Fix hang when using AbstractClassicEntityProducer

[carterkozak]

When AsyncDataProducer.available returns zero, the caller may
not execute the produce method again until
DataStreamChannel.requestOutput has been invoked.

This approach sets available to Integer.MAX_VALUE to avoid this
case. It would be more efficient to call requestOutput after
writes, but I'm not certain if invoking the method unnecessarily
would affect performance, and the MAX_VALUE approach is used by
other producers.

I was able to reproduce this issue in a test by sending a dummy 4gb
stream using AbstractClassicEntityProducer. Overriding available with
a non-zero value stopped the hangs.
Again, it may be more efficient to track available() invocations and
call requestOutput() from a subtype of ContentOutputStream, but I
haven't dug in quite that far yet :-)

[ok2c]

@cakofony Always returning Integer.MAX_VALUE from AsyncDataProducer#available does not sound right me. Simple producers do that if they are self-contained and can always produce more data if the underlying channel is ready. This is not the case with complex producers such as AsyncDataProducer that may not always have data to write to the channel. By returning Integer.MAX_VALUE from AsyncDataProducer#available one can cause the I/O reactor spin busily and produce no data.

We need to think of a better fix. How reliably can you reproduce the issue and would it be possible to make a junit?

[ok2c]

@cakofony Did you have any luck reproducing the issue with an unit / integration test?

[carterkozak]

Apologies, I've been busy, I will take a crack at it this week!