Support for ``wsgi.file_wrapper``
---------------------------------

Waitress supports the `WSGI file_wrapper protocol
<http://www.python.org/dev/peps/pep-0333/#optional-platform-specific-file-handling>`_
.  Here's a usage example::

    import os

    here = os.path.dirname(os.path.abspath(__file__))

    def myapp(environ, start_response):
        f = open(os.path.join(here, 'myphoto.jpg'), 'rb')
        headers = [('Content-Type', 'image/jpeg')]
        start_response(
            '200 OK',
            headers
            )
        return environ['wsgi.file_wrapper'](f, 32768)

The file wrapper constructor is accessed via
``environ['wsgi.file_wrapper']``.  The signature of the file wrapper
constructor is ``(filelike_object, block_size)``.  Both arguments must be
passed as positional (not keyword) arguments.  The result of creating a file
wrapper should be **returned** as the ``app_iter`` from a WSGI application.

The object passed as ``filelike_object`` to the wrapper must be a file-like
object which supports *at least* the ``read()`` method, and the ``read()``
method must support an optional size hint argument and the ``read()`` method
*must* return **bytes** objects (never unicode).  It *should* support the
``seek()`` and ``tell()`` methods.  If it does not, normal iteration over the
filelike object using the provided block_size is used (and copying is done,
negating any benefit of the file wrapper).  It *should* support a ``close()``
method.

The specified ``block_size`` argument to the file wrapper constructor will be
used only when the ``filelike_object`` doesn't support ``seek`` and/or
``tell`` methods.  Waitress needs to use normal iteration to serve the file
in this degenerate case (as per the WSGI pec), and this block size will be
used as the iteration chunk size.  The ``block_size`` argument is optional;
if it is not passed, a default value``32768`` is used.

Waitress will set a ``Content-Length`` header on the behalf of an application
when a file wrapper with a sufficiently filelike object is used if the
application hasn't already set one.

The machinery which handles a file wrapper currently doesn't do anything
particularly special using fancy system calls (it doesn't use ``sendfile``
for example); using it currently just prevents the system from needing to
copy data to a temporary buffer in order to send it to the client.  No
copying of data is done when a WSGI app returns a file wrapper that wraps a
sufficiently filelike object.  It may do something fancier in the future.