Change session-based probabilistic sampling computation (#698)

* new sampler impl

* add the UInt32QuadXorTraceIdRatioSampler to better align with the experience on web and ios.

* add comment

* typo

splunk-otel-android/src/main/java/com/splunk/rum/RumInitializer.java

@@ -31,11 +31,11 @@
 import android.os.Looper;
 import androidx.annotation.NonNull;
 import androidx.annotation.Nullable;
+import com.splunk.rum.internal.UInt32QuadXorTraceIdRatioSampler;
 import io.opentelemetry.android.GlobalAttributesSpanAppender;
 import io.opentelemetry.android.OpenTelemetryRum;
 import io.opentelemetry.android.OpenTelemetryRumBuilder;
 import io.opentelemetry.android.RuntimeDetailsExtractor;
-import io.opentelemetry.android.SessionIdRatioBasedSampler;
 import io.opentelemetry.android.instrumentation.activity.VisibleScreenTracker;
 import io.opentelemetry.android.instrumentation.anr.AnrDetector;
 import io.opentelemetry.android.instrumentation.crash.CrashReporter;
@@ -57,8 +57,10 @@
 import io.opentelemetry.sdk.trace.export.BatchSpanProcessor;
 import io.opentelemetry.sdk.trace.export.SimpleSpanProcessor;
 import io.opentelemetry.sdk.trace.export.SpanExporter;
+import io.opentelemetry.sdk.trace.samplers.Sampler;
 import java.time.Duration;
 import java.util.Collection;
+import java.util.concurrent.atomic.AtomicReference;
 import java.util.function.Function;
 import java.util.function.Supplier;
 import java.util.logging.Level;
@@ -149,13 +151,20 @@ SplunkRum initialize(
                                         .build()));
 
         // Set up the sampler, if enabled
+        // TODO: Make this better...
+        // This holder is required because we cannot reasonably get the session id until after
+        // OpenTelemetryRum has been created. So this is spackled into place below.
+        AtomicReference<Supplier<String>> sessionSupplierHolder = new AtomicReference<>(() -> null);
         if (builder.sessionBasedSamplerEnabled) {
             otelRumBuilder.addTracerProviderCustomizer(
                     (tracerProviderBuilder, app) -> {
-                        SessionIdRatioBasedSampler sampler =
-                                new SessionIdRatioBasedSampler(
+                        Sampler sampler =
+                                UInt32QuadXorTraceIdRatioSampler.create(
                                         builder.sessionBasedSamplerRatio,
-                                        otelRumBuilder.getSessionId());
+                                        () -> {
+                                            Supplier<String> supplier = sessionSupplierHolder.get();
+                                            return supplier == null ? null : supplier.get();
+                                        });
                         return tracerProviderBuilder.setSampler(sampler);
                     });
         }
@@ -210,6 +219,8 @@ SplunkRum initialize(
 
         OpenTelemetryRum openTelemetryRum = otelRumBuilder.build();
 
+        sessionSupplierHolder.set(openTelemetryRum::getRumSessionId);
+
         initializationEvents.recordInitializationSpans(
                 builder.getConfigFlags(),
                 openTelemetryRum.getOpenTelemetry().getTracer(RUM_TRACER_NAME));

splunk-otel-android/src/main/java/com/splunk/rum/internal/SessionUtils.java

@@ -0,0 +1,38 @@
+/*
+ * Copyright Splunk Inc.
+ *
+ * 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.
+ */
+
+package com.splunk.rum.internal;
+
+import android.util.Log;
+
+public class SessionUtils {
+
+    /** Performs an unsigned 32-bit conversion of the hex session id to a long. */
+    static long convertToUInt32(String sessionId) {
+        long acc = 0L;
+        for (int i = 0; i < sessionId.length(); i += 8) {
+            long chunk = 0;
+            try {
+                String chunkString = sessionId.substring(i, i + 8);
+                chunk = Long.parseUnsignedLong(chunkString, 16);
+            } catch (NumberFormatException e) {
+                Log.w("SplunkRum", "Error parsing session id into long: " + sessionId);
+            }
+            acc = acc ^ chunk;
+        }
+        return acc;
+    }
+}

splunk-otel-android/src/main/java/com/splunk/rum/internal/UInt32QuadXorTraceIdRatioSampler.java

