Add flush method to Database trait

src/database/any.rs

@@ -233,6 +233,10 @@ impl Database for AnyDatabase {
     fn increment_last_index(&mut self, keychain: KeychainKind) -> Result<u32, Error> {
         impl_inner_method!(AnyDatabase, self, increment_last_index, keychain)
     }
+
+    fn flush(&mut self) -> Result<(), Error> {
+        impl_inner_method!(AnyDatabase, self, flush)
+    }
 }
 
 impl BatchOperations for AnyBatch {

src/database/keyvalue.rs

@@ -367,6 +367,10 @@ impl Database for Tree {
             Ok(val)
         })
     }
+
+    fn flush(&mut self) -> Result<(), Error> {
+        Ok(Tree::flush(self).map(|_| ())?)
+    }
 }
 
 impl BatchDatabase for Tree {

src/database/memory.rs

@@ -419,6 +419,10 @@ impl Database for MemoryDatabase {
 
         Ok(*value)
     }
+
+    fn flush(&mut self) -> Result<(), Error> {
+        Ok(())
+    }
 }
 
 impl BatchDatabase for MemoryDatabase {

src/database/mod.rs

@@ -134,6 +134,9 @@ pub trait Database: BatchOperations {
     ///
     /// It should insert and return `0` if not present in the database
     fn increment_last_index(&mut self, keychain: KeychainKind) -> Result<u32, Error>;
+
+    /// Force changes to be written to disk, returning the number of bytes written
+    fn flush(&mut self) -> Result<(), Error>;
 }
 
 /// Trait for a database that supports batch operations