Make SnappyOutputStream.close() idempotent; guard against writing to closed stream

[JoshRosen]

This patch will fix #107, an issue where SnappyOutputStream.close() is not idempotent.

The first commit is a failing regression test which shows how a non-idempotent close() can lead to stream corruption issues.

[JoshRosen]

Here's a failing run of the regression test: https://travis-ci.org/xerial/snappy-java/jobs/62514893

[JoshRosen]

Even though this patch makes close() idempotent, I suppose there's still the potential for stream corruption if a user continues to call write() methods on a SnappyOutputStream after calling close() on it. Do you think that we should also add a !closed check in rawWrite() to guard against this case? Alternatively, I guess that we could null out the references to the buffers to be absolutely sure that we can't access them after close().

[xerial]

Thanks for the fix.

I think checking !close has negligible overhead.
And also nulling out the buffers is also good for GC.

Please add both.

[JoshRosen]

I've updated this patch to include nulling of output buffers and if(closed) checks.

[xerial]

Is this necessary?

[JoshRosen]

The goal here was to test that the now weakly-referencable buffers get reclaimed by GC.

[xerial]

Looks good! Thank you for the improvements.