Make traceparent header creation thread-safe (#2747)

Author: eyalkoren

CHANGELOG.asciidoc

@@ -51,6 +51,7 @@ version of classes that are available also in the parent (bootstrap) class loade
 * Avoid warning when log correlation option is provided in remote config - {pull}2765[#2765]
 * Update `async-profiler` to 1.8.8 to avoid missing symbol spam - {pull}2775[#2775]
 * Fix container ID discovery for containers managed through AWS Fargate - {pull}2772[#2772]
+* Make `traceparent` header computation thread-safe - {pull}2747[#2747]
 
 [[release-notes-1.x]]
 === Java Agent version 1.x

apm-agent-core/src/main/java/co/elastic/apm/agent/impl/transaction/TraceContext.java

@@ -586,10 +586,14 @@ <C> boolean propagateTraceContext(C carrier, BinaryHeaderSetter<C> headerSetter)
      */
     StringBuilder getOutgoingTraceParentTextHeader() {
         if (outgoingTextHeader.length() == 0) {
-            // for unsampled traces, propagate the ID of the transaction in calls to downstream services
-            // such that the parentID of those transactions point to a transaction that exists
-            // remember that we do report unsampled transactions
-            fillTraceParentHeader(outgoingTextHeader, isSampled() ? id : transactionId);
+            synchronized (outgoingTextHeader) {
+                if (outgoingTextHeader.length() == 0) {
+                    // for unsampled traces, propagate the ID of the transaction in calls to downstream services
+                    // such that the parentID of those transactions point to a transaction that exists
+                    // remember that we do report unsampled transactions
+                    fillTraceParentHeader(outgoingTextHeader, isSampled() ? id : transactionId);
+                }
+            }
         }
         return outgoingTextHeader;
     }