fix(ecstore): parse ARN region and id in display order (#5790)

* test(ecstore): pin ARN display/parse round-trip field order

* fix(ecstore): parse ARN region and id in display order

crates/ecstore/src/bucket/target/arn.rs

@@ -56,11 +56,59 @@ impl FromStr for ARN {
         if parts.len() != 6 {
             return Err(std::io::Error::new(std::io::ErrorKind::InvalidInput, "Invalid ARN format"));
         }
+        // Display emits `arn:rustfs:{type}:{region}:{id}:{bucket}`; read the
+        // segments back in the same order so parse(display(a)) == a.
         Ok(ARN {
             arn_type: BucketTargetType::from_str(parts[2]).unwrap_or_default(),
-            id: parts[3].to_string(),
-            region: parts[4].to_string(),
+            region: parts[3].to_string(),
+            id: parts[4].to_string(),
             bucket: parts[5].to_string(),
         })
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    /// Display emits `arn:rustfs:{type}:{region}:{id}:{bucket}` (madmin layout);
+    /// FromStr must read the same positions back so parse(display(a)) == a.
+    #[test]
+    fn from_str_round_trips_display_with_region_and_id() {
+        let arn = ARN::new(
+            BucketTargetType::ReplicationService,
+            "depl-123".to_string(),
+            "us-east-1".to_string(),
+            "bucket-a".to_string(),
+        );
+
+        let parsed = ARN::from_str(&arn.to_string()).expect("display output must parse");
+
+        assert_eq!(parsed.arn_type, arn.arn_type);
+        assert_eq!(parsed.region, arn.region, "region must survive display->parse round-trip");
+        assert_eq!(parsed.id, arn.id, "id must survive display->parse round-trip");
+        assert_eq!(parsed.bucket, arn.bucket);
+    }
+
+    #[test]
+    fn from_str_reads_region_then_id_in_display_order() {
+        let parsed = ARN::from_str("arn:rustfs:replication:us-east-1:depl-123:bucket-a").expect("valid ARN must parse");
+
+        assert_eq!(parsed.arn_type, BucketTargetType::ReplicationService);
+        assert_eq!(parsed.region, "us-east-1");
+        assert_eq!(parsed.id, "depl-123");
+        assert_eq!(parsed.bucket, "bucket-a");
+    }
+
+    /// RustFS commonly generates ARNs with an empty region:
+    /// `arn:rustfs:replication::<deployment_id>:<bucket>`.
+    #[test]
+    fn from_str_handles_empty_region_segment() {
+        let parsed = ARN::from_str("arn:rustfs:replication::depl-123:bucket-a").expect("valid ARN must parse");
+
+        assert_eq!(parsed.arn_type, BucketTargetType::ReplicationService);
+        assert_eq!(parsed.region, "", "region segment is empty in this form");
+        assert_eq!(parsed.id, "depl-123");
+        assert_eq!(parsed.bucket, "bucket-a");
+    }
+}