nuageinit: Add guards against empty user data

[jlduran]

Summary

Add guards against attempting to process an empty user data file.

PR: 290395

[kevans]

libexec/nuageinit/nuageinit

665–669

I would probably recommend something like this instead of the below change; in particular, it defends against the additional case below where we try to concatenate nil in the citype ~= "postnet" case. OTOH, I'm looking at that and content probably suffers from the same problem. I wondered if it was better to move the new condition up here, but I guess we probably still want to write the blank output to /var/cache/nuageinit?

[jlduran]

libexec/nuageinit/nuageinit

665–669

OK, yes, it is better.
However, I'm not sure if we want /var/cache/nuageinit with just a newline?

[kevans]

libexec/nuageinit/nuageinit

665–669

Right, maybe not- if it doesn't make sense to preserve an empty user_data, then you might as well preserve the nil-ness (so that you know it's empty and not just a blank line) and just move the check up. I wasn't really sure if it's feasible to reach this on a non-first boot and need to do something still with the user_data file written, but I guess the below open doesn't truncate it (and there's definitely no value in adding an extra blank line to it).

[jlduran]

• Guard earlier against line being empty
• Guard against content being empty

[jlduran]

At this point I'm lost against the file specifications:

• Guard against the file being empty or first line being empty
• Guard against the contents being empty

[jlduran]

These are the current checks:

• There is no file
• The file is empty
• The file contains a number of new lines (and nothing else)
• The file has the first line empty, and then are valid contents <- I'm not sure if this is valid?

[jlduran]

libexec/nuageinit/nuageinit

666

This checks that the first line is not blank (no white spaces, tabs, non-printable characters, etc.).

If you think this blank-string check could be useful in other parts of nuageinit, I can create a function in nuage.lua.

[jlduran]

• The first line of the user data file cannot be empty