Skip Datadog tags a trace context merge never reads

dd-trace-core/src/main/java/datadog/trace/core/CoreTracer.java

@@ -1949,7 +1949,7 @@ protected static final AgentSpan startSpan(
       if (parentSpanContext != null && parentSpanContext.isRemote()) {
         // Preserve AppSec/IAST request context before dropping the remote parent: when
         // extract=IGNORE or RESTART the TagContext parent is nulled before buildSpanContext
-        // can copy requestContextDataAppSec/Iast into DDSpanContext.
+        // can copy requestContextDataAppSec/Iast onto the trace collector.
         if (parentSpanContext instanceof TagContext) {
           TagContext tc = (TagContext) parentSpanContext;
           if (builderRequestContextDataAppSec == null) {

dd-trace-core/src/main/java/datadog/trace/core/propagation/ptags/PTagsCodec.java

@@ -361,8 +361,20 @@ protected int appendSuffix(StringBuilder sb, PTags ptags, int size, SamplingStat
 
   protected abstract boolean isEmpty(StringBuilder sb, int size);
 
+  /**
+   * Whether values of {@code tagKey} are validated. {@link #validateTagValue} checks only these, so
+   * a key missing here goes unvalidated everywhere rather than only where this is consulted.
+   */
+  static boolean hasValidatedValues(TagKey tagKey) {
+    return tagKey.equals(DECISION_MAKER_TAG)
+        || tagKey.equals(TRACE_ID_TAG)
+        || tagKey.equals(TRACE_SOURCE_TAG);
+  }
+
   protected static boolean validateTagValue(TagKey tagKey, TagValue tagValue) {
-    if (tagKey.equals(DECISION_MAKER_TAG) && !validateDecisionMakerTag(tagValue)) {
+    if (!hasValidatedValues(tagKey)) {
+      return true;
+    } else if (tagKey.equals(DECISION_MAKER_TAG) && !validateDecisionMakerTag(tagValue)) {
       return false;
     } else if (tagKey.equals(TRACE_ID_TAG) && !validateTraceId(tagValue)) {
       return false;

dd-trace-core/src/main/java/datadog/trace/core/propagation/ptags/PTagsFactory.java

@@ -955,7 +955,8 @@ public void updateW3CTracestate(String tracestate) {
     @Override
     public void updateW3CTracestateFrom(W3CTracestate source, int overridingPriority) {
       if (source.rawOtelTraceState == null) {
-        setW3CTracestate(source.tracestate, W3CPTagsCodec.extractOtelTraceState(source.tracestate));
+        // the parse already looked for an otel member, so there is none
+        setW3CTracestate(source.tracestate, null);
         return;
       }
       // Keep the otel member unparsed; its consistency checks wait for the first read

dd-trace-core/src/main/java/datadog/trace/core/propagation/ptags/W3CPTagsCodec.java

@@ -199,6 +199,11 @@ private static <R> R parse(String value, Result<R> result) {
       } else {
         TagKey tagKey = TagKey.from(Encoding.W3C, value, tagPos, tagKeyEndsAt);
         if (tagKey != null) {
+          if (!result.keepsDatadogTags() && !hasValidatedValues(tagKey)) {
+            // Its value is never read, and it can't be invalid, so it changes nothing
+            tagPos = nextTagPos;
+            continue;
+          }
           TagValue tagValue = tagValue(Encoding.W3C, tagKey, value, tagValuePos, tagValueEndsAt);
           if (!tagKey.equals(UPSTREAM_SERVICES_DEPRECATED_TAG)) {
             if (!validateTagValue(tagKey, tagValue)) {
@@ -1027,6 +1032,12 @@ private static int memberContributionSize(
 
   /** What a parse of a tracestate header turns into, at each of its outcomes. */
   private interface Result<R> {
+    /**
+     * Whether the {@code dd} member's tags are kept. If not, they're only checked for validity,
+     * which decides between {@link #parsed} and {@link #withoutDatadogTags}.
+     */
+    boolean keepsDatadogTags();
+
     /** The header is malformed, so nothing from it is kept. */
     R rejected();
 
@@ -1068,6 +1079,11 @@ private static final class PTagsResult implements Result<PropagationTags> {
       this.tagsFactory = tagsFactory;
     }
 
+    @Override
+    public boolean keepsDatadogTags() {
+      return true;
+    }
+
     @Override
     public PropagationTags rejected() {
       return tagsFactory.empty();
@@ -1137,6 +1153,11 @@ public PropagationTags parsed(
 
   private static final Result<W3CTracestate> MERGE_RESULT =
       new Result<W3CTracestate>() {
+        @Override
+        public boolean keepsDatadogTags() {
+          return false;
+        }
+
         @Override
         public W3CTracestate rejected() {
           return W3CTracestate.NONE;