tape: improve locking (lock media-sets)

- new helper: lock_media_set()

- MediaPool: lock media set

- Expose Inventory::new() to avoid double loading

- do not lock pool on restore (only lock media-set)

- change pool lock name to ".pool-{name}"

src/api2/tape/restore.rs

@@ -66,8 +66,8 @@ use crate::{
         TapeRead,
         MediaId,
         MediaCatalog,
-        MediaPool,
         Inventory,
+        lock_media_set,
         file_formats::{
             PROXMOX_BACKUP_MEDIA_LABEL_MAGIC_1_0,
             PROXMOX_BACKUP_SNAPSHOT_ARCHIVE_MAGIC_1_0,
@@ -161,11 +161,14 @@ pub fn restore(
         bail!("no permissions on /tape/drive/{}", drive);
     }
 
-    let status_path = Path::new(TAPE_STATUS_DIR);
-    let inventory = Inventory::load(status_path)?;
-
     let media_set_uuid = media_set.parse()?;
 
+    let status_path = Path::new(TAPE_STATUS_DIR);
+
+    let _lock = lock_media_set(status_path, &media_set_uuid, None)?;
+
+    let inventory = Inventory::load(status_path)?;
+
     let pool = inventory.lookup_media_set_pool(&media_set_uuid)?;
 
     let privs = user_info.lookup_privs(&auth_id, &["tape", "pool", &pool]);
@@ -192,8 +195,6 @@ pub fn restore(
 
             set_tape_device_state(&drive, &worker.upid().to_string())?;
 
-            let _lock = MediaPool::lock(status_path, &pool)?;
-
             let members = inventory.compute_media_set_members(&media_set_uuid)?;
 
             let media_list = members.media_list();