I agree that being able to limit output size is useful and desirable, but
I'm not keen on copying the max_length/unconsumed_tail approach used by
zlib's decompressor class. It feels awkward to use, and it complicates
the implementation of the existing decompress() method, which is already
unwieldy enough.

As an alternative, I propose a thin wrapper around the underlying C API:

    def decompress_into(self, src, dst, src_start=0, dst_start=0): ...

This would store decompressed data in a caller-provided bytearray, and
return a pair of integers indicating the end points of the consumed and
produced data in the respective buffers.

The implementation should be extremely simple - it does not need to do
any memory allocation or reference management.

I think it could also be useful for optimizing the implementation of
BZ2File and LZMAFile. I plan to write a prototype and run some benchmarks
some time in the next few weeks.

(Aside: if implemented for zlib, this could also be a nicer (I think)
 solution for the problem raised in issue 5804.)