backup: flock snapshot on backup start

An flock on the snapshot dir itself is used in addition to the group dir
lock. The lock is used to avoid races with forget and prune, while
having more granularity than the group lock (i.e. the group lock is
necessary to prevent more than one backup per group, but the snapshot
lock still allows backups unrelated to the currently running to be
forgotten/pruned).

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

src/backup/datastore.rs

@@ -334,15 +334,20 @@ impl DataStore {
     /// Creates a new backup snapshot inside a BackupGroup
     ///
     /// The BackupGroup directory needs to exist.
-    pub fn create_backup_dir(&self, backup_dir: &BackupDir) ->  Result<(PathBuf, bool), io::Error> {
+    pub fn create_locked_backup_dir(&self, backup_dir: &BackupDir)
+        -> Result<(PathBuf, bool, DirLockGuard), Error>
+    {
         let relative_path = backup_dir.relative_path();
         let mut full_path = self.base_path();
         full_path.push(&relative_path);
 
+        let lock = ||
+            lock_dir_noblock(&full_path, "snapshot", "internal error - tried creating snapshot that's already in use");
+
         match std::fs::create_dir(&full_path) {
-            Ok(_) => Ok((relative_path, true)),
-            Err(ref e) if e.kind() == io::ErrorKind::AlreadyExists => Ok((relative_path, false)),
-            Err(e) => Err(e)
+            Ok(_) => Ok((relative_path, true, lock()?)),
+            Err(ref e) if e.kind() == io::ErrorKind::AlreadyExists => Ok((relative_path, false, lock()?)),
+            Err(e) => Err(e.into())
         }
     }