@@ -0,0 +1,112 @@
+/*
+ * Copyright Splunk Inc.
+ *
+ * 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.
+ */
+
+package com.splunk.rum.internal;
+
+import io.opentelemetry.api.common.Attributes;
+import io.opentelemetry.api.trace.SpanKind;
+import io.opentelemetry.context.Context;
+import io.opentelemetry.sdk.trace.data.LinkData;
+import io.opentelemetry.sdk.trace.samplers.Sampler;
+import io.opentelemetry.sdk.trace.samplers.SamplingResult;
+import java.util.List;
+import java.util.Locale;
+import java.util.function.Supplier;
+
+/**
+ * This class is very similar to the SessionIdRatioBasedSampler from upstream, but exists in order
+ * to perform a trace id into a long calculation in a way that is more consistent with iOS and js.
+ *
+ * <p>This class should be considered a stop-gap measure until this problem is correctly spec'd in
+ * otel.
+ *
+ * <p>This class is internal and is hence not for public use. Its APIs are unstable and can change
+ * at any time.
+ */
+public class UInt32QuadXorTraceIdRatioSampler implements Sampler {
+    static final SamplingResult POSITIVE_SAMPLING_RESULT = SamplingResult.recordAndSample();
+
+    static final SamplingResult NEGATIVE_SAMPLING_RESULT = SamplingResult.drop();
+    private final long idUpperBound;
+    private final String description;
+    private final Supplier<String> sessionIdSupplier;
+    private final Object lock = new Object();
+    private String lastSeenSessionId = "";
+    private SamplingResult lastSamplingResult = NEGATIVE_SAMPLING_RESULT;
+
+    public static Sampler create(double ratio, Supplier<String> sessionIdSupplier) {
+        // Taken directly mostly from the TraceIdRatioBasedSampler in upstream, with a modification
+        // to the upper bound to make it within UInt32.
+        if (ratio < 0.0 || ratio > 1.0) {
+            throw new IllegalArgumentException("ratio must be in range [0.0, 1.0]");
+        }
+        long idUpperBound;
+        // Special case the limits, to avoid any possible issues with lack of precision across
+        // double/long boundaries. For probability == 0.0, we use Long.MIN_VALUE as this guarantees
+        // that we will never sample a trace, even in the case where the id == Long.MIN_VALUE, since
+        // Math.Abs(Long.MIN_VALUE) == Long.MIN_VALUE.
+        if (ratio == 0.0) {
+            idUpperBound = Long.MIN_VALUE;
+        } else if (ratio == 1.0) {
+            idUpperBound = Long.MAX_VALUE;
+        } else {
+            // ratio * UInt32 max value
+            idUpperBound = (long) (ratio * 0xFFFFFFFFL);
+        }
+        String description =
+                String.format(
+                        Locale.getDefault(), "UInt32QuadXorTraceIdRatioSampler{ratio:%f}", ratio);
+        return new UInt32QuadXorTraceIdRatioSampler(idUpperBound, sessionIdSupplier, description);
+    }
+
+    private UInt32QuadXorTraceIdRatioSampler(
+            long idUpperBound, Supplier<String> sessionIdSupplier, String description) {
+        this.idUpperBound = idUpperBound;
+        this.sessionIdSupplier = sessionIdSupplier;
+        this.description = description;
+    }
+
+    @Override
+    public SamplingResult shouldSample(
+            Context parentContext,
+            String traceId,
+            String name,
+            SpanKind spanKind,
+            Attributes attributes,
+            List<LinkData> parentLinks) {
+        String sessionId = sessionIdSupplier.get();
+        if (sessionId == null) {
+            return POSITIVE_SAMPLING_RESULT; // Have to return true because we may not have a
+            // session yet
+        }
+        synchronized (lock) {
+            if (lastSeenSessionId.equals(sessionId)) {
+                return lastSamplingResult;
+            }
+            lastSeenSessionId = sessionId;
+            lastSamplingResult =
+                    SessionUtils.convertToUInt32(sessionId) < idUpperBound
+                            ? POSITIVE_SAMPLING_RESULT
+                            : NEGATIVE_SAMPLING_RESULT;
+            return lastSamplingResult;
+        }
+    }
+
+    @Override
+    public String getDescription() {
+        return description;
+    }
+}

