Re: [PATCH v2 0/3] Improving apt cache

[Uladzimir Bely <ubely@ilbers.de>]

In mail from четверг, 19 января 2023 г. 17:52:37 +03 user Roberto A. Foglietta 
wrote:
> On Thu, 19 Jan 2023 at 08:36, Uladzimir Bely <ubely@ilbers.de> wrote:
> > I got time to get back to this patches and checked this moment. And it
> > really
> > does not work as I expected.
> > 
> > Original patch includes the following:
> > 
> > -        --chroot-setup-commands="cp -n --no-preserve=owner
> > ${ext_deb_dir}/
> > *.deb -t ${deb_dir}/ || :" \
> > +        --chroot-setup-commands="ln -Pf ${ext_deb_dir}/*.deb -t
> > ${deb_dir}/
> > 2>/dev/null || :" \
> > 
> > This results to to failing "ReproTest" in CI with the following error (for
> > libhello, for example):
> > 
> > sbuild-build-depends-dose3-dummy:armhf : Depends: dose-distcheck:amd64 but
> > it
> > is not installable
> > E: Unable to correct problems, you have held broken packages.
> > 
> > To debug it, I removed `2>/dev/null` and found, that hardlinks simply
> > don't
> > work and the following errors are now seen earlier:
> > 
> > ln: failed to create hard link
> > '/var/cache/apt/archives/adduser_3.118_all.deb'
> > => '/home/builder/libhello/rootfs/var/cache/apt/archives/
> > adduser_3.118_all.deb': Invalid cross-device link
> > ... #tons of similar errors...
> > ln: failed to create hard link '/var/cache/apt/archives/
> > zlib1g_1%3a1.2.11.dfsg-2+deb11u2_mipsel.deb' => '/home/builder/libhello/
> > rootfs/var/cache/apt/archives/zlib1g_1%3a1.2.11.dfsg-2+deb11u2_mipsel.deb'
> > :
> > 
> > Invalid cross-device link
> > 
> > I: Finished running 'ln -Pf /home/builder/libhello/rootfs/var/cache/apt/
> > archives/*.deb -t /var/cache/apt/archives/ || :'.
> > 
> > So, it works for network builds (when missing packages always can be
> > downloaded by apt), but it fails for local builds from apt-cache (when, at
> > first network build, sbuild dependencies are simply not exported to
> > download
> > directory due to non-working hardlinks, plus with hidden stderr)
> 
> Hi, first of all I do not fully understand why we use ln -P instead of ln
> -Pf in the exporting debs.
> 
>         --finished-build-commands="ln -P ${deb_dir}/*.deb -t
> ${ext_deb_dir}/ 2>/dev/null || :" \
> 
> This code's goal is to build a deb that should be exported otherwise we are
> continuing using the old package. It seems that breaks things but - again -
> I did fully not understand what and why but simply accepted the suggestion.
> Now, I have reverted back to ln -Pf.
> 
> Second, those lines are supposed to fail - and obviously fail in both
> directions: import and export - but the following
> 
> (a) if export fail we will not have the custom packages but we have them
> (b) removing that lines and the code will always fail
> 
> These mean that ln -Pf complains to fail but make a difference and make the
> difference that we want. However, I might not have understood the case in
> which it fails completely so:
> 
> mv build/downloads .
> rm -rf build
> ./build.sh basic-os (DONE)
> ./clean.sh isar
> ./build.sh (DONE)
> ./clean.sh all
> ./build.sh (DONE)
> 
> remove that two lines about ln -Pf that are supposed to do nothing than fail
> 
> ./clean.sh all
> ./build.sh (DONE with the same building time)
> 
> rm -rf build/downloads
> mv downloads build
> 
> ./clean.sh isar
> ./build.sh complete (FAIL)
> 
> put back those two command lines with ln -Pf and
> 
> ./clean.sh all
> ./build.sh (DONE)
> 
> I do not say you are wrong and I see ln -Pf complains on stderr but nothing
> that tin my private fork cannot be solved using a stderr redirection to
> /dev/null
> 
> However there are some other ways to do this thing:
> 
> 1. sbuild uses schroot by default, using chroot instead
> 2. keep the default in schroot, elsewhere it uses upper folder that can be
> populated in advance
> 
> I did not make any changes because I did not identified the issue.
> 
> Best regards, R-

I'll try to make things clear. Isar supports local builds from 'base-apt' repo 
with.

First build user does as usual.

Second build is done with the following changes (at least):

ISAR_USE_CACHED_BASE_REPO = "1"
BB_NO_NETWORK = "1"

In this case every .deb we downloaded from Debian mirrors at first build goes 
to local 'base-apt' and second build use it, but not remote mirrors.

That's the reason of using this "cp/ln magic" inside of sbuild. While building 
some package, multipe dependencies might be downloaded inside of sbuild. And 
we need a way to extract them from "internal" apt-cache in sbuild schroot and 
put to the package workdir, where deb-dl-dir export will pass them to DL_DIR 
later.

When we replace copying to hardlinks, that actually don't work due to 
different filesystems, we come to the situation, when multiple debian packages 
(dependencies of what we are building or some packages that sbuild requires 
itself) are not present in DL_DIR. So, second "cached" build from local repo 
simply fails.

I see the following possible solutions that allows to save space.
1. Since hardlinks don't work, try to use symlinks.
2. Don't use import with sbuild at all, but only export. But this has a 
drawback, when all packages we build with sbuild might download the similar 
dependencies in parallel.