Fix 0RTT test when offloading tasks (java-native-access#548)

Motivation:

We need to ensure the session was added to the cache before creating the
second connection as otherwise the test will timeout due not re-using
sessions.

Modifications:

- Add extra method to QuicClientSessionCache that we can use to check if
a session was added
- Spin until the session was added to the cache and only after that try
to connect the second time

Result:

No more timeouts due a race when testing for 0RTT. Fixes
netty/netty-incubator-codec-quic#544

codec-classes-quic/src/main/java/io/netty/incubator/codec/quic/QuicClientSessionCache.java

@@ -71,6 +71,17 @@ void saveSession(String host, int port, long creationTime, long timeout, byte[]
         }
     }
 
+    // Only used for testing.
+    boolean hasSession(String host, int port) {
+        HostPort hostPort = keyFor(host, port);
+        if (hostPort != null) {
+            synchronized (sessions) {
+                return sessions.containsKey(hostPort);
+            }
+        }
+        return false;
+    }
+
     byte[] getSession(String host, int port) {
         HostPort hostPort = keyFor(host, port);
         if (hostPort != null) {

codec-native-quic/src/test/java/io/netty/incubator/codec/quic/QuicChannelConnectTest.java

@@ -575,15 +575,6 @@ public void userEventTriggered(ChannelHandlerContext ctx, Object evt) throws Exc
     @ParameterizedTest
     @MethodSource("newSslTaskExecutors")
     public void testConnectWith0RTT(Executor executor) throws Throwable {
-        if (executor != ImmediateExecutor.INSTANCE) {
-            // Disable 0RTT test when offloading for now as it sometimes timeout. This is just a workaround and will
-            // need a proper fix.
-            // See https://github.com/netty/netty-incubator-codec-quic/issues/544
-            //
-            // TODO: remove once fixed.
-            shutdown(executor);
-            return;
-        }
         final CountDownLatch readLatch = new CountDownLatch(1);
         Channel server = QuicTestUtils.newServer(QuicTestUtils.newQuicServerBuilder(executor,
                         QuicSslContextBuilder.forServer(
@@ -647,6 +638,16 @@ public void userEventTriggered(ChannelHandlerContext ctx, Object evt) {
                     .connect()
                     .get();
 
+            QuicClientSessionCache cache = ((QuicheQuicSslContext) sslContext).getSessionCache();
+
+            // Let's spin until the session shows up in the cache. This is needed as this might happen a bit after
+            // the connection is already established.
+            // See https://commondatastorage.googleapis.com/chromium-boringssl-docs/ssl.h.html#SSL_CTX_sess_set_new_cb
+            while (!cache.hasSession("localhost", 9999)) {
+                // Check again in 100ms.
+                Thread.sleep(100);
+            }
+
             quicChannel.close().sync();
 
             if (errorRef.get() != null) {