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Ephemeral Network APIs


Hi folks,

i'm writing this so long as it's fresh to my memory, as we discussed it on Friday evening in IRC … despite the subject, I think it should be persisted somewhere ;)

I've been working on "porting" coudinit/net to freebsd:

- https://code.launchpad.net/~i.galic/cloud-init/+git/cloud-init/+merge/358228
- https://lists.launchpad.net/cloud-init/msg00185.html

so my problem is with class EphemeralIPv4Network

"""Context manager which sets up temporary static network configuration.

No operations are performed if the provided interface is already connected. If unconnected, bring up the interface with valid ip, prefix and broadcast.
If router is provided setup a default route for that interface. Upon
context exit, clean up the interface leaving no configuration behind.

Let me emphasize, "No operations are performed if the provided interface is already connected."

However, this is currently not done, or at least not directly:


   def __enter__(self):
"""Perform ephemeral network setup if interface is not connected."""
       if self.router:

it's only that somewhere in _bringup_device()'s exception path, that "nothing" is done.

Given this convoluted code, it's perhaps no wonder that EphemeralIPv4Network is currently suffering from this bug:

- https://bugs.launchpad.net/cloud-init/+bug/1802598

I think from an API point of view, it's important that "No operation is performed". Especially since this class implements __enter__() and __exit__()[1] — and is used by six different sources in this exact way.

Scott brought up the issue of performance, of wasted cycles thru repeated checking if the link is_up(). But i'm convinced that the benefit from a consistent API is far greater than a few wasted calls that are probably easily cached by a modern Kernel.

To summarize:

- EphemeralIPv4Network is currently not implemented to its own spec
- EphemeralIPv4Network has basic functionality bugs[2]

Fixing these issues would make porting cloudinit.net to non-linux platforms easier as well.


[1]: see https://docs.python.org/3/reference/compound_stmts.html#with for that [2]: you could say it's not idempotent, but i would claim it's not functional