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# -*- coding: UTF-8 -*-
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Format of Transported Data
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--------------------------
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In order to communicate with external non-python programs, the xdr module
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implements a very simple protocol à la XDR.
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The send function expect variable length arguments of str type.
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The arguments will be escaped, joint and packed into binary.
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All the arguments have to be joint into a '\\n' delimited str.
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e.g. send(s, "a", "b", "c") -> "a\\nb\\nc"
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If one argument contains '\n' it has to be escaped
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e.g. send(s, "a\\n", "b", "c") -> "a\\\\n\\nb\\nc"
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If their are no arguments, an empty str is sent.
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To be safely and efficiently exchanged, arguments are packed into the
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following representation :
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+-----+-----+-----+-----+-----+---+
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| length n |byte0|byte1|...| n-1 |
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+-----+-----+-----+-----+-----+---+
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|<-2 bytes->|<------n bytes------>|
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The length is encoded into a two bytes unsigned integer in native
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e.g. send(s, "a", "b", "c") -> "\\x05\\x00a\\nb\\nc" (05 00 61 0a 62 0a 63)
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Recieved data has to be unpacked, splitted and unescaped.
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# we should have used xrdlib
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# but i don't know if this add another dependency
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# and a Packer object has to be created each call
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# if we get issues with the current protocol, we should
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_SIZEOF_HEADER = struct.calcsize(_HEADER_FORMAT)
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# 16 bits unsigned integer header in native order representif the length of
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# the following string. The string represents a '\n'-seperated tuple.
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This module implements a simple tuple transmission protocol. It's kept simple
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to be implementable in other languages as well.
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The protocol can only transmit lists of strings, which it splits up into chunks.
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A chunk consists of a LENGTH byte, the actual PAYLOAD, and a STATUS byte.
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A single string is split up into several chunks if it exceeds the maximum length
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which can be specified by the LENGTH byte (255), otherwise a string is one
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The STATUS byte after every chunk tells if the chunk is
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- continued in the next chunk (CONT)
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- the last chunk of a string (NEXT)
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- the last chunk in the transmission (END)
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In order to handle empty lists without getting too complicated, all lists are
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extended by an arbitrary first element which just gets ignored.
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["Hello", "World!"] is transmitted as:
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01 00 01 05 48 65 6C 6C 6F 01 06 57 6F 72 6C 64 21 02
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(1) ? NEXT (5) H e l l o NEXT (6) W o r l d ! END
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class XDRError(RuntimeError):
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_SEND_ERROR = "--SEND ERROR--"
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def send(s, *args):
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Writes data on a socket
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@param s: given socket
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@type s: socket object
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@param *args: variable length arguments
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NB: *args is a tuple and therefor a tuple is transmitted on the socket.
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That's why recv returns a tuple equals to args.
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data = '\n'.join([ str(a).replace('\n', '\\n') for a in args ])
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header = struct.pack(_HEADER_FORMAT, len(data))
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args = ["\0"] + list(args)
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chunks = [ a[i:i + 0xff] for i in range(0, len(a), 0xff) ]
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if (chunks): s.send(_CONT)
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if (args): s.send(_NEXT)
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Reads data on a socket
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@param s: given socket
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@type s: socket object
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@return : incoming data
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header = s.recv(_SIZEOF_HEADER)
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size = struct.unpack(_HEADER_FORMAT, header)[0]
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return tuple([ a.replace('\\n', '\n') for a in data.split('\n') ])
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length = ord(s.recv(1))
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if (length): chunk += s.recv(length)
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if (flag == _CONT): continue
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if (flag == _END): break
added
removed
utils/xdr.py
