RUM-16886: Mark sampled-out network spans as droppable while preserving stats

CoreTracer needs to see all spans to calculate APM client-side stats. Since network spans have their own sampling rate, sampled-out spans are now finished with the `_dd.local.force_drop` tag instead of being dropped. This tells downstream systems to discard the span while still letting CoreTracer process it for stats calculations.

This only applies when the active tracer is the SDK's own `DatadogTracerAdapter`, which routes through `CoreTraceWriter` — the only implementation that honors `_dd.local.force_drop`. When a custom or global `DatadogTracer` is in use, sampled-out spans are dropped via `drop()` as before, preserving backward compatibility.

Author: abrooksv

features/dd-sdk-android-trace/api/apiSurface

@@ -90,13 +90,15 @@ object com.datadog.android.trace.internal._TraceInternalProxy
   fun setSdkV2Compatible(com.datadog.android.trace.api.tracer.DatadogTracerBuilder): com.datadog.android.trace.api.tracer.DatadogTracerBuilder
   fun addThrowable(com.datadog.android.trace.api.span.DatadogSpan, Throwable, Byte)
   fun activateSpan(com.datadog.android.trace.api.tracer.DatadogTracer, com.datadog.android.trace.api.span.DatadogSpan, Boolean): com.datadog.android.trace.api.scope.DatadogScope?
+  fun isSDKBackedTracer(com.datadog.android.trace.api.tracer.DatadogTracer): Boolean
   fun mergeBaggage(String?, String): String
   fun createApmNetworkInstrumentation(String, com.datadog.android.trace.ApmNetworkInstrumentationConfiguration)
+const val FORCE_DROP_SPAN: String
 fun com.datadog.android.core.sampling.Sampler<com.datadog.android.trace.api.span.DatadogSpan>.effectiveSampleRate(com.datadog.android.trace.api.span.DatadogSpan): Float?
 fun com.datadog.android.trace.api.span.DatadogSpanContext?.isDropped(): Boolean
 fun Int?.isDroppedPriority(): Boolean
 data class com.datadog.android.trace.internal.net.RequestTracingState
-  constructor(com.datadog.android.api.instrumentation.network.HttpRequestInfoBuilder, Boolean = false, com.datadog.android.trace.api.span.DatadogSpan? = null, Float? = null)
+  constructor(com.datadog.android.api.instrumentation.network.HttpRequestInfoBuilder, Boolean = false, com.datadog.android.trace.api.span.DatadogSpan? = null, Float? = null, Boolean)
   fun createRequestInfo(): com.datadog.android.api.instrumentation.network.HttpRequestInfo
 fun RequestTracingState?.toAttributesMap(String, String, String): Map<String, Any?>
 class com.datadog.android.trace.internal.net.SessionRebasedSampler : com.datadog.android.core.sampling.Sampler<com.datadog.android.trace.api.span.DatadogSpan>, SpanAwareSampler

features/dd-sdk-android-trace/api/dd-sdk-android-trace.api

@@ -167,33 +167,40 @@ public final class com/datadog/android/trace/internal/_TraceInternalProxy {
 	public static final fun addThrowable (Lcom/datadog/android/trace/api/span/DatadogSpan;Ljava/lang/Throwable;B)V
 	public final fun createApmNetworkInstrumentation (Ljava/lang/String;Lcom/datadog/android/trace/ApmNetworkInstrumentationConfiguration;)Lcom/datadog/android/trace/internal/ApmNetworkInstrumentation;
 	public final fun getPropagationHelper ()Lcom/datadog/android/trace/internal/DatadogPropagationHelper;
+	public final fun isSDKBackedTracer (Lcom/datadog/android/trace/api/tracer/DatadogTracer;)Z
 	public final fun mergeBaggage (Ljava/lang/String;Ljava/lang/String;)Ljava/lang/String;
 	public final fun setSdkV2Compatible (Lcom/datadog/android/trace/api/tracer/DatadogTracerBuilder;)Lcom/datadog/android/trace/api/tracer/DatadogTracerBuilder;
 	public final fun setTraceId128BitGenerationEnabled (Lcom/datadog/android/trace/api/tracer/DatadogTracerBuilder;)Lcom/datadog/android/trace/api/tracer/DatadogTracerBuilder;
 	public final fun setTracingSamplingPriorityIfNecessary (Lcom/datadog/android/trace/api/span/DatadogSpanContext;)V
 }
 
