,7 +126,7 @@ def collect(
         self,
         aggregation_temporality: AggregationTemporality,
         collection_start_nanos: int,
-    ) -> Sequence[DataPointT]:
+    ) -> Optional[Sequence[DataPointT]]:
 
         data_points: List[DataPointT] = []
         with self._lock:
@@ -136,4 +136,8 @@ def collect(
                 )
                 if data_point is not None:
                     data_points.append(data_point)
-        return data_points
+
+        # Returning here None instead of an empty list because the caller
+        # does not consume a sequence and to be consistent with the rest of
+        # collect methods that also return None.
+        return data_points or None

opentelemetry-sdk/src/opentelemetry/sdk/metrics/_internal/export/__init__.py

@@ -322,10 +322,14 @@ def collect(self, timeout_millis: float = 10_000) -> None:
             )
             return
 
-        self._receive_metrics(
-            self._collect(self, timeout_millis=timeout_millis),
-            timeout_millis=timeout_millis,
-        )
+        metrics = self._collect(self, timeout_millis=timeout_millis)
+
+        if metrics is not None:
+
+            self._receive_metrics(
+                metrics,
+                timeout_millis=timeout_millis,
+            )
 
     @final
     def _set_collect_callback(
@@ -515,8 +519,7 @@ def _receive_metrics(
         timeout_millis: float = 10_000,
         **kwargs,
     ) -> None:
-        if metrics_data is None:
-            return
+
         token = attach(set_value(_SUPPRESS_INSTRUMENTATION_KEY, True))
         try:
             with self._export_lock:

opentelemetry-sdk/src/opentelemetry/sdk/metrics/_internal/measurement_consumer.py

@@ -17,7 +17,7 @@
 from abc import ABC, abstractmethod
 from threading import Lock
 from time import time_ns
-from typing import Iterable, List, Mapping
+from typing import Iterable, List, Mapping, Optional
 
 # This kind of import is needed to avoid Sphinx errors.
 import opentelemetry.sdk.metrics
@@ -51,7 +51,7 @@ def collect(
         self,
         metric_reader: "opentelemetry.sdk.metrics.MetricReader",
         timeout_millis: float = 10_000,
-    ) -> Iterable[Metric]:
+    ) -> Optional[Iterable[Metric]]:
         pass
 
 
@@ -94,7 +94,7 @@ def collect(
         self,
         metric_reader: "opentelemetry.sdk.metrics.MetricReader",
         timeout_millis: float = 10_000,
-    ) -> Iterable[Metric]:
+    ) -> Optional[Iterable[Metric]]:
 
         with self._lock:
             metric_reader_storage = self._reader_storages[metric_reader]
@@ -123,4 +123,6 @@ def collect(
                 for measurement in measurements:
                     metric_reader_storage.consume_measurement(measurement)
 
-        return self._reader_storages[metric_reader].collect()
+            result = self._reader_storages[metric_reader].collect()
+
+        return result

opentelemetry-sdk/src/opentelemetry/sdk/metrics/_internal/metric_reader_storage.py

@@ -15,7 +15,7 @@
 from logging import getLogger
 from threading import RLock
 from time import time_ns
-from typing import Dict, List
+from typing import Dict, List, Optional
 
 from opentelemetry.metrics import (
     Asynchronous,
@@ -119,7 +119,7 @@ def consume_measurement(self, measurement: Measurement) -> None:
         ):
             view_instrument_match.consume_measurement(measurement)
 
-    def collect(self) -> MetricsData:
+    def collect(self) -> Optional[MetricsData]:
         # Use a list instead of yielding to prevent a slow reader from holding
         # SDK locks
 
@@ -152,16 +152,21 @@ def collect(self) -> MetricsData:
 
                 for view_instrument_match in view_instrument_matches:
 
+                    data_points = view_instrument_match.collect(
+                        aggregation_temporality, collection_start_nanos
+                    )
+
+                    if data_points is None:
+                        continue
+
                     if isinstance(
                         # pylint: disable=protected-access
                         view_instrument_match._aggregation,
                         _SumAggregation,
                     ):
                         data = Sum(
                             aggregation_temporality=aggregation_temporality,
-                            data_points=view_instrument_match.collect(
-                                aggregation_temporality, collection_start_nanos
-                            ),
+                            data_points=data_points,
                             is_monotonic=isinstance(
                                 instrument, (Counter, ObservableCounter)
                             ),
@@ -171,20 +176,14 @@ def collect(self) -> MetricsData:
                         view_instrument_match._aggregation,
                         _LastValueAggregation,
                     ):
-                        data = Gauge(
-                            data_points=view_instrument_match.collect(
-                                aggregation_temporality, collection_start_nanos
-                            )
-                        )
+                        data = Gauge(data_points=data_points)
                     elif isinstance(
                         # pylint: disable=protected-access
                         view_instrument_match._aggregation,
                         _ExplicitBucketHistogramAggregation,
                     ):
                         data = Histogram(
-                            data_points=view_instrument_match.collect(
-                                aggregation_temporality, collection_start_nanos
-                            ),
+                            data_points=data_points,
                             aggregation_temporality=aggregation_temporality,
                         )
                     elif isinstance(
@@ -200,9 +199,7 @@ def collect(self) -> MetricsData:
                         _ExponentialBucketHistogramAggregation,
                     ):
                         data = ExponentialHistogram(
-                            data_points=view_instrument_match.collect(
-                                aggregation_temporality, collection_start_nanos
-                            ),
+                            data_points=data_points,
                             aggregation_temporality=aggregation_temporality,
                         )
 
