mark_used_chunks: simply ignore vanished files

In case a prune operation removed a file in the meantime.

src/backup/datastore.rs

@@ -460,15 +460,27 @@ impl DataStore {
             worker.check_abort()?;
             tools::fail_on_shutdown()?;
 
+            let full_path = self.chunk_store.relative_path(&path);
+            match std::fs::File::open(&full_path) {
+                Ok(file) => {
                     if let Ok(archive_type) = archive_type(&path) {
                         if archive_type == ArchiveType::FixedIndex {
-                    let index = self.open_fixed_reader(&path)?;
+                            let index = FixedIndexReader::new(file)?;
                             self.index_mark_used_chunks(index, &path, status, worker)?;
                         } else if archive_type == ArchiveType::DynamicIndex {
-                    let index = self.open_dynamic_reader(&path)?;
+                            let index = DynamicIndexReader::new(file)?;
                             self.index_mark_used_chunks(index, &path, status, worker)?;
                         }
                     }
+                }
+                Err(err) => {
+                    if err.kind() == std::io::ErrorKind::NotFound {
+                        // simply ignore vanished files
+                    } else {
+                        return Err(err.into());
+                    }
+                }
+            }
             done += 1;
 
             let percentage = done*100/image_count;