[augeas-devel] Robust lens parsing

[Matthew Booth]

I just hit an issue where a grub.conf contained the following 2 lines:

serial -unit=0 -speed=115200
terminal -timeout=5 serial console

Those lines are incorrect. They should use '--' instead of '-' for 
options. I just checked the grub source and grub will be silently 
ignoring the options. Grub effectively reads them as:

serial
terminal serial console

However, the effect of the above on augeas is that grub.conf isn't 
parsed at all. I think much better behaviour would be to localise errors 
in the tree wherever possible according to syntax, and parse the rest as 
normal.

For a worked example, take the following grub.conf fragment:

terminal -timeout=5 serial console
default=0
frobnicate --foobaz
hiddenmenu

The above could be parsed as:

* terminal
   * #error = -timeout=5
   * serial
   * console
* default = 0
* #error = frobnicate --foobaz
* hiddenmenu

This is, of course, dependent on implementing this behaviour in each 
lens. The advantage of this is that arbitrary errors which don't cause 
failure can be ignored if desired, or highlighted otherwise. 
Specifically for virt-v2v, a misconfigured grub serial console won't 
prevent me from working out which kernel is supposed to be booted.

Firstly, do people agree this is the right thing to do?

Secondly, I only dip occasionally into lens development. I believe that 
implementing the above will require the use of a fall-through or a 
wildcard match. Wouldn't this cause a problem with common prefixes? I'm 
not sure how I'd go about implementing this.

Thanks,

Matt
-- 
Matthew Booth, RHCA, RHCSS
Red Hat Engineering, Virtualisation Team

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