Allow applications to yield empty strings, and require middleware

to respect applications' block boundaries, so that asynchronous
applications can potentially pause their output while waiting for
(non-browser) I/O.  See these posts and the associated threads
for the discussion:

http://mail.python.org/pipermail/web-sig/2004-September/000868.html
http://mail.python.org/pipermail/web-sig/2004-September/000880.html

pep-0333.txt

@@ -246,9 +246,10 @@ server.
         result = application(environ, start_response)
         try:
             for data in result:
+                if data:	# don't send headers until body appears
 	                write(data)
             if not headers_sent:
-                write('')   # force headers to be sent
+                write('')   # send headers now if body was empty
         finally:
             if hasattr(result,'close'):
                 result.close()
@@ -579,10 +580,11 @@ such output, so that users who cannot fix a broken application are not
 forced to bear the pain of its error.)
 
 The ``start_response`` callable **must not** actually transmit the
-HTTP headers.  It must store them until the first ``write`` call,
-or until after the first iteration of the application return value.
-This is to ensure that buffered applications can replace their 
-originally intended output with error output, up until the last
+HTTP headers.  It must store them until the first ``write`` call that
+is passed a non-empty string, or until after the first iteration of
+the application return value that yields a non-empty string.  This is
+to ensure that buffered and asynchronous applications can replace
+their originally intended output with error output, up until the last
 possible moment.
 
 The ``exc_info`` argument, if supplied, must be a Python
@@ -690,11 +692,12 @@ fashion.  These blocks may be broken to coincide with mulitpart
 boundaries (for "server push"), or just before time-consuming 
 tasks (such as reading another block of an on-disk file).
 
-WSGI servers and gateways **must not** delay the transmission
-of any block; they **must** either fully transmit the block to
-the client, or guarantee that they will continue transmission
-even while the application is producing its next block.  A
-server/gateway may provide this guarantee in one of two ways:
+WSGI servers, gateways, and middleware **must not** delay the 
+transmission of any block; they **must** either fully transmit
+the block to the client, or guarantee that they will continue
+transmission even while the application is producing its next block.
+A server/gateway or middleware may provide this guarantee in one of
+three ways:
 
 1. Send the entire block to the operating system (and request 
    that any O/S buffers be flushed) before returning control
@@ -704,6 +707,9 @@ server/gateway may provide this guarantee in one of two ways:
    to be transmitted while the application produces the next
    block.
    
+3. (Middleware only) send the entire block to its parent
+   gateway/server
+
 By providing this guarantee, WSGI allows applications to ensure
 that transmission will not become stalled at an arbitrary point
 in their output data.  This is critical for proper functioning
@@ -711,6 +717,34 @@ of e.g. multipart "server push" streaming, where data between
 multipart boundaries should be transmitted in full to the client.
 
 
+Middleware Handling of Block Boundaries
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+In order to better support asynchronous applications and servers,
+middleware components **must not** block iteration waiting for
+multiple values from an application iterable.  If the middleware
+needs to accumulate more data from the application before it can
+produce any output, it **must** yield an empty string.
+
+To put this requirement another way, a middleware component **must
+yield at least one value** each time its underlying application 
+yields a value.  If the middleware cannot yield any other value,
+it must yield an empty string.
+
+This requirement ensures that asynchronous applications and servers
+can conspire to reduce the number of threads that are required
+to run a given number of application instances simultaneously.
+
+Note also that this requirement means that middleware **must**
+return an iterable as soon as its underlying application returns
+an iterable.  It is also forbidden for middleware to use the
+``write()`` callable to transmit data that is yielded by an
+underlying application.  Middleware may only use their parent
+server's ``write()`` callable to transmit data that the 
+underlying application sent using a middleware-provided ``write()``
+callable.
+
+
 The ``write()`` Callable
 ~~~~~~~~~~~~~~~~~~~~~~~~