Improve random directory generation:

* make sure it does not exist
* avoid negative numbers

extensions/vertx-core/runtime/src/main/java/io/quarkus/vertx/core/runtime/VertxCoreRecorder.java

@@ -246,8 +246,7 @@ private static VertxOptions convertToVertxOptions(VertxConfiguration conf, Vertx
                 }
             }
 
-            long random = UUID.randomUUID().getMostSignificantBits();
-            File cache = new File(tmp, Long.toString(random));
+            File cache = getRandomDirectory(tmp);
             LOGGER.debugf("Vert.x Cache configured to: %s", cache.getAbsolutePath());
             fileCacheDir = cache.getAbsolutePath();
             if (shutdown != null) {
@@ -291,6 +290,16 @@ public void run() {
         return options;
     }
 
+    private static File getRandomDirectory(File tmp) {
+        long random = Math.abs(UUID.randomUUID().getMostSignificantBits());
+        File cache = new File(tmp, Long.toString(random));
+        if (cache.isDirectory()) {
+            // Do not reuse an existing directory.
+            return getRandomDirectory(tmp);
+        }
+        return cache;
+    }
+
     private static int calculateDefaultIOThreads() {
         //we only allow one event loop per 10mb of ram at the most
         //its hard to say what this number should be, but it is also obvious