StoreBackend::backup_space(): add for statically allocated memory

src/lib.rs

@@ -12,8 +12,9 @@ use iter::Iter;
 pub mod no_flash;
 mod test;
 
+/// Backend interface for `Store`
 pub trait StoreBackend {
-    type Data: Sized + AsRef<[u8]> + Clone;
+    type Data: Sized + AsRef<[u8]>;
 
     /// Memory-mapped
     fn data(&self) -> &Self::Data;
@@ -22,10 +23,17 @@ pub trait StoreBackend {
     }
 
     type Error;
+    /// erase flash
     fn erase(&mut self) -> Result<(), Self::Error>;
+    /// program flash
     fn program(&mut self, offset: usize, payload: &[u8]) -> Result<(), Self::Error>;
     /// called after repeated `program()` invocations to allow for eg. cache flushing
     fn program_done(&mut self) {}
+
+    /// need for automatic compaction
+    ///
+    /// leaves memory management to the implementer
+    fn backup_space(&self) -> &'static mut [u8];
 }
 
 /// Simple Flash Key Value Store
@@ -114,7 +122,8 @@ impl<B: StoreBackend> Store<B> {
     }
 
     fn compact(&mut self) -> Result<(), Error<B::Error>> {
-        let old_data = self.backend.data().clone();
+        let old_data = self.backend.backup_space();
+        old_data.copy_from_slice(self.backend.data().as_ref());
 
         self.erase()?;
 

src/no_flash.rs

@@ -30,4 +30,8 @@ impl StoreBackend for NoFlash {
     fn program(&mut self, _: usize, _: &[u8]) -> Result<(), Self::Error> {
         Err(NoFlashError)
     }
+
+    fn backup_space(&self) -> &'static mut [u8] {
+        &mut []
+    }
 }

src/test.rs

@@ -3,6 +3,7 @@
 use crate::{Error, no_flash, Store, StoreBackend, WriteError};
 
 const SIZE: usize = 1024;
+static mut BACKUP_SPACE: [u8; SIZE] = [0; SIZE];
 
 pub struct TestBackend {
     data: [u8; SIZE],
@@ -39,6 +40,11 @@ impl StoreBackend for TestBackend {
         self.data[offset..end].copy_from_slice(payload);
         Ok(())
     }
+
+
+    fn backup_space(&self) -> &'static mut [u8] {
+        unsafe { &mut BACKUP_SPACE }
+    }
 }
 
 fn make_store() -> Store<TestBackend> {