tree 674d8b7601a8d03da023dafcf9d2d60623ba8510
parent 5f355ed4ad60118eb4331f0ea5b018d3fa16070b
author Jonathan Nieder <jrn@google.com> 1547491686 -0800
committer Jonathan Nieder <jrn@google.com> 1547492128 -0800

dfs: Remove synchronization in BlockBasedFile#LazyChannel

As explained in 'The "Double-Checked Locking is Broken"
Declaration'[*], Java's memory model does not support double-checked
locking:

 class LazyReadableChannel {
   private ReachableChannel rc = null;

   public ReadableChannel get() {
     if (rc == null) {
       synchronized (this) {
         if (rc == null) {
           rc = new ReadableChannel();
	 }
       }
     }
     return rc;
   }
 }

With JDK 5 and newer, there is a formal memory model that ensures this
works if "rc" is volatile, but it is still not thread-safe without
that.

Fortunately, this ReadableChannelSupplier is never passed between
threads, so it does not need to be thread-safe.  Simplify by removing
the synchronization.

[*] https://www.cs.umd.edu/~pugh/java/memoryModel/DoubleCheckedLocking.html

Change-Id: I0698ee6618d734fc129dd4f63fc047c1c17c94a9
Signed-off-by: Jonathan Nieder <jrn@google.com>