+public final class com/datadog/android/trace/internal/domain/event/MetaKeysKt {
+	public static final field FORCE_DROP_SPAN Ljava/lang/String;
+}
+
 public final class com/datadog/android/trace/internal/net/ApmNetworkInstrumentationExtKt {
 	public static final fun effectiveSampleRate (Lcom/datadog/android/core/sampling/Sampler;Lcom/datadog/android/trace/api/span/DatadogSpan;)Ljava/lang/Float;
 	public static final fun isDropped (Lcom/datadog/android/trace/api/span/DatadogSpanContext;)Z
 	public static final fun isDroppedPriority (Ljava/lang/Integer;)Z
 }
 
 public final class com/datadog/android/trace/internal/net/RequestTracingState {
-	public fun <init> (Lcom/datadog/android/api/instrumentation/network/HttpRequestInfoBuilder;ZLcom/datadog/android/trace/api/span/DatadogSpan;Ljava/lang/Float;)V
-	public synthetic fun <init> (Lcom/datadog/android/api/instrumentation/network/HttpRequestInfoBuilder;ZLcom/datadog/android/trace/api/span/DatadogSpan;Ljava/lang/Float;ILkotlin/jvm/internal/DefaultConstructorMarker;)V
+	public fun <init> (Lcom/datadog/android/api/instrumentation/network/HttpRequestInfoBuilder;ZLcom/datadog/android/trace/api/span/DatadogSpan;Ljava/lang/Float;Z)V
+	public synthetic fun <init> (Lcom/datadog/android/api/instrumentation/network/HttpRequestInfoBuilder;ZLcom/datadog/android/trace/api/span/DatadogSpan;Ljava/lang/Float;ZILkotlin/jvm/internal/DefaultConstructorMarker;)V
 	public final fun component1 ()Lcom/datadog/android/api/instrumentation/network/HttpRequestInfoBuilder;
 	public final fun component2 ()Z
 	public final fun component3 ()Lcom/datadog/android/trace/api/span/DatadogSpan;
 	public final fun component4 ()Ljava/lang/Float;
-	public final fun copy (Lcom/datadog/android/api/instrumentation/network/HttpRequestInfoBuilder;ZLcom/datadog/android/trace/api/span/DatadogSpan;Ljava/lang/Float;)Lcom/datadog/android/trace/internal/net/RequestTracingState;
-	public static synthetic fun copy$default (Lcom/datadog/android/trace/internal/net/RequestTracingState;Lcom/datadog/android/api/instrumentation/network/HttpRequestInfoBuilder;ZLcom/datadog/android/trace/api/span/DatadogSpan;Ljava/lang/Float;ILjava/lang/Object;)Lcom/datadog/android/trace/internal/net/RequestTracingState;
+	public final fun component5 ()Z
+	public final fun copy (Lcom/datadog/android/api/instrumentation/network/HttpRequestInfoBuilder;ZLcom/datadog/android/trace/api/span/DatadogSpan;Ljava/lang/Float;Z)Lcom/datadog/android/trace/internal/net/RequestTracingState;
+	public static synthetic fun copy$default (Lcom/datadog/android/trace/internal/net/RequestTracingState;Lcom/datadog/android/api/instrumentation/network/HttpRequestInfoBuilder;ZLcom/datadog/android/trace/api/span/DatadogSpan;Ljava/lang/Float;ZILjava/lang/Object;)Lcom/datadog/android/trace/internal/net/RequestTracingState;
 	public final fun createRequestInfo ()Lcom/datadog/android/api/instrumentation/network/HttpRequestInfo;
 	public fun equals (Ljava/lang/Object;)Z
 	public final fun getRequestInfoBuilder ()Lcom/datadog/android/api/instrumentation/network/HttpRequestInfoBuilder;
 	public final fun getSampleRate ()Ljava/lang/Float;
 	public final fun getSpan ()Lcom/datadog/android/trace/api/span/DatadogSpan;
 	public fun hashCode ()I
+	public final fun isSDKBacked ()Z
 	public final fun isSampled ()Z
 	public fun toString ()Ljava/lang/String;
 }

features/dd-sdk-android-trace/src/main/kotlin/com/datadog/android/trace/internal/ApmNetworkInstrumentation.kt