@@ -216,32 +213,38 @@ def collect(self) -> MetricsData:
                         )
                     )
 
-                if instrument.instrumentation_scope not in (
-                    instrumentation_scope_scope_metrics
-                ):
-                    instrumentation_scope_scope_metrics[
-                        instrument.instrumentation_scope
-                    ] = ScopeMetrics(
-                        scope=instrument.instrumentation_scope,
-                        metrics=metrics,
-                        schema_url=instrument.instrumentation_scope.schema_url,
-                    )
-                else:
-                    instrumentation_scope_scope_metrics[
-                        instrument.instrumentation_scope
-                    ].metrics.extend(metrics)
-
-        return MetricsData(
-            resource_metrics=[
-                ResourceMetrics(
-                    resource=self._sdk_config.resource,
-                    scope_metrics=list(
-                        instrumentation_scope_scope_metrics.values()
-                    ),
-                    schema_url=self._sdk_config.resource.schema_url,
+                if metrics:
+
+                    if instrument.instrumentation_scope not in (
+                        instrumentation_scope_scope_metrics
+                    ):
+                        instrumentation_scope_scope_metrics[
+                            instrument.instrumentation_scope
+                        ] = ScopeMetrics(
+                            scope=instrument.instrumentation_scope,
+                            metrics=metrics,
+                            schema_url=instrument.instrumentation_scope.schema_url,
+                        )
+                    else:
+                        instrumentation_scope_scope_metrics[
+                            instrument.instrumentation_scope
+                        ].metrics.extend(metrics)
+
+            if instrumentation_scope_scope_metrics:
+
+                return MetricsData(
+                    resource_metrics=[
+                        ResourceMetrics(
+                            resource=self._sdk_config.resource,
+                            scope_metrics=list(
+                                instrumentation_scope_scope_metrics.values()
+                            ),
+                            schema_url=self._sdk_config.resource.schema_url,
+                        )
+                    ]
                 )
-            ]
-        )
+
+            return None
 
     def _handle_view_instrument_match(
         self,

opentelemetry-sdk/tests/metrics/integration_test/test_console_exporter.py

@@ -72,3 +72,19 @@ def test_console_exporter(self):
 
         self.assertEqual(metrics["attributes"], {"a": "b"})
         self.assertEqual(metrics["value"], 1)
+
+    def test_console_exporter_no_export(self):
+
+        output = StringIO()
+        exporter = ConsoleMetricExporter(out=output)
+        reader = PeriodicExportingMetricReader(
+            exporter, export_interval_millis=100
+        )
+        provider = MeterProvider(metric_readers=[reader])
+        provider.shutdown()
+
+        output.seek(0)
+        actual = "".join(output.readlines())
+        expected = ""
+
+        self.assertEqual(actual, expected)

opentelemetry-sdk/tests/metrics/integration_test/test_disable_default_views.py

@@ -31,15 +31,7 @@ def test_disable_default_views(self):
         counter.add(10, {"label": "value1"})
         counter.add(10, {"label": "value2"})
         counter.add(10, {"label": "value3"})
-        self.assertEqual(
-            (
-                reader.get_metrics_data()
-                .resource_metrics[0]
-                .scope_metrics[0]
-                .metrics
-            ),
-            [],
-        )
+        self.assertIsNone(reader.get_metrics_data())
 
     def test_disable_default_views_add_custom(self):
         reader = InMemoryMetricReader()

opentelemetry-sdk/tests/metrics/integration_test/test_exporter_concurrency.py

@@ -74,6 +74,15 @@ class TestExporterConcurrency(ConcurrencyTestBase):
     > be called again only after the current call returns.
 
     https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/metrics/sdk.md#exportbatch
+
+    This test also tests that a thread that calls the a
+    ``MetricReader.collect`` method using an asynchronous instrument is able
+    to perform two actions in the same thread lock space (without it being
+    interrupted by another thread):
+
+    1. Consume the measurement produced by the callback associated to the
+       asynchronous instrument.
+    2. Export the measurement mentioned in the step above.
     """
 
     def test_exporter_not_called_concurrently(self):
@@ -84,7 +93,11 @@ def test_exporter_not_called_concurrently(self):
         )
         meter_provider = MeterProvider(metric_readers=[reader])
 
+        counter_cb_counter = 0
+
         def counter_cb(options: CallbackOptions):
+            nonlocal counter_cb_counter
+            counter_cb_counter += 1
             yield Observation(2)
 
         meter_provider.get_meter(__name__).create_observable_counter(
@@ -97,6 +110,7 @@ def test_many_threads():
 
         self.run_with_many_threads(test_many_threads, num_threads=100)
 
+        self.assertEqual(counter_cb_counter, 100)
         # no thread should be in export() now
         self.assertEqual(exporter.count_in_export, 0)
         # should be one call for each thread