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/index.rs

@@ -0,0 +1,44 @@
+pub trait IndexFile {
+    fn index_count(&self) -> usize;
+    fn index_digest(&self, pos: usize) -> Option<&[u8; 32]>;
+}
+
+pub struct IndexIterator {
+    pos: usize,
+    count: usize,
+    reader: Box<dyn IndexFile + Send>,
+}
+
+impl Iterator for IndexIterator {
+    type Item = [u8; 32];
+
+    fn next(&mut self) -> Option<Self::Item> {
+        if self.pos == self.count {
+            return None;
+        }
+
+        let digest = self.reader.index_digest(self.pos).unwrap();
+        self.pos += 1;
+        Some(*digest)
+    }
+}
+
+impl From<Box<dyn IndexFile + Send>> for IndexIterator {
+    fn from(file: Box<dyn IndexFile + Send>) -> Self {
+        Self {
+            pos: 0,
+            count: file.index_count(),
+            reader: file,
+        }
+    }
+}
+
+impl<T: IndexFile + Send + 'static> From<Box<T>> for IndexIterator {
+    fn from(file: Box<T>) -> Self {
+        Self {
+            pos: 0,
+            count: file.index_count(),
+            reader: file,
+        }
+    }
+}