attr-sql: Handle concurrent insertion of identities

If the same identity is added concurrently by two threads (or by the
pool utility) INSERT might fail even though the SELECT was unsuccessful
before.

We are currently not able to lock the identities table in a portable way
(something like SELECT ... FOR UPDATE on MySQL).

src/libhydra/plugins/attr_sql/sql_attribute.c

@@ -50,19 +50,24 @@ static u_int get_identity(private_sql_attribute_t *this, identification_t *id)
 {
 	enumerator_t *e;
 	u_int row;
+	int try = 0;
 
+retry:
 	/* look for peer identity in the identities table */
 	e = this->db->query(this->db,
 						"SELECT id FROM identities WHERE type = ? AND data = ?",
 						DB_INT, id->get_type(id), DB_BLOB, id->get_encoding(id),
 						DB_UINT);
-
 	if (e && e->enumerate(e, &row))
 	{
 		e->destroy(e);
 		return row;
 	}
 	DESTROY_IF(e);
+	if (try > 0)
+	{
+		return 0;
+	}
 	/* not found, insert new one */
 	if (this->db->execute(this->db, &row,
 				  "INSERT INTO identities (type, data) VALUES (?, ?)",
@@ -70,7 +75,12 @@ static u_int get_identity(private_sql_attribute_t *this, identification_t *id)
 	{
 		return row;
 	}
-	return 0;
+	/* the INSERT could fail due to the UNIQUE constraint, if the identity was
+	 * added concurrently by another thread or the pool utility,
+	 * therefore try finding it again. a nicer fix would be to use locking
+	 * on the database, but our API currently not supports that */
+	try++;
+	goto retry;
 }
 
 /**