Enable thread-local buffer cache for Thread that is used by EventExec…

…utor by default (netty#12158)

Motivation:

netty#12109 changed the default of using the thread local cache for buffers only for FastThreadLocalThread. While this is good it also means that it would affect everyone that use a custom ThreadFactory when construct the EventLoop / EventExecutor

Modifications:

Make use of ThreadExecutorMap to check if the Thread is used by any EventExecutor and if so also enable the cache.

Result:

Fixes netty#12109 (comment)

buffer/src/main/java/io/netty/buffer/PooledByteBufAllocator.java

@@ -511,13 +511,19 @@ protected synchronized PoolThreadCache initialValue() {
             final PoolArena<ByteBuffer> directArena = leastUsedArena(directArenas);
 
             final Thread current = Thread.currentThread();
-            if (useCacheForAllThreads || current instanceof FastThreadLocalThread) {
+            final EventExecutor executor = ThreadExecutorMap.currentExecutor();
+
+            if (useCacheForAllThreads ||
+                    // If the current thread is a FastThreadLocalThread we will always use the cache
+                    current instanceof FastThreadLocalThread ||
+                    // The Thread is used by an EventExecutor, let's use the cache as the chances are good that we
+                    // will allocate a lot!
+                    executor != null) {
                 final PoolThreadCache cache = new PoolThreadCache(
                         heapArena, directArena, smallCacheSize, normalCacheSize,
                         DEFAULT_MAX_CACHED_BUFFER_CAPACITY, DEFAULT_CACHE_TRIM_INTERVAL);
 
                 if (DEFAULT_CACHE_TRIM_INTERVAL_MILLIS > 0) {
-                    final EventExecutor executor = ThreadExecutorMap.currentExecutor();
                     if (executor != null) {
                         executor.scheduleAtFixedRate(trimTask, DEFAULT_CACHE_TRIM_INTERVAL_MILLIS,
                                 DEFAULT_CACHE_TRIM_INTERVAL_MILLIS, TimeUnit.MILLISECONDS);