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feat(incus-vm/nixos): bind-mount HDD store over /nix/store for result symlinks

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The VM image bakes the NixOS closure into /nix/store (ext4 ro on sda2).
The nix-shared profile mounts the ThinkStation HDD store at /nix-host/nix
via 9p. nix.settings.store redirects daemon writes to /nix-host/nix/store,
but result symlinks still resolve against /nix/store (the ext4 partition) -
causing "No such file or directory" when running built binaries.

Fix: add a systemd.mounts entry that bind-mounts /nix-host/nix/store over
/nix/store after local-fs.target (when the 9p share is up) and before
nix-daemon.service. This makes /nix/store point at the live HDD store,
so both nix daemon internals and result symlinks work correctly.

The provisioner also pre-applies the bind mount before nixos-rebuild switch
so the first rebuild (which produces a new system derivation) can activate
successfully - without the bind mount, activation aborts because it looks
for the newly built path in the stale ext4 /nix/store.
This commit is contained in:
Ben Potter 2026-04-24 21:22:29 +00:00
parent 668f776d87
commit fde4f8dbb9
1 changed files with 36 additions and 1 deletions
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37
registry/bpmct/templates/incus-vm/nixos.tf
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@ -79,11 +79,37 @@ resource "null_resource" "provision_nixos" {
nix.settings.allowed-users = [ "*" ];
nix.settings.store = "local?root=/nix-host&state=/nix/var/nix&log=/nix/var/log/nix";
# Create the mountpoint for the virtiofs share (Incus mounts it here).
# Create the mountpoint for the virtiofs/9p share (Incus mounts it here).
system.activationScripts.nix-host-dir = ''
mkdir -p /nix-host
'';
# Bind-mount /nix-host/nix/store over /nix/store so that result symlinks
# from nix-build (which point to /nix/store/...) resolve correctly.
#
# Background: the VM image bakes the NixOS closure into /nix/store on sda2
# (ext4, read-only). The nix-shared profile mounts the ThinkStation HDD
# share at /nix-host/nix via 9p. nix.settings.store redirects nix daemon
# writes to /nix-host/nix/store, but the result symlinks still say
# /nix/store/... — which points at the stale ext4 partition. The bind mount
# below shadows the ext4 mount with the live HDD store, so both nix internals
# and result symlinks work correctly.
#
# We order after local-fs.target (the 9p virtio share is mounted as part of
# local-fs) and before nix-daemon so the daemon always sees the unified store.
systemd.mounts = [
{
what = "/nix-host/nix/store";
where = "/nix/store";
type = "none";
options = "bind";
after = [ "local-fs.target" ];
before = [ "nix-daemon.service" ];
wantedBy = [ "multi-user.target" ];
requiredBy = [ "nix-daemon.service" ];
}
];
systemd.services.coder-agent = {
description = "Coder Agent";
after = [ "network-online.target" ];
@ -145,6 +171,15 @@ NIXMOD_EOF
fi"
echo "Running nixos-rebuild switch (this may take a few minutes)..."
# Pre-apply the bind mount before nixos-rebuild so the newly built system
# derivation lands in /nix/store (via the HDD store) and activation can
# find it. Without this, nixos-rebuild writes to /nix-host/nix/store but
# activation checks /nix/store (the ext4 ro partition) and aborts.
incus exec "$REMOTE:$INSTANCE" -- \
env PATH=/run/current-system/sw/bin /run/current-system/sw/bin/bash -c \
"if [ -d /nix-host/nix/store ]; then \
/run/current-system/sw/bin/mount --bind /nix-host/nix/store /nix/store 2>/dev/null && echo 'Bind-mounted /nix-host/nix/store -> /nix/store' || echo 'Bind mount skipped (already mounted or not needed)'; \
fi"
incus exec "$REMOTE:$INSTANCE" -- \
env PATH=/run/current-system/sw/bin /run/current-system/sw/bin/bash -l -c \
"nixos-rebuild switch; EC=\$?; [ \$EC -eq 0 ] || [ \$EC -eq 4 ] || exit \$EC"