@@ -115,7 +115,7 @@ class ApmNetworkInstrumentation internal constructor(
         }
 
         if (sdkCore == null) {
-            return RequestTracingState(requestInfoBuilder)
+            return RequestTracingState(requestInfoBuilder = requestInfoBuilder, isSDKBacked = false)
         }
 
         val tracer = tracerProvider.provideTracer(
@@ -125,7 +125,7 @@ class ApmNetworkInstrumentation internal constructor(
         )
 
         if (tracer == null || !request.isTraceable(sdkCore)) {
-            return RequestTracingState(requestInfoBuilder)
+            return RequestTracingState(requestInfoBuilder = requestInfoBuilder, isSDKBacked = false)
         }
 
         val span = tracer.buildSpan(
@@ -155,7 +155,8 @@ class ApmNetworkInstrumentation internal constructor(
             span = span,
             isSampled = isSampled,
             sampleRate = traceSampler.effectiveSampleRate(span),
-            requestInfoBuilder = tracedRequestInfoBuilder
+            requestInfoBuilder = tracedRequestInfoBuilder,
+            isSDKBacked = tracer is DatadogTracerAdapter
         )
     }
 
@@ -167,17 +168,19 @@ class ApmNetworkInstrumentation internal constructor(
      * @param response the HTTP response information.
      */
     fun onResponseSucceeded(requestTracingState: RequestTracingState, response: HttpResponseInfo) {
-        if (requestTracingState.isSampled) {
-            requestTracingState.span?.setTag(DatadogTracingConstants.Tags.KEY_HTTP_STATUS, response.statusCode)
-            if (response.statusCode in HttpURLConnection.HTTP_BAD_REQUEST until HttpURLConnection.HTTP_INTERNAL_ERROR) {
-                requestTracingState.span?.isError = true
-            }
-            if (response.statusCode == HttpURLConnection.HTTP_NOT_FOUND && redacted404ResourceName) {
-                requestTracingState.span?.resourceName = RESOURCE_NAME_404
-            }
+        requestTracingState.span?.setTag(DatadogTracingConstants.Tags.KEY_HTTP_STATUS, response.statusCode)
+        if (response.statusCode in HttpURLConnection.HTTP_BAD_REQUEST until HttpURLConnection.HTTP_INTERNAL_ERROR) {
+            requestTracingState.span?.isError = true
+        }
+        if (response.statusCode == HttpURLConnection.HTTP_NOT_FOUND && redacted404ResourceName) {
+            requestTracingState.span?.resourceName = RESOURCE_NAME_404
         }
         requestTracingState.onRequestIntercepted(response, null)
-        requestTracingState.span?.finishRumAware(requestTracingState.isSampled, canSendSpan)
+        requestTracingState.span?.finishRumAware(
+            requestTracingState.isSampled,
+            canSendSpan,
+            requestTracingState.isSDKBacked
+        )
     }
 
     /**
@@ -188,8 +191,8 @@ class ApmNetworkInstrumentation internal constructor(
      * @param throwable the exception that caused the failure.
      */
     fun onResponseFailed(requestTracingState: RequestTracingState, throwable: Throwable) {
+        requestTracingState.span?.isError = true
         if (requestTracingState.isSampled) {
-            requestTracingState.span?.isError = true
             requestTracingState.span?.setTag(DatadogTracingConstants.Tags.KEY_ERROR_MSG, throwable.message)
             requestTracingState.span?.setTag(DatadogTracingConstants.Tags.KEY_ERROR_TYPE, throwable.javaClass.name)
             requestTracingState.span?.setTag(
@@ -198,7 +201,11 @@ class ApmNetworkInstrumentation internal constructor(
             )
         }
         requestTracingState.onRequestIntercepted(null, throwable)
-        requestTracingState.span?.finishRumAware(requestTracingState.isSampled, canSendSpan)
+        requestTracingState.span?.finishRumAware(
+            requestTracingState.isSampled,
+            canSendSpan,
+            requestTracingState.isSDKBacked
+        )
     }
 
     /**

features/dd-sdk-android-trace/src/main/kotlin/com/datadog/android/trace/internal/_TraceInternalProxy.kt

@@ -93,6 +93,13 @@ object _TraceInternalProxy {
         return (tracer as? DatadogTracerAdapter)?.activateSpan(span, asyncPropagating)
     }
 
