Re: Option parsing: Allow the same option multiple times

On 19/06/2023 08:18, Stefano Brivio wrote:
...
  On Mon, 19 Jun 2023 14:49:42 +1000
 David Gibson <david(a)gibson.dropbear.id.au> wrote:

  On Fri, Jun 16, 2023 at 11:04:00PM +0200, Stefano
Brivio wrote:
  On Thu, 15 Jun 2023 14:57:37 +0200
 Paul Holzinger <pholzing(a)redhat.com> wrote:

  Hi all,

 following up on a quick discussion with Stefano on IRC.

 passt/pasta currently rejects most (not all) options when specified
 multiple times, i.e. pasta -I eth0 -I eth1 ... fails. I think it makes
 more sense to just use the last one instead.

 My use case: In podman I added a new containers.conf option[1] which
 allows users to set default pasta cli options. However users can also
 add options on the podman cli with podman run --net=pasta:... For me it
 would make the most sense to just append those to the config options and
 then let pasta deal with it. This allows some form of overwrite
 mechanism, i.e. by default I may have "-I" , "eth0" in
containers.conf
 but for one specific container I want to use a different interface name
 and set --net=pasta:-I,eth1 on the cli. Then podman should just hand "-I
 eth0 -I eth1" to pasta and then pasta picks the last one.

 If we keep the current behavior it means I am forced to parse the
 options in podman and dedup them which is hard to maintain as podman
 would need to keep up with pasta upstream. 
 I had to do something similar in
libpod/networking_pasta_linux.go,
 which, if you change this, could also be simplified a bit.

  I am willing to send a patch to change this so
please let me know if
 anyone would object to that. 
 For the record, as I mentioned on IRC, I think it
makes sense.

 Maybe it's less "correct" as a behaviour, and it would make it a bit
 harder for users to spot (unlikely) mistakes on the command line, but
 making integrations simpler probably outweighs this.

 I don't have a good idea for sentences like "This option can be
 specified zero (for defaults) to two times (once for IPv4, once for IPv6)."
 that are currently in the man page... maybe we could switch from
 "This option can be specified" to "This option specifies one to
two..."
 and similar. 
 So, I'm actually a bit hesitant about applying this treatment
(allow
 multiple, last one wins) to -a specifically.  The reason being that we
 have draft plans to allow multiple addresses within the guest/ns.
 That might logically lead to allowing arbitrary numbers of -a options
 in future, where *all* the addresses apply.  If we'd previously
 allowed multiple options, but only the last one applies, that might be
 a breaking semantic change. 
 Uh oh, right... and it might also apply to -g /
--gateway eventually.

 On the other hand: are two semantic changes (both of which not breaking
 any reasonable or expected previous usage) in a relatively short period
 of time so much worse than one (we plan to change -a handling anyway)?

 I guess handling -a differently in Podman's options would still be
 additional effort for Paul. In my opinion, as long as we keep man pages
 updated, we should still prioritise making our lives (and the code!)
 easier.
 
If you plan to make changes like that I agree that supporting it might 
result in possible breaking changes for some users.

However I like to point out that the current behavior is inconsistent 
and only works for some options which is the reason I thought this would 
work in the first place, namely it works fine with `--ns-mac-addr` right 
now.


Another option for Podman would be to discard the options from the conf 
file when cli options are set so it is either or. Easy to do but I think 
it is not so great for users as this means a user would need to type all 
options again if they just want to change a single one.