Fix for buffer-pruned positions in MMR store

The MMR storage has a buffer for all changes so they can either
be discarded without side effects or committed to disk. Pruning
can also happen in the buffer if an input directly spends an
output in a block (not likely), or a fork has an input spending
a previous output within it.

When the buffer gets flushed to storage, the pruned element was
just skipped, causing an offset within the underlying storage and
a shorter file than expected. This fix writes zeroes instead, so
the size is consistent. Note that the zeroes will be removed with
all other pruned elements on next compaction.

Fixes #444

store/src/sumtree.rs

@@ -394,20 +394,27 @@ where
 	/// data has been successfully written to disk.
 	pub fn sync(&mut self) -> io::Result<()> {
 		// truncating the storage file if a rewind occurred
-		if let Some((pos, _, _)) = self.rewind {
 		let record_len = 32 + T::sum_len() as u64;
+		if let Some((pos, _, _)) = self.rewind {
 			self.hashsum_file.truncate(pos * record_len)?;
 		}
+
 		for elem in &self.buffer.elems {
-			if let Some(ref hs) = *elem {
-				if let Err(e) = self.hashsum_file.append(&ser::ser_vec(&hs).unwrap()[..]) {
+			let res = if let Some(ref hs) = *elem {
+				self.hashsum_file.append(&ser::ser_vec(&hs).unwrap()[..])
+			} else {
+				// the element has alredy been pruned in the buffer, we just insert
+				// zeros until compaction to avoid wrong hashum store offsets 
+				self.hashsum_file.append(&vec![0; record_len as usize])
+			};
+
+			if let Err(e) = res {
 				return Err(io::Error::new(
 					io::ErrorKind::Interrupted,
 					format!("Could not write to log storage, disk full? {:?}", e),
 				));
 			}
 		}
-		}
 
 		self.buffer_index = self.buffer_index + self.buffer.len();
 		self.buffer.clear();
@@ -422,7 +429,7 @@ where
 		if let Some((_, _, bi)) = self.rewind {
 			self.buffer_index = bi;
 		}
-		self.buffer = VecBackend::new();
+		self.buffer.clear();
 		self.remove_log.discard();
 		self.rewind = None;
 	}