+    /**
+     * Returns true if [tracer] is the SDK's built-in tracer, meaning it routes spans through
+     * CoreTraceWriter which honors the FORCE_DROP_SPAN tag. Custom tracer implementations
+     * do not honor that tag, so callers must call drop() instead of setTag+finish() for them.
+     */
+    fun isSDKBackedTracer(tracer: DatadogTracer): Boolean = tracer is DatadogTracerAdapter
+
     /**
      * Merges two baggage headers into a single string representation. The [oldHeader] represents the
      * previously existing baggage header, and the [newHeader] represents the new baggage information

features/dd-sdk-android-trace/src/main/kotlin/com/datadog/android/trace/internal/data/CoreTraceWriter.kt

@@ -19,6 +19,7 @@ import com.datadog.android.event.NoOpEventMapper
 import com.datadog.android.trace.internal.RumContextPropagator
 import com.datadog.android.trace.internal.RumContextPropagator.Companion.extractRumContext
 import com.datadog.android.trace.internal.domain.event.ContextAwareMapper
+import com.datadog.android.trace.internal.domain.event.FORCE_DROP_SPAN
 import com.datadog.android.trace.internal.storage.ContextAwareSerializer
 import com.datadog.android.trace.model.SpanEvent
 import com.datadog.trace.api.sampling.PrioritySampling
@@ -45,7 +46,10 @@ internal class CoreTraceWriter(
         sdkCore.getFeature(Feature.TRACING_FEATURE_NAME)
             ?.withWriteContext { datadogContext, writeScope ->
                 val writeSpans = trace
-                    .filter { it.getTraceSamplingPriority() !in DROP_SAMPLING_PRIORITIES }
+                    .filter {
+                        it.getTraceSamplingPriority() !in DROP_SAMPLING_PRIORITIES &&
+                            it.getTag(FORCE_DROP_SPAN) == null
+                    }
                     .map { it.extractRumContext(rumContextPropagator) }
                 // TODO RUM-4092 Add the capability in the serializer to handle multiple spans in one payload
                 writeScope {

features/dd-sdk-android-trace/src/main/kotlin/com/datadog/android/trace/internal/domain/event/MetaKeys.kt

@@ -6,6 +6,13 @@
 
 package com.datadog.android.trace.internal.domain.event
 
+import com.datadog.android.lint.InternalApi
+
+/**
+ * Internal tag which indicates the span should be created locally but never sent to the backend.
+ */
+@InternalApi
+const val FORCE_DROP_SPAN: String = "_dd.local.force_drop"
 internal const val TRACE_ID_META_KEY = "_dd.p.id"
 internal const val APPLICATION_VARIANT_KEY = "variant"
 internal const val COMPUTE_STATS_META_KEY = "_dd.compute_stats"

features/dd-sdk-android-trace/src/main/kotlin/com/datadog/android/trace/internal/net/ApmNetworkInstrumentationExt.kt

@@ -23,6 +23,7 @@ import com.datadog.android.trace.internal.ApmNetworkInstrumentation.Companion.ZE
 import com.datadog.android.trace.internal.ParentContextSource
 import com.datadog.android.trace.internal._TraceInternalProxy
 import com.datadog.android.trace.internal._TraceInternalProxy.propagationHelper
+import com.datadog.android.trace.internal.domain.event.FORCE_DROP_SPAN
 import java.util.Locale
 
 internal val FeatureSdkCore?.isRumEnabled: Boolean
@@ -71,8 +72,12 @@ internal fun DatadogSpan.sample(
     }
 }
 
-internal fun DatadogSpan.finishRumAware(isSampled: Boolean, canSendSpan: Boolean) {
-    if (canSendSpan && isSampled) {
+internal fun DatadogSpan.finishRumAware(isSampled: Boolean, canSendSpan: Boolean, isSDKBacked: Boolean) {
+    if (canSendSpan && (isSDKBacked || isSampled)) {
+        if (isSDKBacked && !isSampled) {
+            // SDK-backed tracer: tag the span so CoreTraceWriter suppresses it instead of calling drop()
+            setTag(FORCE_DROP_SPAN, true)
+        }
         finish()
     } else {
         drop()

features/dd-sdk-android-trace/src/main/kotlin/com/datadog/android/trace/internal/net/RequestTracingState.kt

