Restricting API access

Removed SpanLinkAccessor interface
SpanLinkAccessor was intended to limit how the span is modified in DDSpanContext#processTagsAndBaggage, but the trade-off was introducing a public getLinks method on DDSpan.

Reduced visibility of processTagsAndBaggage methods on DDSpanContext
Since processTagsAndBaggage is really just a helper routine of DDSpan.processTagsAndBaggage and the usage is a bit sensitive, I decided to reduce the visibility

Author: dougqh

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

@@ -51,7 +51,7 @@
  * <p>Spans are created by the {@link CoreTracer#buildSpan}. This implementation adds some features
  * according to the DD agent.
  */
-public class DDSpan implements AgentSpan, CoreSpan<DDSpan>, SpanLinkAccessor, AttachableWrapper {
+public class DDSpan implements AgentSpan, CoreSpan<DDSpan>, AttachableWrapper {
   private static final Logger log = LoggerFactory.getLogger(DDSpan.class);
 
   static DDSpan create(
@@ -884,8 +884,7 @@ public TraceConfig traceConfig() {
     return context.getTraceCollector().getTraceConfig();
   }
 
-  @Override
-  public List<? extends AgentSpanLink> getLinks() {
+  List<? extends AgentSpanLink> getLinks() {
     return this.links;
   }
 
@@ -908,8 +907,7 @@ public void addLink(AgentSpanLink link) {
     // so just add to the list
 
     // If links still points to EMPTY inside the synchronized block, then construct a new
-    // CopyOnWriteArrayList
-    // containing the newly added link
+    // CopyOnWriteArrayList containing the newly added link
 
     List<AgentSpanLink> links = this.links;
     if (links != EMPTY) {

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

@@ -24,6 +24,7 @@
 import datadog.trace.api.sampling.PrioritySampling;
 import datadog.trace.api.sampling.SamplingMechanism;
 import datadog.trace.bootstrap.instrumentation.api.AgentSpanContext;
+import datadog.trace.bootstrap.instrumentation.api.AppendableSpanLinks;
 import datadog.trace.bootstrap.instrumentation.api.Baggage;
 import datadog.trace.bootstrap.instrumentation.api.ProfilerContext;
 import datadog.trace.bootstrap.instrumentation.api.ProfilingContextIntegration;
@@ -1086,19 +1087,23 @@ public <T> void setMetaStruct(final String field, final T value) {
     }
   }
 
-  public void earlyProcessTags(SpanLinkAccessor links) {
+  void earlyProcessTags(AppendableSpanLinks links) {
     synchronized (unsafeTags) {
       TagsPostProcessorFactory.eagerProcessor().processTags(unsafeTags, this, links);
     }
   }
 
-  public void processTagsAndBaggage(
-      final MetadataConsumer consumer, int longRunningVersion, SpanLinkAccessor links) {
+  void processTagsAndBaggage(
+      final MetadataConsumer consumer, int longRunningVersion, DDSpan restrictedSpan) {
+	// NOTE: The span is passed for the sole purpose of allowing updating & reading of the span links
+	// This is a compromise to avoid...
+	// - creating an extra wrapper object that would create significant allocation
+	// - implementing an interface to read the spans that require making the read method public
     synchronized (unsafeTags) {
       // Tags
-      TagsPostProcessorFactory.lazyProcessor().processTags(unsafeTags, this, links);
+      TagsPostProcessorFactory.lazyProcessor().processTags(unsafeTags, this, restrictedSpan);
 
-      String linksTag = DDSpanLink.toTag(links.getLinks());
+      String linksTag = DDSpanLink.toTag(restrictedSpan.getLinks());
       if (linksTag != null) {
         unsafeTags.put(SPAN_LINKS, linksTag);
       }