Enable PooledByteBufAllocator to work, event without a cache

Motivation:
`useCacheForAllThreads` may be false which disables memory caching
on non netty threads.  Setting this argument or the system property
makes it impossible to use `PooledByteBufAllocator`.

Modifications:

Delayed the check of `freeSweepAllocationThreshold` in
`PoolThreadCache` to after it knows there will be any caches in
use.  Additionally, check if the caches will have any data in them
(rather than allocating a 0-length array).

A test case is also added that fails without this change.

Results:

Fixes netty#7194

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

@@ -71,10 +71,6 @@ final class PoolThreadCache {
             throw new IllegalArgumentException("maxCachedBufferCapacity: "
                     + maxCachedBufferCapacity + " (expected: >= 0)");
         }
-        if (freeSweepAllocationThreshold < 1) {
-            throw new IllegalArgumentException("freeSweepAllocationThreshold: "
-                    + freeSweepAllocationThreshold + " (expected: > 0)");
-        }
         this.freeSweepAllocationThreshold = freeSweepAllocationThreshold;
         this.heapArena = heapArena;
         this.directArena = directArena;
@@ -119,6 +115,11 @@ final class PoolThreadCache {
         // We only need to watch the thread when any cache is used.
         if (tinySubPageDirectCaches != null || smallSubPageDirectCaches != null || normalDirectCaches != null
                 || tinySubPageHeapCaches != null || smallSubPageHeapCaches != null || normalHeapCaches != null) {
+            // Only check freeSweepAllocationThreshold when there are caches in use.
+            if (freeSweepAllocationThreshold < 1) {
+                throw new IllegalArgumentException("freeSweepAllocationThreshold: "
+                        + freeSweepAllocationThreshold + " (expected: > 0)");
+            }
             freeTask = new Runnable() {
                 @Override
                 public void run() {
@@ -139,7 +140,7 @@ public void run() {
 
     private static <T> MemoryRegionCache<T>[] createSubPageCaches(
             int cacheSize, int numCaches, SizeClass sizeClass) {
-        if (cacheSize > 0) {
+        if (cacheSize > 0 && numCaches > 0) {
             @SuppressWarnings("unchecked")
             MemoryRegionCache<T>[] cache = new MemoryRegionCache[numCaches];
             for (int i = 0; i < cache.length; i++) {
@@ -154,7 +155,7 @@ private static <T> MemoryRegionCache<T>[] createSubPageCaches(
 
     private static <T> MemoryRegionCache<T>[] createNormalCaches(
             int cacheSize, int maxCachedBufferCapacity, PoolArena<T> area) {
-        if (cacheSize > 0) {
+        if (cacheSize > 0 && maxCachedBufferCapacity > 0) {
             int max = Math.min(area.chunkSize, maxCachedBufferCapacity);
             int arraySize = Math.max(1, log2(max / area.pageSize) + 1);
 

buffer/src/test/java/io/netty/buffer/PooledByteBufAllocatorTest.java

@@ -77,6 +77,24 @@ public void testPooledUnsafeHeapBufferAndUnsafeDirectBuffer() {
         heapBuffer.release();
     }
 
+    @Test
+    public void testWithoutUseCacheForAllThreads() {
+        assertFalse(Thread.currentThread() instanceof FastThreadLocalThread);
+
+        PooledByteBufAllocator pool = new PooledByteBufAllocator(
+                /*preferDirect=*/ false,
+                /*nHeapArena=*/ 1,
+                /*nDirectArena=*/ 1,
+                /*pageSize=*/8192,
+                /*maxOrder=*/ 11,
+                /*tinyCacheSize=*/ 0,
+                /*smallCacheSize=*/ 0,
+                /*normalCacheSize=*/ 0,
+                /*useCacheForAllThreads=*/ false);
+        ByteBuf buf = pool.buffer(1);
+        buf.release();
+    }
+
     @Test
     public void testArenaMetricsNoCache() {
         testArenaMetrics0(new PooledByteBufAllocator(true, 2, 2, 8192, 11, 0, 0, 0), 100, 0, 100, 100);