[Libguestfs] [PATCH v2] v2v: factor out bootloader handling

[Pino Toscano]

On Monday, 15 August 2016 16:26:51 CEST Richard W.M. Jones wrote:
> On Mon, Aug 15, 2016 at 04:48:29PM +0200, Pino Toscano wrote:
> > Create an object hierarchy to represent different bootloaders for Linux
> > guests, moving the separate handling of grub1 and grub2 in different
> > classes: this isolates the code for each type of bootloader together,
> > instead of scattering it all around.
> > 
> > This is mostly code refactoring, with no actual behaviour change.
> > ---
> >  po/POTFILES-ml       |   1 +
> >  v2v/Makefile.am      |   2 +
> >  v2v/bootloaders.ml   | 317 +++++++++++++++++++++++++++++++++++++++++++++++++++
> >  v2v/bootloaders.mli  |  44 +++++++
> 
> Is this really "bootloaders"?  It's more like "linux_bootloaders",
> unless this might one day be extended to include NTLDR.

OK, make sense -- I will rename it.

> > +class virtual bootloader = object
> > +  method virtual name : string
> > +  method virtual augeas_device_patterns : string list
> > +  method virtual list_kernels : unit -> string list
> > +  method virtual set_default_kernel : string -> unit
> > +  method set_augeas_configuration () = false
> > +  method virtual configure_console : unit -> unit
> > +  method virtual remove_console : unit -> unit
> > +  method update () = ()
> > +end
> 
> I think you'll also find that you don't need to use inheritance at
> all.  You can just declare bootloader as a class type and declare two
> (unrelated) classes which implement the class type.  They won't be
> able to inherit the two trivial non-virtual functions, but IMHO that's
> a feature.

Using inheritance helps here to spot typos in methods, and also when
implementing subclasses it'll be spotted if a method is not implemented.

> I still don't much see the point versus an implementation which used a
> type Bootloader.t declared as:
> 
>   type t = Grub1 of <any local data grub1 needs>
>          | Grub2 of <any local data grub2 needs>
> 
> (and not exposed through the mli file).  As this is to some extent my
> personal preference, I'll let it slide.

I did this approach as well, and it turns out that:
* every method is basically a giant "match t with Grub1 _ | Grub2 _",
  which makes it ugly to read: this is also because the code shared
  between one bootloader implementation and another is not really much,
  and currently only the "remove_hd_prefix" function is
* the implementation of a bootloader is scattered all around, so
  looking at what is done for a bootloader means looking everywhere
* passing a data type when calling every method is basically like
  what is done in C to mimim OOP; hence if I have to do something that
  resembles OOP but not actually doing it, then it will be a suboptimal
  implementation than OO (IMHO at least)

Thanks,
-- 
Pino Toscano
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