\section{\module{asynchat} ---
        Asynchronous socket command/response handler}

\declaremodule{standard}{asynchat}
\modulesynopsis{Support for asynchronous command/response protocols.}
\moduleauthor{Sam Rushing}{[email protected]}
\sectionauthor{Steve Holden}{[email protected]}

This module builds on the \refmodule{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. \refmodule{asynchat} defines the abstract class
\class{async_chat} that you subclass, providing implementations of the
\method{collect_incoming_data()} and \method{found_terminator()}
methods. It uses the same asynchronous loop as \refmodule{asyncore}, and
the two types of channel, \class{asyncore.dispatcher} and
\class{asynchat.async_chat}, can freely be mixed in the channel map.
Typically an \class{asyncore.dispatcher} server channel generates new
\class{asynchat.async_chat} channel objects as it receives incoming
connection requests.

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

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

 Unlike \class{asyncore.dispatcher}, \class{async_chat} allows you to define
 a first-in-first-out queue (fifo) of \emph{producers}. A producer need have
 only one method, \method{more()}, which should return data to be transmitted
 on the channel. The producer indicates exhaustion (\emph{i.e.} that it contains
 no more data) by having its \method{more()} method return the empty string. At
 this point the \class{async_chat} object removes the producer from the fifo
 and starts using the next producer, if any. When the producer fifo is empty
 the \method{handle_write()} method does nothing. You use the channel object's
 \method{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 \class{async_chat} subclass your
 input methods \method{collect_incoming_data()} and
 \method{found_terminator()} must handle the data that the channel receives
 asynchronously. The methods are described below.
\end{classdesc}

\begin{methoddesc}{close_when_done}{}
 Pushes a \code{None} on to the producer fifo. When this producer is
 popped off the fifo it causes the channel to be closed.
\end{methoddesc}

\begin{methoddesc}{collect_incoming_data}{data}
 Called with \var{data} holding an arbitrary amount of received data.
 The default method, which must be overridden, raises a \exception{NotImplementedError} exception.
\end{methoddesc}

\begin{methoddesc}{discard_buffers}{}
 In emergencies this method will discard any data held in the input and/or
 output buffers and the producer fifo.
\end{methoddesc}

\begin{methoddesc}{found_terminator}{}
 Called when the incoming data stream  matches the termination condition
 set by \method{set_terminator}. The default method, which must be overridden,
 raises a \exception{NotImplementedError} exception. The buffered input data should
 be available via an instance attribute.
\end{methoddesc}

\begin{methoddesc}{get_terminator}{}
 Returns the current terminator for the channel.
\end{methoddesc}

\begin{methoddesc}{handle_close}{}
 Called when the channel is closed. The default method silently closes
 the channel's socket.
\end{methoddesc}

\begin{methoddesc}{handle_read}{}
 Called when a read event fires on the channel's socket in the
 asynchronous loop. The default method checks for the termination
 condition established by \method{set_terminator()}, which can be either
 the appearance of a particular string in the input stream or the receipt
 of a particular number of characters. When the terminator is found,
 \method{handle_read} calls the \method{found_terminator()} method after
 calling \method{collect_incoming_data()} with any data preceding the
 terminating condition.
\end{methoddesc}

\begin{methoddesc}{handle_write}{}
 Called when the application may write data to the channel.
 The default method calls the \method{initiate_send()} method, which in turn
 will call \method{refill_buffer()} to collect data from the producer
 fifo associated with the channel.
\end{methoddesc}

\begin{methoddesc}{push}{data}
 Creates a \class{simple_producer} object (\emph{see below}) containing the data and
 pushes it on to the channel's \code{producer_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.
\end{methoddesc}

\begin{methoddesc}{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
 \method{more()} method and send the data to the remote endpoint.
\end{methoddesc}

\begin{methoddesc}{readable}{}
 Should return \code{True} for the channel to be included in the set of
 channels tested by the \cfunction{select()} loop for readability.
\end{methoddesc}

\begin{methoddesc}{refill_buffer}{}
 Refills the output buffer by calling the \method{more()} method of the
 producer at the head of the fifo. If it is exhausted then the
 producer is popped off the fifo and the next producer is activated.
 If the current producer is, or becomes, \code{None} then the channel
 is closed.
\end{methoddesc}

\begin{methoddesc}{set_terminator}{term}
 Sets the terminating condition to be recognised on the channel. \code{term}
 may be any of three types of value, corresponding to three different ways
 to handle incoming protocol data.

 \begin{tableii}{l|l}{}{term}{Description}
   \lineii{\emph{string}}{Will call \method{found_terminator()} when the
               string is found in the input stream}
   \lineii{\emph{integer}}{Will call \method{found_terminator()} when the
               indicated number of characters have been received}
   \lineii{\code{None}}{The channel continues to collect data forever}
 \end{tableii}

 Note that any data following the terminator will be available for reading by
 the channel after \method{found_terminator()} is called.
\end{methoddesc}

\begin{methoddesc}{writable}{}
 Should return \code{True} as long as items remain on the producer fifo,
 or the channel is connected and the channel's output buffer is non-empty.
\end{methoddesc}

\subsection{asynchat - Auxiliary Classes and Functions}

\begin{classdesc}{simple_producer}{data\optional{, buffer_size=512}}
 A \class{simple_producer} takes a chunk of data and an optional buffer size.
 Repeated calls to its \method{more()} method yield successive chunks of the
 data no larger than \var{buffer_size}.
\end{classdesc}

\begin{methoddesc}{more}{}
 Produces the next chunk of information from the producer, or returns the empty string.
\end{methoddesc}

\begin{classdesc}{fifo}{\optional{list=None}}
 Each channel maintains a \class{fifo} holding data which has been pushed by the
 application but not yet popped for writing to the channel.
 A \class{fifo} is a list used to hold data and/or producers until they are required.
 If the \var{list} argument is provided then it should contain producers or
 data items to be written to the channel.
\end{classdesc}

\begin{methoddesc}{is_empty}{}
 Returns \code{True} iff the fifo is empty.
\end{methoddesc}

\begin{methoddesc}{first}{}
 Returns the least-recently \method{push()}ed item from the fifo.
\end{methoddesc}

\begin{methoddesc}{push}{data}
 Adds the given data (which may be a string or a producer object) to the
 producer fifo.
\end{methoddesc}

\begin{methoddesc}{pop}{}
 If the fifo is not empty, returns \code{True, first()}, deleting the popped
 item. Returns \code{False, None} for an empty fifo.
\end{methoddesc}

The \module{asynchat} module also defines one utility function, which may be
of use in network and textual analysis operations.

\begin{funcdesc}{find_prefix_at_end}{haystack, needle}
 Returns \code{True} if string \var{haystack} ends with any non-empty
 prefix of string \var{needle}.
\end{funcdesc}

\subsection{asynchat Example \label{asynchat-example}}

The following partial example shows how HTTP requests can be read with
\class{async_chat}. A web server might create an \class{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
\code{Content-Length:} header is used to set a numeric terminator to
read the right amount of data from the channel.

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

\begin{verbatim}
class http_request_handler(asynchat.async_chat):

   def __init__(self, conn, addr, sessions, log):
       asynchat.async_chat.__init__(self, conn=conn)
       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()
\end{verbatim}

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