Handle older systemd-repart without partno output in JSON

Older systemd-repart versions (ex. the one in c9s) do not
include the partno field in the JSON output. Make the field
optional and fall back to the array index when absent

Signed-off-by: Pragyan Poudyal <pragyanpoudyal41999@gmail.com>

Author: Johan-Liebert1

crates/lib/src/install/baseline.rs

@@ -210,7 +210,10 @@ struct RepartPartition {
     #[serde(rename = "type")]
     partition_type: String,
     /// 0-indexed partition number
-    partno: u32,
+    /// absent in older systemd-repart
+    /// versions, in c9s
+    #[serde(default)]
+    partno: Option<u32>,
     #[allow(dead_code)]
     fs: Option<String>,
 }
@@ -338,9 +341,11 @@ fn parse_repart_layout(partitions: &[RepartPartition]) -> Result<PartitionLayout
     let mut boot_partno = None;
     let mut rootpn = None;
 
-    for part in partitions {
+    for (idx, part) in partitions.iter().enumerate() {
         // repart partno is 0-indexed, lsblk/sfdisk use 1-indexed
-        let partno = part.partno + 1;
+        // Fall back to array indexing for older systemd-repart without partno
+        // like the version in c9s
+        let partno = part.partno.unwrap_or(idx as u32) + 1;
 
         match part.partition_type.as_str() {
             "esp" => esp_partno = Some(partno),