Only flush leaf_set if we have a prunable backend. (#2735)

store/src/pmmr.rs

@@ -261,10 +261,10 @@ impl<T: PMMRable> PMMRBackend<T> {
 	/// Syncs all files to disk. A call to sync is required to ensure all the
 	/// data has been successfully written to disk.
 	pub fn sync(&mut self) -> io::Result<()> {
-		self.hash_file
-			.flush()
+		Ok(())
+			.and(self.hash_file.flush())
 			.and(self.data_file.flush())
-			.and(self.leaf_set.flush())
+			.and(self.sync_leaf_set())
 			.map_err(|e| {
 				io::Error::new(
 					io::ErrorKind::Interrupted,
@@ -273,11 +273,19 @@ impl<T: PMMRable> PMMRBackend<T> {
 			})
 	}
 
+	// Sync the leaf_set if this is a prunable backend.
+	fn sync_leaf_set(&mut self) -> io::Result<()> {
+		if !self.prunable {
+			return Ok(());
+		}
+		self.leaf_set.flush()
+	}
+
 	/// Discard the current, non synced state of the backend.
 	pub fn discard(&mut self) {
 		self.hash_file.discard();
-		self.leaf_set.discard();
 		self.data_file.discard();
+		self.leaf_set.discard();
 	}
 
 	/// Takes the leaf_set at a given cutoff_pos and generates an updated
@@ -344,8 +352,7 @@ impl<T: PMMRable> PMMRBackend<T> {
 		self.data_file.replace(Path::new(&tmp_prune_file_data))?;
 
 		// 6. Write the leaf_set to disk.
-		// Optimize the bitmap storage in the process.
-		self.leaf_set.flush()?;
+		self.sync_leaf_set()?;
 
 		// 7. cleanup rewind files
 		self.clean_rewind_files()?;