17.7. "asynchat" --- Asynchronous socket command/response handler
*****************************************************************

**Source code:** Lib/asynchat.py

======================================================================

This module builds on the "asyncore" infrastructure, simplifying
asynchronous clients and servers and making it easier to handle
protocols whose elements are terminated by arbitrary strings, or are
of variable length. "asynchat" defines the abstract class "async_chat"
that you subclass, providing implementations of the
"collect_incoming_data()" and "found_terminator()" methods. It uses
the same asynchronous loop as "asyncore", and the two types of
channel, "asyncore.dispatcher" and "asynchat.async_chat", can freely
be mixed in the channel map. Typically an "asyncore.dispatcher" server
channel generates new "asynchat.async_chat" channel objects as it
receives incoming connection requests.

class asynchat.async_chat

   This class is an abstract subclass of "asyncore.dispatcher". To
   make practical use of the code you must subclass "async_chat",
   providing meaningful "collect_incoming_data()" and
   "found_terminator()" methods. The "asyncore.dispatcher" methods can
   be used, although not all make sense in a message/response context.

   Like "asyncore.dispatcher", "async_chat" defines a set of events
   that are generated by an analysis of socket conditions after a
   "select()" call. Once the polling loop has been started the
   "async_chat" object's methods are called by the event-processing
   framework with no action on the part of the programmer.

   Two class attributes can be modified, to improve performance, or
   possibly even to conserve memory.

   ac_in_buffer_size

      The asynchronous input buffer size (default "4096").

   ac_out_buffer_size

      The asynchronous output buffer size (default "4096").

   Unlike "asyncore.dispatcher", "async_chat" allows you to define a
   first-in-first-out queue (fifo) of *producers*. A producer need
   have only one method, "more()", which should return data to be
   transmitted on the channel. The producer indicates exhaustion
   (*i.e.* that it contains no more data) by having its "more()"
   method return the empty string. At this point the "async_chat"
   object removes the producer from the fifo and starts using the next
   producer, if any. When the producer fifo is empty the
   "handle_write()" method does nothing. You use the channel object's
   "set_terminator()" method to describe how to recognize the end of,
   or an important breakpoint in, an incoming transmission from the
   remote endpoint.

   To build a functioning "async_chat" subclass your  input methods
   "collect_incoming_data()" and "found_terminator()" must handle the
   data that the channel receives asynchronously. The methods are
   described below.

async_chat.close_when_done()

   Pushes a "None" on to the producer fifo. When this producer is
   popped off the fifo it causes the channel to be closed.

async_chat.collect_incoming_data(data)

   Called with *data* holding an arbitrary amount of received data.
   The default method, which must be overridden, raises a
   "NotImplementedError" exception.

async_chat.discard_buffers()

   In emergencies this method will discard any data held in the input
   and/or output buffers and the producer fifo.

async_chat.found_terminator()

   Called when the incoming data stream  matches the termination
   condition set by "set_terminator()". The default method, which must
   be overridden, raises a "NotImplementedError" exception. The
   buffered input data should be available via an instance attribute.

async_chat.get_terminator()

   Returns the current terminator for the channel.

async_chat.push(data)

   Pushes data on to the channel's fifo to ensure its transmission.
   This is all you need to do to have the channel write the data out
   to the network, although it is possible to use your own producers
   in more complex schemes to implement encryption and chunking, for
   example.

async_chat.push_with_producer(producer)

   Takes a producer object and adds it to the producer fifo associated
   with the channel.  When all currently-pushed producers have been
   exhausted the channel will consume this producer's data by calling
   its "more()" method and send the data to the remote endpoint.

async_chat.set_terminator(term)

   Sets the terminating condition to be recognized on the channel.
   "term" may be any of three types of value, corresponding to three
   different ways to handle incoming protocol data.

   +-------------+-----------------------------------------------+
   | term        | Description                                   |
   +=============+===============================================+
   | *string*    | Will call "found_terminator()" when the       |
   |             | string is found in the input stream           |
   +-------------+-----------------------------------------------+
   | *integer*   | Will call "found_terminator()" when the       |
   |             | indicated number of characters have been      |
   |             | received                                      |
   +-------------+-----------------------------------------------+
   | "None"      | The channel continues to collect data forever |
   +-------------+-----------------------------------------------+

   Note that any data following the terminator will be available for
   reading by the channel after "found_terminator()" is called.


17.7.1. asynchat - Auxiliary Classes
====================================

class asynchat.fifo([list=None])

   A "fifo" holding data which has been pushed by the application but
   not yet popped for writing to the channel.  A "fifo" is a list used
   to hold data and/or producers until they are required.  If the
   *list* argument is provided then it should contain producers or
   data items to be written to the channel.

   is_empty()

      Returns "True" if and only if the fifo is empty.

   first()

      Returns the least-recently "push()"ed item from the fifo.

   push(data)

      Adds the given data (which may be a string or a producer object)
      to the producer fifo.

   pop()

      If the fifo is not empty, returns "True, first()", deleting the
      popped item.  Returns "False, None" for an empty fifo.


17.7.2. asynchat Example
========================

The following partial example shows how HTTP requests can be read with
"async_chat".  A web server might create an "http_request_handler"
object for each incoming client connection. Notice that initially the
channel terminator is set to match the blank line at the end of the
HTTP headers, and a flag indicates that the headers are being read.

Once the headers have been read, if the request is of type POST
(indicating that further data are present in the input stream) then
the "Content-Length:" header is used to set a numeric terminator to
read the right amount of data from the channel.

The "handle_request()" method is called once all relevant input has
been marshalled, after setting the channel terminator to "None" to
ensure that any extraneous data sent by the web client are ignored.

   class http_request_handler(asynchat.async_chat):

       def __init__(self, sock, addr, sessions, log):
           asynchat.async_chat.__init__(self, sock=sock)
           self.addr = addr
           self.sessions = sessions
           self.ibuffer = []
           self.obuffer = ""
           self.set_terminator("\r\n\r\n")
           self.reading_headers = True
           self.handling = False
           self.cgi_data = None
           self.log = log

       def collect_incoming_data(self, data):
           """Buffer the data"""
           self.ibuffer.append(data)

       def found_terminator(self):
           if self.reading_headers:
               self.reading_headers = False
               self.parse_headers("".join(self.ibuffer))
               self.ibuffer = []
               if self.op.upper() == "POST":
                   clen = self.headers.getheader("content-length")
                   self.set_terminator(int(clen))
               else:
                   self.handling = True
                   self.set_terminator(None)
                   self.handle_request()
           elif not self.handling:
               self.set_terminator(None)  # browsers sometimes over-send
               self.cgi_data = parse(self.headers, "".join(self.ibuffer))
               self.handling = True
               self.ibuffer = []
               self.handle_request()
