union store: introduce common type for the union store implmentation

Summary:
We will be implmenting multiple union stores and therefore, it makes
sense to encapsulate the common logic in its own type. This also abstracts the
usage of `RefCell` within the union store.

Reviewed By: jsgf

Differential Revision: D7884651

fbshipit-source-id: a74b6d9df5ee0d7d04359219e276fd5713b3a00b

lib/revisionstore/src/lib.rs

@@ -5,6 +5,8 @@
 #[macro_use]
 extern crate failure;
 
+mod unionstore;
+
 pub mod error;
 pub mod datastore;
 pub mod key;

lib/revisionstore/src/unionstore.rs

@@ -0,0 +1,46 @@
+// Copyright Facebook, Inc. 2018
+// Union store
+
+use std::cell::RefCell;
+use std::vec::IntoIter;
+
+pub struct UnionStore<T> {
+    stores: RefCell<Vec<T>>,
+}
+
+pub struct UnionStoreIterator<T>(IntoIter<T>);
+
+impl<T> UnionStore<T> {
+    pub fn new() -> UnionStore<T> {
+        UnionStore {
+            stores: RefCell::new(vec![]),
+        }
+    }
+
+    pub fn add(&mut self, item: T)
+    where
+        T: Clone,
+    {
+        self.stores.borrow_mut().push(item)
+    }
+}
+
+impl<'a, T> IntoIterator for &'a UnionStore<T>
+where
+    T: Clone,
+{
+    type Item = T;
+    type IntoIter = UnionStoreIterator<T>;
+
+    fn into_iter(self) -> Self::IntoIter {
+        UnionStoreIterator(self.stores.borrow().clone().into_iter())
+    }
+}
+
+impl<T> Iterator for UnionStoreIterator<T> {
+    type Item = T;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        self.0.next()
+    }
+}