Translate a sampling.priority tag set to a positive value to forceKeep, overriding any locked sampling priority (#11621)

Translate a sampling.priority tag set to a positive value to forceKeep, overriding any locked sampling priority

Co-authored-by: devflow.devflow-routing-intake <devflow.devflow-routing-intake@kubernetes.us1.ddbuild.io>

Author: mcculls

dd-trace-core/src/main/java/datadog/trace/core/taginterceptor/TagInterceptor.java

@@ -5,7 +5,6 @@
 import static datadog.trace.api.DDTags.ORIGIN_KEY;
 import static datadog.trace.api.DDTags.SPAN_TYPE;
 import static datadog.trace.api.sampling.PrioritySampling.USER_DROP;
-import static datadog.trace.api.sampling.PrioritySampling.USER_KEEP;
 import static datadog.trace.bootstrap.instrumentation.api.InstrumentationTags.SERVLET_CONTEXT;
 import static datadog.trace.bootstrap.instrumentation.api.ServiceNameSources.SPLIT_BY_SERVLET_CONTEXT;
 import static datadog.trace.bootstrap.instrumentation.api.ServiceNameSources.SPLIT_BY_TAGS;
@@ -327,8 +326,11 @@ private boolean interceptSamplingPriority(DDSpanContext span, Object value) {
     if (ruleFlags.isEnabled(FORCE_SAMPLING_PRIORITY)) {
       Number samplingPriority = getOrTryParse(value);
       if (null != samplingPriority) {
-        span.setSamplingPriority(
-            samplingPriority.intValue() > 0 ? USER_KEEP : USER_DROP, SamplingMechanism.MANUAL);
+        if (samplingPriority.intValue() > 0) {
+          span.forceKeep(SamplingMechanism.MANUAL);
+        } else {
+          span.setSamplingPriority(USER_DROP, SamplingMechanism.MANUAL);
+        }
       }
       return true;
     }

dd-trace-core/src/test/java/datadog/trace/core/taginterceptor/TagInterceptorTest.java

@@ -1,6 +1,7 @@
 package datadog.trace.core.taginterceptor;
 
 import static datadog.trace.api.DDTags.ANALYTICS_SAMPLE_RATE;
+import static datadog.trace.api.TracePropagationStyle.DATADOG;
 import static datadog.trace.api.config.TracerConfig.SPLIT_BY_TAGS;
 import static datadog.trace.junit.utils.config.WithConfigExtension.injectSysConfig;
 import static org.junit.jupiter.api.Assertions.assertEquals;
@@ -16,11 +17,14 @@
 
 import datadog.trace.api.DDSpanTypes;
 import datadog.trace.api.DDTags;
+import datadog.trace.api.DDTraceId;
 import datadog.trace.api.ProductTraceSource;
 import datadog.trace.api.remoteconfig.ServiceNameCollector;
 import datadog.trace.api.remoteconfig.ServiceNameCollectorTestBridge;
 import datadog.trace.api.sampling.PrioritySampling;
+import datadog.trace.api.sampling.SamplingMechanism;
 import datadog.trace.bootstrap.instrumentation.api.AgentSpan;
+import datadog.trace.bootstrap.instrumentation.api.AgentSpanContext;
 import datadog.trace.bootstrap.instrumentation.api.InstrumentationTags;
 import datadog.trace.bootstrap.instrumentation.api.Tags;
 import datadog.trace.common.sampling.AllSampler;
@@ -29,7 +33,10 @@
 import datadog.trace.core.CoreSpan;
 import datadog.trace.core.CoreTracer;
 import datadog.trace.core.DDCoreJavaSpecification;
+import datadog.trace.core.DDSpan;
 import datadog.trace.core.DDSpanContext;
+import datadog.trace.core.propagation.ExtractedContext;
+import datadog.trace.core.propagation.PropagationTags;
 import datadog.trace.junit.utils.config.WithConfig;
 import datadog.trace.junit.utils.tabletest.ConfigDefaultsConverter;
 import datadog.trace.junit.utils.tabletest.DDTagsConverter;
@@ -415,6 +422,55 @@ void setPrioritySamplingViaTag(String scenario, String tag, Object value, Intege
     assertEquals(expected, span.getSamplingPriority());
   }
 
+  @Test
+  void samplingPriorityPositiveTagOverridesLockedPriority() {
+    CoreTracer tracer = tracerBuilder().writer(new ListWriter()).build();
+    AgentSpan span = tracer.buildSpan("datadog", "test").start();
+
+    // Simulate upstream propagation of x-datadog-sampling-priority: -1 (USER_DROP)
+    span.setSamplingPriority(PrioritySampling.USER_DROP, SamplingMechanism.UNKNOWN);
+    assertEquals((int) PrioritySampling.USER_DROP, span.getSamplingPriority());
+
+    // positive sampling.priority overrides the propagated locked priority
+    span.setTag(Tags.SAMPLING_PRIORITY, 2);
+    assertEquals((int) PrioritySampling.USER_KEEP, span.getSamplingPriority());
+  }
+
+  @Test
+  void samplingPriorityNonPositiveTagDoesNotOverrideLockedPriority() {
+    CoreTracer tracer = tracerBuilder().writer(new ListWriter()).build();
+    AgentSpan span = tracer.buildSpan("datadog", "test").start();
+
+    // Simulate upstream propagation of x-datadog-sampling-priority: 2 (USER_KEEP)
+    span.setSamplingPriority(PrioritySampling.USER_KEEP, SamplingMechanism.UNKNOWN);
+    assertEquals((int) PrioritySampling.USER_KEEP, span.getSamplingPriority());
+
+    // non-positive sampling.priority respects the propagated locked priority
+    span.setTag(Tags.SAMPLING_PRIORITY, 0);
+    assertEquals((int) PrioritySampling.USER_KEEP, span.getSamplingPriority());
+  }
+
+  @Test
+  void samplingPriorityPositiveTagOverridesDecisionMakerFromUpstreamPropagation() {
+    CoreTracer tracer = tracerBuilder().writer(new ListWriter()).build();
+    // Upstream service dropped with LOCAL_USER_RULE and propagated _dd.p.dm=-3
+    PropagationTags propagationTags =
+        PropagationTags.factory()
+            .fromHeaderValue(PropagationTags.HeaderType.DATADOG, "_dd.p.dm=-3");
+    AgentSpanContext extracted =
+        new ExtractedContext(
+            DDTraceId.from(123), 456L, PrioritySampling.USER_DROP, null, propagationTags, DATADOG);
+    DDSpan span = (DDSpan) tracer.buildSpan("datadog", "test").asChildOf(extracted).start();
+
+    // positive sampling.priority overrides locked priority and sets _dd.p.dm to MANUAL
+    span.setTag(Tags.SAMPLING_PRIORITY, 2);
+
+    assertEquals((int) PrioritySampling.USER_KEEP, span.getSamplingPriority());
+    assertEquals(
+        "_dd.p.dm=-4",
+        span.context().getPropagationTags().headerValue(PropagationTags.HeaderType.DATADOG));
+  }
+
   @ParameterizedTest
   @ValueSource(booleans = {true, false})
   void setErrorFlagWhenErrorTagReported(boolean error) throws Exception {