From 40fc417db7c49b65db3a6ce247497cb6b063c427 Mon Sep 17 00:00:00 2001
From: Douglas Q Hawkins <douglas.hawkins@datadoghq.com>
Date: Thu, 12 Mar 2026 11:11:04 -0400
Subject: [PATCH] Avoid creation of empty CopyOnWriteArrayList for span links

This change shares an "EMPTY" list between span when there are no span links.

This avoids the construction of a CopyOnWriteArrayList, empty Object[], and internal lock Object for the common case where there are no links.

This elimination of the CopyOnWriteArrayList in the empty case, reduced allocation by 5% and GC time by 7% in a span creation stress test.
---
 .../main/java/datadog/trace/core/DDSpan.java  | 40 +++++++++++++++++--
 1 file changed, 36 insertions(+), 4 deletions(-)

diff --git a/dd-trace-core/src/main/java/datadog/trace/core/DDSpan.java b/dd-trace-core/src/main/java/datadog/trace/core/DDSpan.java
index d460d5ea1bb..90ea74feebb 100644
--- a/dd-trace-core/src/main/java/datadog/trace/core/DDSpan.java
+++ b/dd-trace-core/src/main/java/datadog/trace/core/DDSpan.java
@@ -116,7 +116,8 @@ static DDSpan create(
    */
   private volatile int longRunningVersion = 0;
 
-  protected final List<AgentSpanLink> links;
+  private static final List<AgentSpanLink> EMPTY = Collections.emptyList();
+  protected volatile List<AgentSpanLink> links;
 
   /**
    * Spans should be constructed using the builder, not by calling the constructor directly.
@@ -144,7 +145,7 @@ private DDSpan(
       context.getTraceCollector().touch(); // external clock: explicitly update lastReferenced
     }
 
-    this.links = links == null ? new CopyOnWriteArrayList<>() : new CopyOnWriteArrayList<>(links);
+    this.links = links == null || links.isEmpty() ? EMPTY : new CopyOnWriteArrayList<>(links);
   }
 
   public boolean isFinished() {
@@ -879,8 +880,39 @@ public TraceConfig traceConfig() {
 
   @Override
   public void addLink(AgentSpanLink link) {
-    if (link != null) {
-      this.links.add(link);
+    if (link == null) {
+      return;
+    }
+
+    // If links are initially null / empty, then the shared placeholder List EMPTY is used.
+    // Bacause EMPTY is shared, EMPTY is safe for reading, but not for writing.
+    // On write - if links is the EMPTY placeholder, then need to create a CopyOnWriteArrayList
+    // owned by this DDSpan
+
+    // Creation of the CopyOnWriteArrayList is done via double checking locking using volatile &
+    // synchronized
+
+    // If before or inside the synchronized block, links no longer points to EMPTY,
+    // then this thread or another thread has already handled the list construction,
+    // 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
+
+    List<AgentSpanLink> links = this.links;
+    if (links != EMPTY) {
+      links.add(link);
+      return;
+    }
+
+    synchronized (this) {
+      links = this.links;
+      if (links != EMPTY) {
+        links.add(link);
+      } else {
+        this.links = new CopyOnWriteArrayList<>(Collections.singletonList(link));
+      }
     }
   }