>     # Using zlib's interface
>     while not d.eof:
>         compressed = d.unconsumed_tail or f.read(8192)
>         if not compressed:
>             raise ValueError('End-of-stream marker not found')
>         output = d.decompress(compressed, 8192)
>         # <process output>

This is not usable with bzip2. Bzip2 uses large block size and unconsumed_tail 
can be non empty but decompress() will return b''. With zlib you possible can 
see the same effect on some input when read by one byte.

> A related, but orthogonal proposal: We might want to make unconsumed_tail
> a memoryview (provided the input data is know to be immutable), to avoid
> creating an unnecessary copy of the data.

It looks interesting. However the data should be copied anyway if the input 
data is not a bytes object.