Fix broken span ordering

instrumentation/reactor/reactor-netty/reactor-netty-1.0/javaagent/src/main/java/io/opentelemetry/javaagent/instrumentation/reactornetty/v1_0/HttpResponseReceiverInstrumenter.java

@@ -24,8 +24,7 @@ public final class HttpResponseReceiverInstrumenter {
   // this method adds several stateful listeners that execute the instrumenter lifecycle during HTTP
   // request processing
   // it should be used just before one of the response*() methods is called - after this point the
-  // HTTP
-  // request is no longer modifiable by the user
+  // HTTP request is no longer modifiable by the user
   @Nullable
   public static HttpClient.ResponseReceiver<?> instrument(HttpClient.ResponseReceiver<?> receiver) {
     // receiver should always be an HttpClientFinalizer, which both extends HttpClient and

instrumentation/reactor/reactor-netty/reactor-netty-1.0/javaagent/src/main/java/io/opentelemetry/javaagent/instrumentation/reactornetty/v1_0/InstrumentationContexts.java

@@ -9,17 +9,23 @@
 
 import io.opentelemetry.context.Context;
 import io.opentelemetry.instrumentation.api.instrumenter.http.HttpClientResend;
+import java.util.Queue;
+import java.util.concurrent.ArrayBlockingQueue;
+import java.util.logging.Level;
+import java.util.logging.Logger;
 import javax.annotation.Nullable;
 import reactor.netty.http.client.HttpClientRequest;
 import reactor.netty.http.client.HttpClientResponse;
 
 final class InstrumentationContexts {
 
+  static final Logger logger = Logger.getLogger(InstrumentationContexts.class.getName());
+
   private volatile Context parentContext;
   // on retries, reactor-netty starts the next resend attempt before it ends the previous one (i.e.
   // it calls the callback functions in that order); thus for a short moment there can be 2
   // coexisting HTTP client spans
-  private final ConstantSizeStack<RequestAndContext> clientContexts = new ConstantSizeStack<>(2);
+  private final Queue<RequestAndContext> clientContexts = new ArrayBlockingQueue<>(2, true);
 
   void initialize(Context parentContext) {
     this.parentContext = HttpClientResend.initialize(parentContext);
@@ -41,13 +47,19 @@ Context startClientSpan(HttpClientRequest request) {
     Context context = null;
     if (instrumenter().shouldStart(parentContext, request)) {
       context = instrumenter().start(parentContext, request);
-      clientContexts.push(new RequestAndContext(request, context));
+      if (!clientContexts.offer(new RequestAndContext(request, context))) {
+        // should not ever happen in reality
+        String message =
+            "Could not instrument HTTP client request; not enough space in the request queue";
+        logger.log(Level.FINE, message);
+        instrumenter().end(context, request, null, new IllegalStateException(message));
+      }
     }
     return context;
   }
 
   void endClientSpan(@Nullable HttpClientResponse response, @Nullable Throwable error) {
-    RequestAndContext requestAndContext = clientContexts.pop();
+    RequestAndContext requestAndContext = clientContexts.poll();
     if (requestAndContext != null) {
       instrumenter().end(requestAndContext.context, requestAndContext.request, response, error);
     }