Allow up to 16383 callbacks on a Deferred.

Change-Id: I9f9a7fb5110f693db6813c0b8a62f0d03b896a1e

NEWS

@@ -5,6 +5,11 @@ StumbleUpon's Async Library - User visible changes.
 Noteworthy changes:
   - Optimization: the internal state of a Deferred is now captured
     in a plain `int' instead of using an enum.
+  - The maximum number of callbacks allowed on a Deferred has been
+    significantly increased, from 128 to 16383, to allow for some
+    use cases where a large number of operations are waiting on a
+    single asynchronous operation to complete.  This implementation
+    cannot support more than this many callbacks without refactoring.
 
 
 * Version 1.1.0 (2011-09-27) [cf62d8d]

src/Deferred.java

@@ -439,11 +439,14 @@ public final class Deferred<T> {
 
   /**
    * Maximum length of the callback chain we allow.
-   * Set this to some aggressive limit to quickly detect problems in code
+   * Set this to some arbitrary limit to quickly detect problems in code
    * that creates potentially infinite chains.  I can't imagine a practical
    * case that would require a chain with more callbacks than this.
    */
-  private static final short MAX_CALLBACK_CHAIN_LENGTH = 128;
+  private static final short MAX_CALLBACK_CHAIN_LENGTH = Short.MAX_VALUE >> 1;
+  // NOTE: The current implementation cannot support more than this many
+  // callbacks because indexes used to access the `callbacks' array are
+  // of type `short' to save memory.
 
   /**
    * How many entries do we create in the callback+errback chain by default.