Lucas, it is indeed impossible for peek() to return more than the buffer
size and remain compatible with non-seekable raw streams. That's why it
/never/ returns more than the buffer size.

As for the fact that peek() doesn't behave as documented, I disagree.
Here is what the docstring says:

        """Returns buffered bytes without advancing the position.

        The argument indicates a desired minimal number of bytes; we
        do at most one raw read to satisfy it.  We never return more
        than self.buffer_size.
        """

Please note : "a desired /minimal/ number of bytes" (minimal, not
maximal). Furthermore, "We never return more than self.buffer_size." The
behaviour looks ok to me.