splunk-otel-android/src/test/java/com/splunk/rum/internal/SessionUtilsTest.java

@@ -0,0 +1,30 @@
+/*
+ * Copyright Splunk Inc.
+ *
+ * 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.
+ */
+
+package com.splunk.rum.internal;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+import org.junit.jupiter.api.Test;
+
+class SessionUtilsTest {
+
+    @Test
+    void testConvert() {
+        long result = SessionUtils.convertToUInt32("c06947ed1f53b1a69be3c6899bc11a3e");
+        assertEquals(3742903036L, result);
+    }
+}

splunk-otel-android/src/test/java/com/splunk/rum/internal/UInt32QuadXorTraceIdRatioSamplerTest.java

@@ -0,0 +1,80 @@
+/*
+ * Copyright Splunk Inc.
+ *
+ * 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.
+ */
+
+package com.splunk.rum.internal;
+
+import static com.splunk.rum.internal.UInt32QuadXorTraceIdRatioSampler.NEGATIVE_SAMPLING_RESULT;
+import static com.splunk.rum.internal.UInt32QuadXorTraceIdRatioSampler.POSITIVE_SAMPLING_RESULT;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import io.opentelemetry.api.common.Attributes;
+import io.opentelemetry.api.trace.Span;
+import io.opentelemetry.context.Context;
+import io.opentelemetry.sdk.trace.samplers.Sampler;
+import io.opentelemetry.sdk.trace.samplers.SamplingResult;
+import java.util.Collections;
+import org.junit.jupiter.api.Test;
+
+class UInt32QuadXorTraceIdRatioSamplerTest {
+
+    private final Context parentContext = Context.root().with(Span.getInvalid());
+
+    @Test
+    void sampleInclude() {
+        Sampler sampler =
+                UInt32QuadXorTraceIdRatioSampler.create(
+                        0.5, () -> "4777abcd3f7777abcdefc6899bc11a3e");
+        SamplingResult result =
+                sampler.shouldSample(
+                        parentContext,
+                        null,
+                        null,
+                        null,
+                        Attributes.empty(),
+                        Collections.emptyList());
+        assertEquals(POSITIVE_SAMPLING_RESULT.getDecision(), result.getDecision());
+    }
+
+    @Test
+    void sampleDrop() {
+        Sampler sampler =
+                UInt32QuadXorTraceIdRatioSampler.create(
+                        0.5, () -> "9777abcd3f7777abcdefc6899bc11a3e");
+        SamplingResult result =
+                sampler.shouldSample(
+                        parentContext,
+                        null,
+                        null,
+                        null,
+                        Attributes.empty(),
+                        Collections.emptyList());
+        assertEquals(NEGATIVE_SAMPLING_RESULT.getDecision(), result.getDecision());
+    }
+
+    @Test
+    void nullSessionMeansAlwaysPositive() {
+        Sampler sampler = UInt32QuadXorTraceIdRatioSampler.create(0.00000001, () -> null);
+        SamplingResult result =
+                sampler.shouldSample(
+                        parentContext,
+                        null,
+                        null,
+                        null,
+                        Attributes.empty(),
+                        Collections.emptyList());
+        assertEquals(POSITIVE_SAMPLING_RESULT.getDecision(), result.getDecision());
+    }
+}