backup: use shared flock for base snapshot

To allow other reading operations on the base snapshot as well. No
semantic changes with this patch alone, as all other locks on snapshots
are exclusive.

Signed-off-by: Stefan Reiter <s.reiter@proxmox.com>

src/api2/backup.rs

@@ -16,7 +16,7 @@ use crate::backup::*;
 use crate::api2::types::*;
 use crate::config::acl::PRIV_DATASTORE_BACKUP;
 use crate::config::cached_user_info::CachedUserInfo;
-use crate::tools::fs::lock_dir_noblock;
+use crate::tools::fs::lock_dir_noblock_shared;
 
 mod environment;
 use environment::*;
@@ -144,7 +144,7 @@ async move {
 
         // lock last snapshot to prevent forgetting/pruning it during backup
         let full_path = datastore.snapshot_path(&last.backup_dir);
-        Some(lock_dir_noblock(&full_path, "snapshot", "base snapshot is already locked by another operation")?)
+        Some(lock_dir_noblock_shared(&full_path, "snapshot", "base snapshot is already locked by another operation")?)
     } else {
         None
     };