Reuse trace context parsing on the span link path

When a trace context's trace id doesn't match, build it from the
interpreter the merge already ran instead of interpreting its headers
again, and reuse the propagation tags parse result rather than
allocating one per tracestate. Also drop a duplicate empty check and a
pass-through overload.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: dougqh

dd-trace-core/src/main/java/datadog/trace/core/propagation/HttpCodec.java

@@ -223,11 +223,13 @@ public <C> TagContext extract(
       ExtractionCache<C> extractionCache = new ExtractionCache<>(carrier, getter);
 
       for (final Extractor extractor : this.extractors) {
+        TagContext extracted;
         if (context != null && extractor instanceof W3CHttpCodec.TraceContextExtractor) {
           // Merging a trace context only needs a few of its parts, so skip building it whole
+          W3CHttpCodec.TraceContextExtractor traceContextExtractor =
+              (W3CHttpCodec.TraceContextExtractor) extractor;
           W3CHttpCodec.TraceContextMerge merge =
-              ((W3CHttpCodec.TraceContextExtractor) extractor)
-                  .extractForMerge(extractionCache, extractionCache);
+              traceContextExtractor.extractForMerge(extractionCache, extractionCache);
           if (merge == null) {
             continue;
           }
@@ -236,8 +238,10 @@ public <C> TagContext extract(
             continue;
           }
           // A mismatched trace context becomes a span link, which needs all of it
+          extracted = traceContextExtractor.completeExtraction();
+        } else {
+          extracted = extractor.extract(extractionCache, extractionCache);
         }
-        TagContext extracted = extractor.extract(extractionCache, extractionCache);
         // Check if context is valid
         if (extracted instanceof ExtractedContext) {
           ExtractedContext extractedContext = (ExtractedContext) extracted;

dd-trace-core/src/main/java/datadog/trace/core/propagation/TagContextExtractor.java

@@ -21,6 +21,11 @@ public <C> TagContext extract(final C carrier, final AgentPropagation.ContextVis
     return interpret(carrier, getter).build();
   }
 
+  /** This thread's interpreter, as the last {@link #interpret} left it. */
+  protected final ContextInterpreter interpreter() {
+    return this.ctxInterpreter.get();
+  }
+
   /** Runs this thread's interpreter over the carrier, ready to build. */
   protected final <C> ContextInterpreter interpret(
       final C carrier, final AgentPropagation.ContextVisitor<C> getter) {

dd-trace-core/src/main/java/datadog/trace/core/propagation/W3CHttpCodec.java

@@ -164,6 +164,11 @@ <C> TraceContextMerge extractForMerge(
         final C carrier, final AgentPropagation.ContextVisitor<C> getter) {
       return ((W3CContextInterpreter) interpret(carrier, getter)).buildForMerge();
     }
+
+    /** Builds the whole context that {@link #extractForMerge} last returned the merge parts of. */
+    TagContext completeExtraction() {
+      return interpreter().build();
+    }
   }
 
   private static class W3CContextInterpreter extends ContextInterpreter {

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

@@ -33,9 +33,16 @@ public class W3CPTagsCodec extends PTagsCodec {
   private static final int MAX_ALLOWED_CHAR = 126;
   @VisibleForTesting public static final int MAX_MEMBER_COUNT = 32;
 
+  /** Reused across parses; the codec belongs to one factory, so this is set once. */
+  private volatile PTagsResult ptagsResult;
+
   @Override
   PropagationTags fromHeaderValue(PTagsFactory tagsFactory, String value) {
-    return parse(value, new PTagsResult(tagsFactory));
+    PTagsResult result = ptagsResult;
+    if (result == null || result.tagsFactory != tagsFactory) {
+      ptagsResult = result = new PTagsResult(tagsFactory);
+    }
+    return parse(value, result);
   }
 
   /**
@@ -1152,12 +1159,7 @@ public W3CTracestate parsed(
 
   /** Creates tags that preserve unmanaged W3C tracestate members without sampling state. */
   static W3CPTags empty(PTagsFactory factory, String original) {
-    return empty(factory, original, null);
-  }
-
-  private static W3CPTags empty(
-      PTagsFactory factory, String original, OtelTraceState otelTraceState) {
-    return empty(factory, original, 0, -1, -1, otelTraceState);
+    return empty(factory, original, 0, -1, -1, null);
   }
 
   private static W3CPTags empty(

dd-trace-core/src/main/java/datadog/trace/core/propagation/ptags/W3CTracestate.java

@@ -31,9 +31,6 @@ public final class W3CTracestate {
 
   /** Parses a {@code tracestate} header value, which may be null. */
   public static W3CTracestate parse(String value) {
-    if (value == null || value.isEmpty()) {
-      return NONE;
-    }
     return W3CPTagsCodec.parseForMerge(value);
   }