Lightnion requires python 3.6 or later

[plcp]

We need dict to keep insertion order due to implementation details in lightnion/cell.

For example using python 3.5 we are randomly unable to validate versions cells during negotiation, failing with traces like:

Traceback (most recent call last):
  File "examples/link.py", line 13, in <module>
    link = lnn.link.initiate(sys_argv.addr, sys_argv.port)
  File "/vagrant/lightnion/link.py", line 203, in initiate
    version = negotiate_version(peer, versions, as_initiator=True)
  File "/vagrant/lightnion/link.py", line 132, in negotiate_version
    lnn.cell.versions.send(peer, lnn.cell.versions.pack(versions))
  File "/vagrant/lightnion/cell/versions.py", line 35, in pack
    length=len(versions))
  File "/vagrant/lightnion/cell/view.py", line 559, in set
    self.finalize(truncate=True)
  File "/vagrant/lightnion/cell/view.py", line 566, in finalize
    self._view, self.raw))
RuntimeError: Invalid payload for <lightnion.cell.view.fields object at 0x7f46a55b84e0> view: b'\x00\x04\x07\x00'

…or…

Traceback (most recent call last):
  File "examples/link.py", line 13, in <module>
    link = lnn.link.initiate(sys_argv.addr, sys_argv.port)
  File "/vagrant/lightnion/link.py", line 203, in initiate
    version = negotiate_version(peer, versions, as_initiator=True)
  File "/vagrant/lightnion/link.py", line 132, in negotiate_version
    lnn.cell.versions.send(peer, lnn.cell.versions.pack(versions))
  File "/vagrant/lightnion/cell/versions.py", line 35, in pack
    length=len(versions))
  File "/vagrant/lightnion/cell/view.py", line 559, in set
    self.finalize(truncate=True)
  File "/vagrant/lightnion/cell/view.py", line 566, in finalize
    self._view, self.raw))
RuntimeError: Invalid payload for <lightnion.cell.view.fields object at 0x7f7ece53f518> view: b'\x00\x04\x07'

…or…

Traceback (most recent call last):
  File "examples/link.py", line 13, in <module>
    link = lnn.link.initiate(sys_argv.addr, sys_argv.port)
  File "/vagrant/lightnion/link.py", line 203, in initiate
    version = negotiate_version(peer, versions, as_initiator=True)
  File "/vagrant/lightnion/link.py", line 132, in negotiate_version
    lnn.cell.versions.send(peer, lnn.cell.versions.pack(versions))
  File "/vagrant/lightnion/cell/versions.py", line 35, in pack
    length=len(versions))
  File "/vagrant/lightnion/cell/view.py", line 558, in set
    self.write(*kargs, **kwargs)
  File "/vagrant/lightnion/cell/view.py", line 555, in write
    self.raw = self._view.write(self.raw, value, **kwargs)
  File "/vagrant/lightnion/cell/view.py", line 626, in raw
    offset = self._parent.offset(self._field)
  File "/vagrant/lightnion/cell/view.py", line 547, in offset
    return self._view.offset(self.raw, field)
  File "/vagrant/lightnion/cell/view.py", line 303, in offset
    return super().offset(payload, field=field)
  File "/vagrant/lightnion/cell/view.py", line 191, in offset
    width = view.width(payload)
  File "/vagrant/lightnion/cell/view.py", line 406, in width
    offset = self.offset(payload, self.quantity - 1)
  File "/vagrant/lightnion/cell/view.py", line 391, in quantity
    return self.length.cache()
  File "/vagrant/lightnion/cell/common.py", line 5, in cache
    return super().cache() // 2
  File "/vagrant/lightnion/cell/view.py", line 703, in cache
    + 'Have you called .value() of parent view yet?')
RuntimeError: Bounded value unknown at runtime: Have you called .value() of parent view yet?

…or even sending invalid versions cells to the OR and then silently hanging:

send b'\x00\x04\x00\x00\x07\x00\x04\x00\x05'
^CTraceback (most recent call last):
  File "examples/link.py", line 13, in <module>
    link = lnn.link.initiate(sys_argv.addr, sys_argv.port)
  File "/vagrant/lightnion/link.py", line 203, in initiate
    version = negotiate_version(peer, versions, as_initiator=True)
  File "/vagrant/lightnion/link.py", line 134, in negotiate_version
    vercell = lnn.cell.versions.recv(peer)
  File "/vagrant/lightnion/cell/versions.py", line 41, in recv
    answer = peer.recv(_cell.header_legacy_view.width())
  File "/usr/lib/python3.5/ssl.py", line 922, in recv
    return self.read(buflen)
  File "/usr/lib/python3.5/ssl.py", line 799, in read
    return self._sslobj.read(len, buffer)
  File "/usr/lib/python3.5/ssl.py", line 585, in read
    v = self._sslobj.read(len)
KeyboardInterrupt

Having ordered dictionaries in python is only guaranteed since python 3.7 and thus makes us rely on a pretty recent guarantee of the python language.

We still have a lot of python 3.4 and python 3.5 out there where lightnion breaks badly, either making the python 3.6+ requirement explicit or supporting python3.4+ can be a good thing.

[plcp]

Started a rewrite of cell/view.py with proper semantics, testing, coverage and support for python3.4 and later.