Re: [PATCH 4/4] Remove unnecessary cpio_init function

[David Gibson <david@gibson.dropbear.id.au>]

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On Fri, Apr 05, 2024 at 08:08:53PM +0200, Stefano Brivio wrote:
> On Fri, 22 Mar 2024 13:27:39 +1100
> David Gibson <david@gibson.dropbear.id.au> wrote:
> 
> > The cpio_init function is now only every called with the "discard" option.
> > But, moreover, what it does is create an initial mostly empty archive which
> > will just get overwritten by the final archive.
> > 
> > So, it's entirely unnecessary except for one subtlety.  Our use of realpath
> > when generating the final output requires that a file already exist in the
> > output location.  We can fix that by shuffling some things out of a
> > subshell, removing the need for realpath.
> > 
> > Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
> > ---
> >  mbuto | 31 ++++---------------------------
> >  1 file changed, 4 insertions(+), 27 deletions(-)
> > 
> > diff --git a/mbuto b/mbuto
> > index 550f76e..0c51e18 100755
> > --- a/mbuto
> > +++ b/mbuto
> > @@ -544,28 +544,6 @@ subopts_get() {
> >  
> >  ### CPIO #######################################################################
> >  
> > -# cpio_init() - Source existing CPIO archive, or create if needed
> > -# $1:	Path to CPIO archive, might exist, might be discarded if existing
> > -cpio_init() {
> > -	if [ -f "${OUT}" ] && [ "${1}" != "discard" ]; then
> > -		info "Sourcing CPIO archive from ${OUT}"
> > -
> > -		if ! "${GZIP}" -dfc "${OUT}" |
> > -		   "${CPIO}" --quiet -iD "${wd}"; then
> > -			err "Invalid CPIO archive ${OUT}"
> > -		fi
> > -	else
> 
> This, and the second part of 1/4, remove a functionality which I
> accidentally broke in commit b87e4f2e6595 ("mbuto: Create working
> directory before profiles are sourced"): it was once possible to add
> contents to an existing initramfs archive.

Yeah, I didn't track it to a specific point, but I assumed that it was
built to add to an existing archive in the past.  It looked fairly
thoroughly broken to me.

> I guess I might be the only interactive user of mbuto at the moment, so I
> got slightly annoyed by the fact it didn't work anymore but I didn't
> really investigate further. I used it whenever an initramfs took more than
> 5-10 seconds to build, and I'd keep forgetting to add stuff.

So... in fact the kernel accepts not just a single cpio archive, but
multiple.  So rather than doing anything with archivemount or mangling
the cpio, you could just append a secondary cpio with the additions.

> Note that the help message still describes this mode of operation:
> 
>   "Build initramfs image unless an existing one is passed."
> 
> and in the "Examples":
> 
> 	./mbuto -f kata.img zsh_5.6.2-3_amd64.deb
> 		Install zsh package to pre-existing kata.img

Ah, yes, they should be updated to match.

> but I also realised it's not convenient to have this as default, because,
> especially for initramfs images that can be built faster, one might just
> want to rebuild a single binary and the image, repeatedly.
> 
> Perhaps, instead of dropping this altogether, we could introduce a new
> -K ("keep") option that skips the unconditional wd assignment and calls
> cpio_init() (which becomes cpio_load() or something like that).
> 

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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