initial support for kernel data download (#2765)

* initial support for kernel data download

* fix vec backend for tests

* cleanup after rebase

core/src/core/pmmr/backend.rs

@@ -12,6 +12,8 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+use std::fs::File;
+
 use croaring::Bitmap;
 
 use crate::core::hash::Hash;
@@ -59,6 +61,11 @@ pub trait Backend<T: PMMRable> {
 	/// triggered removal).
 	fn remove(&mut self, position: u64) -> Result<(), String>;
 
+	/// Creates a temp file containing the contents of the underlying data file
+	/// from the backend storage. This allows a caller to see a consistent view
+	/// of the data without needing to lock the backend storage.
+	fn data_as_temp_file(&self) -> Result<File, String>;
+
 	/// Release underlying datafiles and locks
 	fn release_files(&mut self);
 

core/src/core/pmmr/rewindable_pmmr.rs

@@ -49,6 +49,11 @@ where
 		}
 	}
 
+	/// Reference to the underlying storage backend.
+	pub fn backend(&'a self) -> &Backend<T> {
+		self.backend
+	}
+
 	/// Build a new readonly PMMR pre-initialized to
 	/// last_pos with the provided backend.
 	pub fn at(backend: &'a B, last_pos: u64) -> RewindablePMMR<'_, T, B> {