lib: Use Device::root_disk() for ESP and parent device lookups

All callers that need the whole-disk device (e.g. to find the ESP
partition) now use root_disk() to traverse from the filesystem
partition to the root disk.

This fixes list_dev_by_dir() consumers (store, bootloader, composefs
boot) that were searching for ESP partitions on a partition device
rather than the whole disk, and simplifies install.rs which had an
open-coded parent walk.

Assisted-by: Claude Code (Opus 4)
Signed-off-by: ckyrouac <ckyrouac@redhat.com>

Author: ckyrouac

crates/lib/src/bootc_composefs/boot.rs

@@ -535,7 +535,7 @@ pub(crate) fn setup_composefs_bls_boot(
             cmdline.add_or_modify(&param);
 
             // Locate ESP partition device
-            let root_dev = bootc_blockdev::list_dev_by_dir(&storage.physical_root)?;
+            let root_dev = bootc_blockdev::list_dev_by_dir(&storage.physical_root)?.root_disk()?;
             let esp_dev = root_dev.find_partition_of_esp()?;
 
             (
@@ -1067,7 +1067,7 @@ pub(crate) fn setup_composefs_uki_boot(
             let bootloader = host.require_composefs_booted()?.bootloader.clone();
 
             // Locate ESP partition device
-            let root_dev = bootc_blockdev::list_dev_by_dir(&storage.physical_root)?;
+            let root_dev = bootc_blockdev::list_dev_by_dir(&storage.physical_root)?.root_disk()?;
             let esp_dev = root_dev.find_partition_of_esp()?;
 
             (sysroot, esp_dev.path(), bootloader, false, None)

crates/lib/src/bootloader.rs

@@ -45,7 +45,7 @@ pub(crate) fn mount_esp_part(root: &Dir, root_path: &Utf8Path, is_ostree: bool)
         root
     };
 
-    let dev = bootc_blockdev::list_dev_by_dir(physical_root)?;
+    let dev = bootc_blockdev::list_dev_by_dir(physical_root)?.root_disk()?;
     if let Some(esp_dev) = dev.find_partition_of_type(bootc_blockdev::ESP) {
         let esp_path = esp_dev.path();
         bootc_mount::mount(&esp_path, &root_path.join(&efi_path))?;

crates/lib/src/install.rs

@@ -2442,42 +2442,9 @@ pub(crate) async fn install_to_filesystem(
     // Find the real underlying backing device for the root.  This is currently just required
     // for GRUB (BIOS) and in the future zipl (I think).
     let device_info = {
-        let dev = bootc_blockdev::list_dev(Utf8Path::new(&inspect.source))?;
-        // Walk the inverse tree to the root device.
-        // In inverse lsblk output, "children" are actually parents.
-        match dev.list_parents()? {
-            None => {
-                tracing::debug!("Backing device: {}", dev.path());
-                dev
-            }
-            Some(parents) => {
-                let mut current = parents;
-                loop {
-                    if current.len() > 1 {
-                        anyhow::bail!(
-                            "Found multiple parent devices {} and {}; not currently supported",
-                            current[0].path(),
-                            current[1].path()
-                        );
-                    }
-                    let mut parent = current.into_iter().next().unwrap();
-                    // In inverse output, children = grandparents
-                    match parent.children.take() {
-                        Some(grandparents) if !grandparents.is_empty() => {
-                            current = grandparents;
-                        }
-                        _ => {
-                            tracing::debug!("Backing device: {}", parent.path());
-                            // Re-query the root device to populate its actual
-                            // children (partitions). The inverse tree only
-                            // carries parent links, not real children.
-                            parent.refresh()?;
-                            break parent;
-                        }
-                    }
-                }
-            }
-        }
+        let dev = bootc_blockdev::list_dev(Utf8Path::new(&inspect.source))?.root_disk()?;
+        tracing::debug!("Backing device: {}", dev.path());
+        dev
     };
 
     let rootarg = format!("root={}", root_info.mount_spec);

crates/lib/src/store/mod.rs

@@ -173,7 +173,7 @@ impl BootedStorage {
                 let composefs = Arc::new(composefs);
 
                 //TODO: this assumes a single ESP on the root device
-                let root_dev = bootc_blockdev::list_dev_by_dir(&physical_root)?;
+                let root_dev = bootc_blockdev::list_dev_by_dir(&physical_root)?.root_disk()?;
                 let esp_dev = root_dev.find_partition_of_esp()?;
                 let esp_mount = mount_esp(&esp_dev.path())?;