Improve the randomness of ThreadLocalRandom for all platform

- Fixes #1765
Java 6 did a poor job of generating seedUniquifier unlike 7, so I implemented platform-independent seedUniquifier generator with configurability

common/src/main/java/io/netty/util/internal/ThreadLocalRandom.java

@@ -22,7 +22,15 @@
 
 package io.netty.util.internal;
 
+import io.netty.util.internal.logging.InternalLogger;
+import io.netty.util.internal.logging.InternalLoggerFactory;
+
+import java.security.SecureRandom;
 import java.util.Random;
+import java.util.concurrent.BlockingQueue;
+import java.util.concurrent.LinkedBlockingQueue;
+import java.util.concurrent.TimeUnit;
+import java.util.concurrent.atomic.AtomicLong;
 
 /**
  * A random number generator isolated to the current thread.  Like the
@@ -50,6 +58,90 @@ import java.util.Random;
  */
 @SuppressWarnings("all")
 public class ThreadLocalRandom extends Random {
+
+    private static final InternalLogger logger = InternalLoggerFactory.getInstance(ThreadLocalRandom.class);
+
+    private static final AtomicLong seedUniquifier = new AtomicLong();
+
+    private static volatile long initialSeedUniquifier;
+
+    public static void setInitialSeedUniquifier(long initialSeedUniquifier) {
+        ThreadLocalRandom.initialSeedUniquifier = initialSeedUniquifier;
+    }
+
+    public static synchronized long getInitialSeedUniquifier() {
+        // Use the value set via the setter.
+        long initialSeedUniquifier = ThreadLocalRandom.initialSeedUniquifier;
+        if (initialSeedUniquifier == 0) {
+            // Use the system property value.
+            ThreadLocalRandom.initialSeedUniquifier = initialSeedUniquifier =
+                    SystemPropertyUtil.getLong("io.netty.initialSeedUniquifier", 0);
+        }
+
+        // Otherwise, generate one.
+        if (initialSeedUniquifier == 0) {
+            // Try to generate a real random number from /dev/random.
+            // Get from a different thread to avoid blocking indefinitely on a machine without much entrophy.
+            final BlockingQueue<Long> queue = new LinkedBlockingQueue<Long>();
+            Thread generatorThread = new Thread("initialSeedUniquifierGenerator") {
+                @Override
+                public void run() {
+                    SecureRandom random = new SecureRandom(); // Get the real random seed from /dev/random
+                    queue.add(random.nextLong());
+                }
+            };
+            generatorThread.start();
+
+            // Get the random seed from the thread with timeout.
+            final long timeoutSeconds = 3;
+            final long deadLine = System.nanoTime() + TimeUnit.SECONDS.toNanos(timeoutSeconds);
+            for (;;) {
+                long waitTime = deadLine - System.nanoTime();
+                if (waitTime <= 0) {
+                    logger.warn(
+                            "Failed to get the secure random number from SecureRandom within {} seconds. " +
+                            "Not enough entrophy?", timeoutSeconds);
+                    break;
+                }
+
+                try {
+                    Long result = queue.poll(waitTime, TimeUnit.NANOSECONDS);
+                    if (result != null) {
+                        initialSeedUniquifier = result;
+                        break;
+                    }
+                } catch (InterruptedException ignore) {
+                    // Ignore
+                }
+            }
+
+            // Just in case the initialSeedUniquifier is zero or some other constant
+            initialSeedUniquifier ^= 0x3255ecdc33bae119L; // just a meaningless random number
+            initialSeedUniquifier ^= Long.reverse(System.nanoTime());
+
+            ThreadLocalRandom.initialSeedUniquifier = initialSeedUniquifier;
+        }
+
+        return initialSeedUniquifier;
+    }
+
+    private static long newSeed() {
+        for (;;) {
+            final long current = seedUniquifier.get();
+            final long actualCurrent = current != 0? current : getInitialSeedUniquifier();
+
+            // L'Ecuyer, "Tables of Linear Congruential Generators of Different Sizes and Good Lattice Structure", 1999
+            final long next = actualCurrent * 181783497276652981L;
+
+            if (seedUniquifier.compareAndSet(current, next)) {
+                if (current == 0 && logger.isDebugEnabled()) {
+                    logger.debug(String.format("-Dio.netty.initialSeedUniquifier: 0x%016x", actualCurrent));
+                }
+                return next ^ System.nanoTime();
+            }
+        }
+    }
+
     // same constants as Random, but must be redeclared because private
     private static final long multiplier = 0x5DEECE66DL;
     private static final long addend = 0xBL;
@@ -73,6 +165,14 @@ public class ThreadLocalRandom extends Random {
     // each other.
     private long pad0, pad1, pad2, pad3, pad4, pad5, pad6, pad7;
 
+    /**
+     * Constructor called only by localRandom.initialValue.
+     */
+    ThreadLocalRandom() {
+        super(newSeed());
+        initialized = true;
+    }
+
     /**
      * The actual ThreadLocal
      */
@@ -83,14 +183,6 @@ public class ThreadLocalRandom extends Random {
                 }
             };
 
-    /**
-     * Constructor called only by localRandom.initialValue.
-     */
-    ThreadLocalRandom() {
-        super();
-        initialized = true;
-    }
-
     /**
      * Returns the current thread's {@code ThreadLocalRandom}.
      *