Merge pull request #5766 from guanqun/db-tests

three small commits for HashDB and MemoryDB

util/src/hashdb.rs

@@ -86,7 +86,7 @@ pub trait HashDB: AsHashDB + Send + Sync {
 	fn emplace(&mut self, key: H256, value: DBValue);
 
 	/// Remove a datum previously inserted. Insertions can be "owed" such that the same number of `insert()`s may
-	/// happen without the data being eventually being inserted into the DB.
+	/// happen without the data being eventually being inserted into the DB. It can be "owed" more than once.
 	///
 	/// # Examples
 	/// ```rust
@@ -100,6 +100,10 @@ pub trait HashDB: AsHashDB + Send + Sync {
 	///   let key = &d.sha3();
 	///   m.remove(key);	// OK - we now owe an insertion.
 	///   assert!(!m.contains(key));
+	///   m.remove(key);	// OK - we now owe two insertions.
+	///   assert!(!m.contains(key));
+	///   m.insert(d);	// OK - still owed.
+	///   assert!(!m.contains(key));
 	///   m.insert(d);	// OK - now it's "empty" again.
 	///   assert!(!m.contains(key));
 	///   m.insert(d);	// OK - now we've

util/src/memorydb.rs

@@ -246,6 +246,10 @@ impl HashDB for MemoryDB {
 	}
 }
 
+#[cfg(test)]
+mod tests {
+	use super::*;
+
 	#[test]
 	fn memorydb_remove_and_purge() {
 		let hello_bytes = b"Hello world!";
@@ -282,10 +286,17 @@ fn consolidate() {
 		let insert_key = other.insert(b"arf");
 		main.emplace(insert_key, DBValue::from_slice(b"arf"));
 
+		let negative_remove_key = other.insert(b"negative");
+		other.remove(&negative_remove_key);	// ref cnt: 0
+		other.remove(&negative_remove_key);	// ref cnt: -1
+		main.remove(&negative_remove_key);	// ref cnt: -1
+
 		main.consolidate(other);
 
 		let overlay = main.drain();
 
 		assert_eq!(overlay.get(&remove_key).unwrap(), &(DBValue::from_slice(b"doggo"), 0));
 		assert_eq!(overlay.get(&insert_key).unwrap(), &(DBValue::from_slice(b"arf"), 2));
+		assert_eq!(overlay.get(&negative_remove_key).unwrap(), &(DBValue::from_slice(b"negative"), -2));
+	}
 }