Re: Why we should avoid new submodules if possible

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Sep 28, 2022 at 10:57:47AM +0100, Daniel P. Berrangé wrote:
> On Wed, Sep 28, 2022 at 05:53:17AM -0400, Michael S. Tsirkin wrote:
> > On Wed, Sep 28, 2022 at 10:37:14AM +0100, Daniel P. Berrangé wrote:
> > > On Wed, Sep 28, 2022 at 05:26:42AM -0400, Michael S. Tsirkin wrote:
> > > > On Tue, Jun 28, 2022 at 12:21:39PM +0200, Thomas Huth wrote:
> > > > > On 28/06/2022 12.03, Michael S. Tsirkin wrote:
> > > > > [...]
> > > > > > For biosbits if we are going this route then I feel a submodule is much
> > > > > > better.  It records which version exactly each qemu version wants.
> > > > > 
> > > > > As far as I know, you can also specify the version when using pip, can't
> > > > > you? So that's not really an advantage here.
> > > > > 
> > > > > On the contrary, submodules have a couple of disadvantages that I really
> > > > > dislike:
> > > > > 
> > > > > - submodules do not get updated automatically when doing a "git checkout",
> > > > > we have to update them via a script instead. This causes e.g. trouble if you
> > > > > rsync your source tree to a machine that has no access to the internet and
> > > > > you forgot to update the submodule before the sync
> > > > > 
> > > > > - the content of submodules is not added to the tarballs that get created on
> > > > > the git forges automatically. There were lots of requests from users in the
> > > > > past that tried to download a tarball from github and then wondered why they
> > > > > couldn't compile QEMU.
> > > > > 
> > > > > - we include the submodule content in our release tarballs, so people get
> > > > > the impression that hte submodule content is part of the QEMU sources. This
> > > > > has two disadvantages:
> > > > >  * We already got bug reports for the code in the submodule,
> > > > >    where people did not understand that they should report that
> > > > >    rather to the original project instead (i.e. you ship it - you
> > > > >    own it)
> > > > >  * People get the impression that QEMU is a huge monster
> > > > >    application if they count the number of code lines, run
> > > > >    their code scanner tools on the tarball contents, etc.
> > > > >    Remember "nemu", for example, where one of the main complaints
> > > > >    was that QEMU has too many lines of code?
> > > > > 
> > > > > - If programs includes code via submodules, this gets a higher
> > > > >   burder for distro maintainers, since they have to patch each
> > > > >   and every package when there is a bug, instead of being able to
> > > > >   fix it in one central place.
> > > > > 
> > > > > So in my opinion we should avoid new submodules if there is an alternative.
> > > > > 
> > > > >  Thomas
> > > > 
> > > > So looking at the latest proposals downloading files from CI,
> > > > checksumming them etc etc. No auto checkout, not added automatically
> > > > either, right?
> > > > 
> > > > This seems to be the only difference:
> > > > - we include the submodule content in our release tarballs
> > > 
> > > That's just one of the issues with submodules. Working with them in general
> > > is not a pleasant experiance.
> > 
> > This is what I asked about at the maintainers summit.
> > I'd like to map the issues and see if there's anything
> > we can do to solve them. In particular we will likely
> > keep using submodules where we historically did
> > so it's time well spent.
> > 
> > I agree generally but the big question is what to replace these with.
> > Below I assume the replacement is a script such as avocado or pytest
> > with its own hashing, calling wget internally etc etc.
> > 
> > 
> > > Thomas pointed out some of the issues, such
> > > as 'git checkout' ignoring submodules, requiring extra steps to sync them.
> > 
> > 
> > Not different from a home grown SCM as part of test script, right?
> 
> We're not building a home grown SCM as part of a test script, so
> this answer is irrelevant.

If you are managing contents of files using code then you have
a content managing system :)

> > > There's also the perenial problem that developers frequently send
> > > patches that mistakenly include submodule changes,
> > 
> > OK, so the thing to do would be to look for ways to exclude submodule changes
> > from git commits.
> 
> If someone wants to make git suck less with submodules great, but needs
> someone to actually do the work.

I have some experience with git, might be able to if I can figure out
what the issues are. Or maybe there's already a flag to do what we want
git is pretty configurable.

> > > I'd really like to see us doing more to eliminate as much use of submodules
> > > as is possible over time. 
> > 
> > Or try to fix the problems, right?
> 
> Again needs someone to actually make it happen.
> 
> Meanwhile  QEMU already has an integrated test harness in the form
> of Avocado that does everything needed. If Avocado had just been
> used for this biosbits test in the first place, the test would
> likely have already been merged to QEMU, instead of us having this
> never ending debate on how to re-invent an alternative to what
> already avocado does.
> 
> With regards,
> Daniel

I just think there's a ton of difference between linux guest images
which we want to keep running indefinitely and a unit test firmware
which we want to version with qemu.


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