[Buildroot] [RFC PATCH 2/3] download/git: recover dirty cache

[Yann E. MORIN <yann.morin.1998@free.fr>]

Ricardo, Arnout, All,

On 2018-04-17 05:56 -0300, Ricardo Martincoski spake thusly:
> On Tue, Apr 17, 2018 at 05:10 AM, Arnout Vandecappelle wrote:
> > On 17-04-18 09:04, Ricardo Martincoski wrote:
> >> Here is a squashed, no commit log, no comments on the code, hackish preview:
> >> https://gitlab.com/RicardoMartincoski/buildroot/commit/f4e20bb62b761bb6c82a29acbd2f8d56d90e4e9c

ACK. I'll vampirise the git-init part, then.

> >  Looks good to me, with the following mods:
> > 
> > * use GIT_DIR
> > * add comment above git init explaining that it is safe
> > * explain why -ff is needed for git clean

ACK ?3

> > * Removing the shallow fetch should be done in a separate patch (I'm not
> > entirely convinced yet that it is needed).

IIRC, Ricardo explained the case in a previous message: there are cases
where we may miss blobs in a tree.

But anyway, the current code only works by chance. Let's assume you want
to use tag v4.17-rc1 from Linus' tree. The code would currently do
something like (without an existing git cache)

    mkdir git
    git init git
    cd git
    git remote add origin https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
    git fetch origin --depth 1 v4.17-rc1
    if ! git fetch origin v4.17-rc1:v4.17-rc1 ; then
        echo "warning"
    fi
    git checkout v4.17-rc1

The if-block is initially there to fetch special refs created by things
like gerrit or github PRs. If you do the above, you get the warning,
i.e. the if-condition is false, i.e. fetching the tag that way fails
with exit-code 1, and this message:

    error: cannot update the ref 'refs/heads/v4.17-rc1': Trying to write
    non-commit object a79c33a10ce2764c62fb8af6cbb571752d55c1c0 to branch
    refs/heads/v4.17-rc1
    From https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux
     ! [new tag]         v4.17-rc1  -> v4.17-rc1  (unable to update local ref)
     * [new tag]         v4.17-rc1  -> v4.17-rc1

But since it is in a if-ciondition, we just print a warning, and
continue.

Then and the checkout succeeds.

But! If you didn't try to fetch the special refs, the checkout would
have failed:

    error: pathspec 'v4.17-rc1' did not match any file(s) known to git.

I.e. we have code that just happens to work by chance (or rather, by
side-effects and luck).

So, even if the missing-blob problem could be solved another way, our
shallow fetch is anyway already borked, but we were lucky so far not to
fall for it...

I still think that this shalow-fetch dance is too risky, and that having
a sane git cache helps so much more in the long run, so much so that we
should just ditch the shallow fetch support.

> These 2 are already covered by Yann's WIP series, with comments and proper
> commit logs:
> https://git.buildroot.org/~ymorin/git/buildroot/log/?h=yem/git-robust

With the last two to be swapped.

Regards,
Yann E. MORIN.

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