bug#42001: “SQLite database is busy”: contention on the store database

[Mathieu Othacehe]

Hey Ludo,

> utimensat(AT_FDCWD, 
> "/gnu/store/whhc80cx1zpi9cj3bil3sp8wwzcf8p9p-image-root/gnu/store/n6l40ygclxfysnc2nxwrslnn5ab6wv3v-profile/lib/modules/5.4.47-gnu/kernel/net/netfilter",
>  [{tv_sec=1592814617, tv_nsec=396083395} /* 
> 2020-06-22T10:30:17.396083395+0200 */, {tv_sec=1, tv_nsec=0} /* 
> 1970-01-01T01:00:01+0100 */], 0) = 0
> chmod("/gnu/store/whhc80cx1zpi9cj3bil3sp8wwzcf8p9p-image-root/gnu/store/n6l40ygclxfysnc2nxwrslnn5ab6wv3v-profile/lib/modules/5.4.47-gnu/kernel/net/netfilter",
>  040555) = 0
> getrusage(RUSAGE_SELF, {ru_utime={tv_sec=1306, tv_usec=334062}, 
> ru_stime={tv_sec=143, tv_usec=819675}, ...}) = 0
> clock_gettime(CLOCK_PROCESS_CPUTIME_ID, {tv_sec=1450, tv_nsec=153748295}) = 0 
> lstat("/gnu/store/whhc80cx1zpi9cj3bil3sp8wwzcf8p9p-image-root/gnu/store/n6l40ygclxfysnc2nxwrslnn5ab6wv3v-profile/lib/modules/5.4.47-gnu/kernel/net/netfilter/xt_connlimit.ko",
>  {st_mode=S_IFLNK|0777, st_size=125, ...}) = 0

As discussed on IRC, those "image-root" derivations do exercise store
deduplication a lot, are could participate to the observed database
contention.

Turns out I used those derivations as "staging" directories that are
passed to GNU Xorriso when producing ISO images and "mke2fs" and
"mkdosfs" when producing raw disk-images. Using the store was also
convenient to ensure root ownership of the image files.

With 1cb9effc3e875ec7bae31bb06069d16ac89f7e1d and
7f75a7ec08975eb6d6e01db61bd6b91f447f655e, I remove those "image-root"
derivations, and proceed to image creation in a single derivation.

The later commit is a bit more tricky because "mke2fs" is not able to
override file permissions. I used "fakeroot" to make sure that the raw
disk-image files are owned by "root" and not "guixbuilder".

Hoping it will help,

Mathieu