Re: A corner case of broken reproducibility

[raingloom]

On Wed, 01 Jun 2022 22:09:11 +0200
Maxime Devos <maximedevos@telenet.be> wrote:

> Ludovic Courtès schreef op wo 01-06-2022 om 18:38 [+0200]:
> > There’s a talk by Lennart Poettering where he explains that,
> > contrary to what one might think, “chown -R $HOME” turns out to be
> > fast enough that systemd-homed can do that unconditionally (off the
> > of my head).  
> 
> Interesting.
> Taking "find $HOME > /dev/null" as an approximation of how long "chown
> -R $HOME" would take:
> 
> $ time find . > /dev/null 
> 
> real  0m7,066s
> user  0m0,427s
> sys   0m1,341s
> 
> Assuming that ‘unconditionally = only chown -R $HOME if the uid of
> $HOME isn't what was expected’ (otherwise, +7sec for every boot),
> that's not too bad.
> 
> That's on a SSD and not a hard disk though, I'd image it would be
> worse on a hard disk.
> 
> Depending on the size of $HOME, it could be a lot longer though:
> 
> $ time find /gnu/store > /dev/null 
> 
> real  1m9,729s
> user  0m4,135s
> sys   0m13,840s
> 
> Might still be acceptable as long as uids aren't switched too often
> ...
> 
> Can we, as-is in Guix, assume we can modify /home/foo though?  E.g.
> what if it's put on a separate storage medium not attached at boot.
> 
> Greetings,
> Maxime.

Could we instead check for existing homes and set uids in
/etc/passwd based on that instead? That's practically O(1), but is a
bit more involved.