[Libguestfs] [PATCH nbdkit 3/6] file: Make the file= parameter into a magic config key.

[Eric Blake]

On 09/07/2018 06:51 AM, Richard W.M. Jones wrote:

> +++ b/plugins/file/nbdkit-file-plugin.pod
> @@ -4,7 +4,7 @@ nbdkit-file-plugin - nbdkit file plugin
>   
>   =head1 SYNOPSIS
>   
> - nbdkit file file=FILENAME
> + nbdkit file FILENAME
>   
>   =head1 DESCRIPTION
>   
> @@ -26,6 +26,9 @@ be used here.
>   
>   This parameter is required.
>   
> +In nbdkit E<ge> 1.7, C<file=> may be omitted if the filename does not
> +start with a C<-> or contain an C<=> character.

This is accurate, but annoying for 'nbdkit file "$name"' when $name is 
not under your control.

With just a bit more code in patch 2/6, you could:
  s|contain an C<=> character|& with no earlier C</> character|

That is, if we assume/enforce that all valid key values that any plugin 
should ever want to support are also valid shell variable names, then we 
can tell that 'a=b' is a key/value, while './a=b' is an attempt to use 
the magic-key functionality (equivalent to 'file=./a=b').  Then, it 
becomes easy to write:

case $name in
   /*) nbdkit file "$name" ;;
   *) nbdkit file "./$name" ;;
esac

without worrying about leading - or contained = in $name.

Of course, you can still always write:
   nbdkit file file="$name"

when untrusted names are a concern, so I don't know if the extra magic 
is worth it.

Otherwise, I'm liking this series.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org