@@ -23,13 +23,15 @@ import com.datadog.android.trace.api.span.DatadogSpan
  * @property isSampled whether the request trace was sampled.
  * @property span the trace span created for this request, or null if not sampled or tracing is disabled.
  * @property sampleRate the sample rate used for the sampling decision.
+ * @property isSDKBacked whether the tracer is the SDK's built-in implementation
  */
 @InternalApi
 data class RequestTracingState(
     val requestInfoBuilder: HttpRequestInfoBuilder,
     val isSampled: Boolean = false,
     val span: DatadogSpan? = null,
-    val sampleRate: Float? = null
+    val sampleRate: Float? = null,
+    val isSDKBacked: Boolean
 ) {
 
     /**

features/dd-sdk-android-trace/src/test/kotlin/com/datadog/android/trace/internal/TraceInternalProxyTest.kt

@@ -8,6 +8,7 @@ package com.datadog.android.trace.internal
 
 import com.datadog.android.trace.ApmNetworkInstrumentationConfiguration
 import com.datadog.android.trace.TracingHeaderType
+import com.datadog.android.trace.api.tracer.DatadogTracer
 import com.datadog.android.utils.forge.Configurator
 import fr.xgouchet.elmyr.Forge
 import fr.xgouchet.elmyr.annotation.StringForgery
@@ -18,8 +19,10 @@ import org.junit.jupiter.api.BeforeEach
 import org.junit.jupiter.api.Test
 import org.junit.jupiter.api.extension.ExtendWith
 import org.junit.jupiter.api.extension.Extensions
+import org.mockito.Mock
 import org.mockito.junit.jupiter.MockitoExtension
 import org.mockito.junit.jupiter.MockitoSettings
+import org.mockito.kotlin.mock
 import org.mockito.quality.Strictness
 
 @Extensions(
@@ -30,6 +33,9 @@ import org.mockito.quality.Strictness
 @ForgeConfiguration(Configurator::class)
 internal class TraceInternalProxyTest {
 
+    @Mock
+    lateinit var mockCustomTracer: DatadogTracer
+
     private lateinit var fakeTracedHosts: Map<String, Set<TracingHeaderType>>
 
     @StringForgery
@@ -83,4 +89,16 @@ internal class TraceInternalProxyTest {
         assertThat(result.sdkInstanceName).isEqualTo(fakeSdkInstanceName)
         assertThat(result.traceOrigin).isEqualTo(fakeTraceOrigin)
     }
+
+    @Test
+    fun `M return false W isSDKBackedTracer() {custom tracer}`() {
+        assertThat(_TraceInternalProxy.isSDKBackedTracer(mockCustomTracer)).isFalse()
+    }
+
+    @Test
+    fun `M return true W isSDKBackedTracer() {SDK DatadogTracerAdapter}`() {
+        val sdkTracer: DatadogTracer = mock<DatadogTracerAdapter>()
+
+        assertThat(_TraceInternalProxy.isSDKBackedTracer(sdkTracer)).isTrue()
+    }
 }

features/dd-sdk-android-trace/src/test/kotlin/com/datadog/android/trace/internal/data/CoreTraceWriterTest.kt

@@ -19,6 +19,7 @@ import com.datadog.android.event.EventMapper
 import com.datadog.android.log.LogAttributes
 import com.datadog.android.trace.internal.RumContextPropagator
 import com.datadog.android.trace.internal.domain.event.ContextAwareMapper
+import com.datadog.android.trace.internal.domain.event.FORCE_DROP_SPAN
 import com.datadog.android.trace.internal.storage.ContextAwareSerializer
 import com.datadog.android.trace.model.SpanEvent
 import com.datadog.android.trace.utils.RumContextTestsUtils.RUM_CONTEXT_ACTION_ID
@@ -319,6 +320,24 @@ internal class CoreTraceWriterTest {
         }
     }
 
+    @Test
+    fun `M not write spans with force drop tag W write()`(forge: Forge) {
+        // GIVEN
+        val ddSpans = createNonEmptyDdSpans(
+            forge = forge,
+            includeDropSamplingPriority = false
+        )
+        ddSpans.forEach { whenever(it.getTag(FORCE_DROP_SPAN)) doReturn true }
+
+        // WHEN
+        testedWriter.write(ddSpans)
+
+        // THEN
+        verifyNoInteractions(mockEventBatchWriter)
+
+        ddSpans.forEach { it.finish() }
+    }
+
     @Test
     fun `M not write non-mapped spans W write()`(forge: Forge) {
         // GIVEN