add IndexFile trait

We want to be able to iterate through digests of index
files, but without always having to distinguish between
dynamic and fixed types, so add a trait we can use as a
trait object.

Unfortunately the iterator needs to yield copies as
iterators cannot yield values with life times when
represented as trait objects (Box<dyn Iterator<Item = ?>>)

Signed-off-by: Wolfgang Bumiller <w.bumiller@proxmox.com>

src/backup/dynamic_index.rs

@@ -1,6 +1,7 @@
 use failure::*;
 
 use crate::tools;
+use super::IndexFile;
 use super::chunk_stat::*;
 use super::chunk_store::*;
 use super::chunker::*;
@@ -196,6 +197,22 @@ impl DynamicIndexReader {
     }
 }
 
+impl IndexFile for DynamicIndexReader {
+    fn index_count(&self) -> usize {
+        self.index_entries
+    }
+
+    fn index_digest(&self, pos: usize) -> Option<&[u8; 32]> {
+        if pos >= self.index_entries {
+            None
+        } else {
+            Some(unsafe {
+                std::mem::transmute(self.chunk_digest(pos).as_ptr())
+            })
+        }
+    }
+}
+
 pub struct BufferedDynamicReader {
     index: DynamicIndexReader,
     archive_size: u64,