Curious unionfs-ro-mount for /gnu/store in installation-image

[Hartmut Goebel <h.goebel@crazy-compilers.com>]

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Hello,

immediately after booting the installation image (build according to [1]
and run according to [2]), the "mount" command spits out *two* unionfs
mounted points:

root@gnu ~# mount
unionfs on / type fuse.unionfs (rw,…)
unionfs on /gnu/store type fuse.unionfs (ro,…)

The first one ("/") is easy to explain: It is created by
gnu/build/linux-boot.scm (mount-root-file-system) if #:volatile-root is
true.

But I'm curious, where the second one comes from. I've been searching
this for two or three hours now and did not find, where this is defined.

- unionfs is used twice only in the whole code:
    1) In gnu/build/linux-boot.scm (mount-root-files-ystem) to make a
unionfs root – this is our first case
    2) in gnu/system/install.scm (make-cow-store) – which is not started
yet.

- gnu/system/install.scm defines the system to be put into the
installation image. Thuns I thought it may be a file-system defined in
install.scm. But related to the store there is only %immutable-store,
which does not use unionfs a ro-bind-mount, not a unionfs.

- I tried to find some hint in the disk-image, but did not. I found
"shepherd-file-system--gnu-store.scm", which implements %immutable-store".

Any ideas where this second unionfs-mount comes from?

[1]
https://www.gnu.org/software/guix/manual/guix.html#Building-the-Installation-Image
[2]
https://www.gnu.org/software/guix/manual/guix.html#Installing-GuixSD-in-a-VM

-- 
Regards
Hartmut Goebel

| Hartmut Goebel          | h.goebel@crazy-compilers.com               |
| www.crazy-compilers.com | compilers which you thought are impossible |


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