Demi Marie Obenour <demiobenour@gmail.com> writes:

> This prevents lengthy and pointless scans of large image files.
> dontFixup is added to the derivations that already used
> unsafeDiscardReferences.  Both dontFixup and unsafeDiscardReferences are
> added to a derivation that builds an ext4 filesystem image.
>
> Signed-off-by: Demi Marie Obenour <demiobenour@gmail.com>
> ---
>  host/initramfs/default.nix             | 3 +++
>  host/rootfs/default.nix                | 1 +
>  img/app/default.nix                    | 2 ++
>  release/checks/integration/default.nix | 2 ++
>  release/combined/default.nix           | 3 +++
>  release/combined/eosimages.nix         | 1 +
>  release/live/default.nix               | 2 ++
>  vm-lib/make-vm.nix                     | 1 +
>  vm/sys/net/default.nix                 | 2 ++
>  9 files changed, 17 insertions(+)

Good idea!

> diff --git a/release/checks/integration/default.nix b/release/checks/integration/default.nix
> index 570918613e083b0c2d7c82486544b6405ae5ea89..582a4be8df3098ddaf318b7dac60690c2f8b2304 100644
> --- a/release/checks/integration/default.nix
> +++ b/release/checks/integration/default.nix
> @@ -37,6 +37,8 @@ let
>  
>    userData = runCommand "user-data.img" {
>      nativeBuildInputs = [ e2fsprogs tar2ext4 ];
> +    unsafeDiscardReferences = true;
> +    dontFixup = true;
>    } ''
>      tar -Pcvf root.tar \
>          --transform=s,^${appimage},test.appimage, ${appimage} \

This is not correct, because 1) unsafeDiscardReferences is an attribute
set; 2) it doesn't do anything without __structuredAttrs.

https://nix.dev/manual/nix/2.26/language/advanced-attributes#adv-attr-unsafeDiscardReferences

So I'm going to make a standalone commit first that adds
__structuredAttrs and unsafeDiscardReferences here, and then apply the
rest of this.  I'll also tweak it to group __structuredAttrs,
unsafeDiscardReferences, and dontFixup, so it's clear they should be
applied together if being copied.