Cannot disable the use of the sandbox with --store /path/

[georgyo]

Describe the bug
If either you or the nix daemon set the real nix store path then setting sandbox = false or sandbox = relaxed have no effect. The same is true for command line flags.

With any of these store paths (non-inclusive)

• --store /path/to/store
• --store "local?real=/path/to/store"

Disables without any error message these flags

• --option sandbox false
• --option sandbox relaxed
• --no-sandbox

Steps To Reproduce

This fails, despite you having full control over the nix store

nix-build \
  --option sandbox relaxed \
  --store $HOME/nix \
  -E 'derivation { name = "foo"; system = builtins.currentSystem; builder = "/usr/bin/bash"; args = [ "-c" "echo '$RANDOM' > $out" ]; __noChroot = true; }'

This succeeds as long as you are trusted or the daemon does not have --store /path/ or --store ...?real=/path

nix-build \
  --option sandbox relaxed \
  -E 'derivation { name = "foo"; system = builtins.currentSystem; builder = "/usr/bin/bash"; args = [ "-c" "echo '$RANDOM' > $out" ]; __noChroot = true; }'

This also succeeds as long as the daemon does not change it's real store

nix-build \
  --option sandbox relaxed \
  --store unix:///nix/var/nix/daemon-socket/socket \
  -E 'derivation { name = "foo"; system = builtins.currentSystem; builder = "/usr/bin/bash"; args = [ "-c" "echo '$RANDOM' > $out" ]; __noChroot = true; }'

BUT It also fails if the daemon was set with a real store path.

# as root
systemctl stop nix-daemon.socket nix-daemon.service
MY_NIX_ROOT=$(mktemp -d)
nix daemon --store $MY_NIX_ROOT

# in another shell.
nix-build \
  --option sandbox relaxed \
  --store unix:///nix/var/nix/daemon-socket/socket \
  -E 'derivation { name = "foo"; system = builtins.currentSystem; builder = "/usr/bin/bash"; args = [ "-c" "echo '$RANDOM' > $out" ]; __noChroot = true; }'
  
 # For completeness this also fails
 nix-build \
  --option sandbox relaxed \
  --store "unix:///nix/var/nix/daemon-socket/socket?real=$MY_NIX_ROOT" \
  -E 'derivation { name = "foo"; system = builtins.currentSystem; builder = "/usr/bin/bash"; args = [ "-c" "echo '$RANDOM' > $out" ]; __noChroot = true; }'

NOTE: $RANDOM is not escaped in these examples so we get a different derivation each time for testing.

NOTE: Note we set the builder to /usr/bin/bash, as that is likely not to be in the nix sandbox. Changing it to /bin/sh would make all these tests pass because that is in the sandbox, but you still could not access files outside the sandbox.

Expected behavior

I would expect all of the above to behave the same. It is surprising you cannot disable the sandbox when you fully own the nix store already.

nix-env --version output

Tested on

• nix-env (Nix) 2.19.4
• nix-env (Nix) 2.23.3
• nix-env (Nix) 2.25.0pre20240813_049d091

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