composefs/bls: Properly reuse kernel + initrd duplicates

Function `find_vmlinuz_initrd_duplicates` returns all the deployment ids
that match the current deployment's kernel + initrd SHA256 sum, but only
would actually exist on disk, every other deployment's BLS Config would
simply be pointing to one single directory.

We were incorrectly picking the first entry out of the returned list,
which might or might not exist as an actual directory containing the
kernel.

Fix the bug by iterating over all directory entries in `/boot` or
`/EFI/Linux` and checking if the deployment id in the list actually
exists on the disk or not

Signed-off-by: Pragyan Poudyal <[email protected]>

Author: Johan-Liebert1

crates/lib/src/bootc_composefs/boot.rs

@@ -504,19 +504,51 @@ pub(crate) fn setup_composefs_bls_boot(
                 });
 
             match find_vmlinuz_initrd_duplicates(&boot_digest)? {
-                Some(symlink_to) => {
-                    let symlink_to = &symlink_to[0];
+                Some(shared_entries) => {
+                    // Multiple deployments could be using the same kernel + initrd, but there
+                    // would be only one available
+                    //
+                    // Symlinking directories themselves would be better, but vfat does not support
+                    // symlinks
+
+                    let mut shared_entry: Option<String> = None;
+
+                    let entries =
+                        Dir::open_ambient_dir(entry_paths.entries_path, ambient_authority())
+                            .context("Opening entries path")?
+                            .entries_utf8()
+                            .context("Getting dir entries")?;
+
+                    for ent in entries {
+                        let ent = ent?;
+                        // We shouldn't error here as all our file names are UTF-8 compatible
+                        let ent_name = ent.file_name()?;
+
+                        if shared_entries.contains(&ent_name) {
+                            shared_entry = Some(ent_name);
+                            break;
+                        }
+                    }
+
+                    let shared_entry = shared_entry.ok_or_else(|| {
+                        anyhow::anyhow!("Could not get symlink to BLS boot entry")
+                    })?;
 
                     match bls_config.cfg_type {
                         BLSConfigType::NonEFI {
                             ref mut linux,
                             ref mut initrd,
                             ..
                         } => {
-                            *linux = entry_paths.abs_entries_path.join(&symlink_to).join(VMLINUZ);
-
-                            *initrd =
-                                vec![entry_paths.abs_entries_path.join(&symlink_to).join(INITRD)];
+                            *linux = entry_paths
+                                .abs_entries_path
+                                .join(&shared_entry)
+                                .join(VMLINUZ);
+
+                            *initrd = vec![entry_paths
+                                .abs_entries_path
+                                .join(&shared_entry)
+                                .join(INITRD)];
                         }
 
                         _ => unreachable!(),