Better support for fixed length strings in SVG message drawing code.

[nickbroon]

If the basic example of a string is changed to a fixed length of 13 (which exactly matches the string length passed to the data record) it works just fine:

import ipfix
import ipfix.vis
from IPython.display import SVG

def draw_message(msg, length=256):
 return SVG(ipfix.vis.MessageBufferRenderer(msg, raster=4).render(length=length))

def draw_template(tmpl):
 ofd = ipfix.vis.OctetFieldDrawing(raster=4)
 ipfix.vis.draw_template(ofd, tmpl)
 return SVG(ofd.render((90,30)))

ipfix.ie.use_iana_default()
vtmpl = ipfix.template.for_specs(262, "wlanSSID[13]")
draw_template(vtmpl)

msg = ipfix.message.MessageBuffer()
msg.begin_export(8304)
msg.add_template(vtmpl)
msg.export_new_set(262)
msg.export_namedict({'wlanSSID': 'ietf-a-v6only'})
draw_message(msg)

If it's changed to 12 it also works just fine, truncating the string to fit.

If it's changed to 14, or any other larger value, it crashes.
I'm guessing that it needs to correctly zero pad the string.

---------------------------------------------------------------------------
ExpatError Traceback (most recent call last)
<ipython-input-3-9dedd178bbfb> in <module>()
 5 msg.export_namedict({'wlanSSID': 'ietf-a-v6only',
 6 'protocolIdentifier': 6})
----> 7 draw_message(msg)

<ipython-input-1-ffa57e3f6760> in draw_message(msg, length)
 5 
 6 def draw_message(msg, length=256):
----> 7 return SVG(ipfix.vis.MessageBufferRenderer(msg, raster=4).render(length=length))
 8 
 9 def draw_template(tmpl):

/usr/lib/python3/dist-packages/IPython/core/display.py in __init__(self, data, url, filename)
 388 data = None
 389 
--> 390 self.data = data
 391 self.url = url
 392 self.filename = None if filename is None else unicode_type(filename)

/usr/lib/python3/dist-packages/IPython/core/display.py in data(self, svg)
 496 from xml.dom import minidom
 497 svg = cast_bytes_py2(svg)
--> 498 x = minidom.parseString(svg)
 499 # get svg tag (should be 1)
 500 found_svg = x.getElementsByTagName('svg')

/usr/lib/python3.5/xml/dom/minidom.py in parseString(string, parser)
 1966 if parser is None:
 1967 from xml.dom import expatbuilder
-> 1968 return expatbuilder.parseString(string)
 1969 else:
 1970 from xml.dom import pulldom

/usr/lib/python3.5/xml/dom/expatbuilder.py in parseString(string, namespaces)
 923 else:
 924 builder = ExpatBuilder()
--> 925 return builder.parseString(string)
 926 
 927 

/usr/lib/python3.5/xml/dom/expatbuilder.py in parseString(self, string)
 221 parser = self.getParser()
 222 try:
--> 223 parser.Parse(string, True)
 224 self._setup_subset(string)
 225 except ParseEscape:

ExpatError: not well-formed (invalid token): line 1, column 4720

[britram]

Argh. Yeah, this code was designed as a hack for a tutorial I did, pretty much the night before. I really don't have the cycles to swap this back in at the moment, but pull requests will be enthusiastically accepted here. :)

[britram]

Note that the SVG drawing code has been moved into its own package visipfix, which will be uploaded to pypi shortly...