fix(server): prevent zfs-init-tank from wiping pool on udev race

zpool create -f was called if `zpool import -d /dev/disk/by-id` failed,
which could happen due to a race with systemd-udev-settle. The disk
would then be visible by the time zpool create ran, silently destroying
all data on an otherwise-intact pool.

Fix: locate the data disk first, retry the import directly against it
as a fallback, then check zdb -l for existing ZFS label metadata before
concluding the disk is blank. Remove -f so zpool create refuses rather
than overwrites if a pool is present.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01ULXzafSDwGhmFGnn3LtDSQ

modules/build-types/server.nix

@@ -44,14 +44,8 @@ in
         exit 0
       fi
 
-      # Pool exists on a device but not yet imported — let the standard
-      # zfs-import-${poolName}.service handle it normally.
-      if zpool import -d /dev/disk/by-id -N "${poolName}" 2>/dev/null; then
-        exit 0
-      fi
-
-      # No pool found at all.  Create it on the Proxmox data disk (scsi1),
-      # which appears as /dev/disk/by-id/scsi-*drive-scsi1 inside the VM.
+      # Locate the data disk first — used for both the fallback import
+      # attempt and, only if the disk is genuinely blank, pool creation.
       DATA_DISK=""
       for candidate in /dev/disk/by-id/scsi-*drive-scsi1; do
         [[ "$candidate" == *-part* ]] && continue
@@ -63,8 +57,31 @@ in
         exit 1
       fi
 
+      # Try importing via the by-id symlink directory first (normal path),
+      # then fall back to scanning the disk directly.  The two-step exists
+      # because of a udev race: systemd-udev-settle.service can clear before
+      # /dev/disk/by-id/ entries are fully populated, causing the first
+      # import to fail even when the pool is intact on the disk.
+      if zpool import -d /dev/disk/by-id -N "${poolName}" 2>/dev/null; then
+        exit 0
+      fi
+      if zpool import -d "$DATA_DISK" -N "${poolName}" 2>/dev/null; then
+        exit 0
+      fi
+
+      # Both import attempts failed.  Before creating a new pool, verify the
+      # disk is genuinely blank — if ZFS label metadata is present the import
+      # failed for some other reason and we must not clobber existing data.
+      if zdb -l "$DATA_DISK" 2>/dev/null | grep -q "name: '${poolName}'"; then
+        echo "zfs-init-${poolName}: $DATA_DISK has ZFS pool '${poolName}' metadata but import failed — refusing to overwrite existing data. Run 'zpool import -d $DATA_DISK ${poolName}' manually to investigate." >&2
+        exit 1
+      fi
+
+      # Disk is genuinely blank: create the pool.  -f is intentionally
+      # omitted so that if we somehow reach this point with an existing pool
+      # on the disk, zpool refuses rather than silently destroying data.
       echo "zfs-init-${poolName}: creating pool on $DATA_DISK"
-      zpool create -f "${poolName}" "$DATA_DISK"
+      zpool create "${poolName}" "$DATA_DISK"
       ${lib.concatMapStrings (ds: ''
         zfs create "${poolName}/${ds}"
       '') poolDatasets}