Fix for #34632 to prevent race condition on adding headers if already added

Author: Paul Carter-Brown

independent-projects/resteasy-reactive/server/vertx/src/main/java/org/jboss/resteasy/reactive/server/vertx/ResteasyReactiveOutputStream.java

@@ -226,10 +226,12 @@ private void prepareWrite(ByteBuf buffer, boolean finished) throws IOException {
         if (!committed) {
             committed = true;
             if (finished) {
-                if (buffer == null) {
-                    context.serverResponse().setResponseHeader(HttpHeaderNames.CONTENT_LENGTH, "0");
-                } else {
-                    context.serverResponse().setResponseHeader(HttpHeaderNames.CONTENT_LENGTH, "" + buffer.readableBytes());
+                if (!context.serverResponse().headWritten()) {
+                    if (buffer == null) {
+                        context.serverResponse().setResponseHeader(HttpHeaderNames.CONTENT_LENGTH, "0");
+                    } else {
+                        context.serverResponse().setResponseHeader(HttpHeaderNames.CONTENT_LENGTH, "" + buffer.readableBytes());
+                    }
                 }
             } else {
                 var contentLengthSet = contentLengthSet();