Re: Parallel guix builds can trample?

[Phil Beadling <phil@beadling.co.uk>]

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Hi all,

The issue also exists when using --with-commit - see below for a refined
test that makes it trivial to demonstrate with any package where the source
is retrieved from git.

On Tue, 18 Jan 2022 at 10:10, Phil <phil@beadling.co.uk> wrote:

>
> > Philip Beadling <phil@beadling.co.uk> writes:
>


> >No, and we probably should do, even in spite of this issue.  We're going
> >to repeat the experiment now using --with-commit, to confirm behaviour
> there.
>
>
Let's try and build 2 package variations - for this experiement we don't
even have to run them at the same time:

mkdir checkout-test
cd checkout-test
mkdir cache1 cache2
XDG_CACHE_HOME=`realpath ./cache1` guix build
--with-commit=simm=*3981f60d81d3c9592d78b3e88545259713a93194
*simm
XDG_CACHE_HOME=`realpath ./cache2` guix build
--with-commit=simm=*ec1357c1fdf78a2ac1408d00c0c3e7d025ab9ade
*simm

Now look at the caches:

$ ll cache1/guix/checkouts

total 4.0K
-rw-rw-r-- 1 phil.beadling phil.beadling  10 Jan 18 12:37
last-expiry-cleanup
drwxrwxr-x 6 phil.beadling phil.beadling 162 Jan 18 12:37
lx4ipn27sk6uvu6b33kelt4tik65oianvl7b7yn5bpng7gpt4pyq
drwxrwxr-x 5 phil.beadling phil.beadling 115 Jan 18 12:37
mqqlbc4ksftu2tkbnxydosm4j6mynxsh5nmhpq3cfrofkktkhexq
drwxrwxr-x 5 phil.beadling phil.beadling 103 Jan 18 12:37
qqzuuoqxvqikivkpenqom43kzk2xr7nlntanbm6fpe7bci6kaenq
drwxrwxr-x 4 phil.beadling phil.beadling 202 Jan 18 12:37
smvbhjf46cbaxxju6fsqj7t6ujktxy5i7vhuhzpvgjt2vhid2vqq
drwxrwxr-x 5 phil.beadling phil.beadling 267 Jan 18 12:37
*xulvhb6agjrsf6x7wknutpn7wkt5i4nesk637czazvfxps357eoq*

$ ll cache2/guix/checkouts

total 4.0K
-rw-rw-r-- 1 phil.beadling phil.beadling  10 Jan 18 12:38
last-expiry-cleanup
drwxrwxr-x 6 phil.beadling phil.beadling 162 Jan 18 12:38
lx4ipn27sk6uvu6b33kelt4tik65oianvl7b7yn5bpng7gpt4pyq
drwxrwxr-x 5 phil.beadling phil.beadling 115 Jan 18 12:38
mqqlbc4ksftu2tkbnxydosm4j6mynxsh5nmhpq3cfrofkktkhexq
drwxrwxr-x 5 phil.beadling phil.beadling 103 Jan 18 12:39
qqzuuoqxvqikivkpenqom43kzk2xr7nlntanbm6fpe7bci6kaenq
drwxrwxr-x 4 phil.beadling phil.beadling 202 Jan 18 12:38
smvbhjf46cbaxxju6fsqj7t6ujktxy5i7vhuhzpvgjt2vhid2vqq
drwxrwxr-x 5 phil.beadling phil.beadling 267 Jan 18 12:38
*xulvhb6agjrsf6x7wknutpn7wkt5i4nesk637czazvfxps357eoq*

$

Despite asking for different variants of the simm package, they are both
stored under the same SHA.  If they both used the same cache directory (the
default) - they'd have overwritten each other.  Thus it is not safe to run
--with-commit or any other variation in parallel for the same user.

To prove I've matched the right SHA - I can back-out the original commit
ids provided to --with-commit:

$ cd
cache1/guix/checkouts/xulvhb6agjrsf6x7wknutpn7wkt5i4nesk637czazvfxps357eoq
&& git rev-parse HEAD && cd -

*3981f60d81d3c9592d78b3e88545259713a93194*
~/checkout-test

$ cd
cache2/guix/checkouts/xulvhb6agjrsf6x7wknutpn7wkt5i4nesk637czazvfxps357eoq
&& git rev-parse HEAD && cd -

*ec1357c1fdf78a2ac1408d00c0c3e7d025ab9ade*
~/checkout-test

$

Whilst my original setup was a bit of a edge-case, I think this refined
example demonstrates that the problem can occur with typical/reasonable use
of guix switches?  I can even think of how this could occur without the
user running in parallel - for example if a package required via nth-order
dependencies to build 2 versions of the same repo - it's possible that a
single "guix build" command could cause the issue?

Thoughts?

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