Avoid duplicate headers when injecting on java.net http client (#10315)

* Avoid duplicate headers when injecting on java.net http client

* Use TreeMap

Author: amarziali

dd-java-agent/instrumentation/java/java-net/java-net-11.0/src/main/java11/datadog/trace/instrumentation/httpclient/HeadersAdvice.java

@@ -4,17 +4,21 @@
 import static datadog.trace.instrumentation.httpclient.HttpHeadersInjectAdapter.KEEP;
 import static datadog.trace.instrumentation.httpclient.HttpHeadersInjectAdapter.SETTER;
 import static datadog.trace.instrumentation.httpclient.JavaNetClientDecorator.DECORATE;
+import static java.lang.String.CASE_INSENSITIVE_ORDER;
 
 import java.net.http.HttpHeaders;
-import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
+import java.util.TreeMap;
 import net.bytebuddy.asm.Advice;
 
 public class HeadersAdvice {
   @Advice.OnMethodExit(onThrowable = Throwable.class, suppress = Throwable.class)
   public static void methodExit(@Advice.Return(readOnly = false) HttpHeaders headers) {
-    final Map<String, List<String>> headerMap = new HashMap<>(headers.map());
+    // Note: adding duplicate keys will throw an IllegalArgumentException so we need to dedupe
+    // case insensitively
+    final Map<String, List<String>> headerMap = new TreeMap<>(CASE_INSENSITIVE_ORDER);
+    headerMap.putAll(headers.map());
     DECORATE.injectContext(getCurrentContext(), headerMap, SETTER);
     headers = HttpHeaders.of(headerMap, KEEP);
   }

dd-java-agent/instrumentation/java/java-net/java-net-11.0/src/test/groovy/datadog/trace/instrumentation/httpclient/JavaHttpClientTest.groovy

@@ -1,8 +1,9 @@
 package datadog.trace.instrumentation.httpclient
 
-import datadog.trace.agent.test.base.HttpClientTest
+
 import static datadog.trace.instrumentation.httpclient.JavaNetClientDecorator.DECORATE
 
+import datadog.trace.agent.test.base.HttpClientTest
 import java.net.http.HttpClient
 import java.net.http.HttpRequest
 import java.net.http.HttpResponse
@@ -42,6 +43,22 @@ abstract class JavaHttpClientTest extends HttpClientTest {
   boolean testRedirects() {
     false
   }
+
+  def 'should not inject duplicate headers'() {
+    when:
+    def status = doRequest("GET", server.address.resolve("/success"),
+      // our codec inject names all lowercase
+      ["X-Datadog-Trace-ID": "0"])
+
+    then:
+    status == 200
+    assertTraces(2) {
+      trace(size(1)) {
+        clientSpan(it, null)
+      }
+      server.distributedRequestTrace(it, trace(0).last())
+    }
+  }
 }
 
 class JavaHttpClientV0Test extends JavaHttpClientTest {