op heads: add a new trait method combining add and remove of op heads

Consider how one would implment the current `OpHeadsStore` interface
for a cloud-based backend. After `OpHeadsStore::add_op_head()` is
called, the set of op heads temporarily contains two heads (typically)
until `OpHeadsStore::remove_op_head()` is called. That's not invalid,
but it's annoying to have to deal with that state more than
necessary. Also, it's unnecessarily inefficient to send the addition
and removal of op heads as separate RPCs. This patch therefore adds a
`update_op_heads()` method that takes a list of old heads to remove
and a single new head to add. Coming patches will start migrating to
that method.

lib/src/op_heads_store.rs

@@ -43,8 +43,15 @@ pub trait OpHeadsStoreLock<'a> {
 pub trait OpHeadsStore: Send + Sync + Debug {
     fn name(&self) -> &str;
 
+    /// Remove the old op heads and add the new one.
+    ///
+    /// The old op heads must not contain the new one.
+    fn update_op_heads(&self, old_ids: &[OperationId], new_id: &OperationId);
+
+    // TODO: migrate callers update_op_heads()
     fn add_op_head(&self, id: &OperationId);
 
+    // TODO: migrate callers update_op_heads()
     fn remove_op_head(&self, id: &OperationId);
 
     fn get_op_heads(&self) -> Vec<OperationId>;

lib/src/repo.rs

@@ -186,7 +186,7 @@ impl ReadonlyRepo {
         let init_operation_id = op_store.write_operation(&init_operation).unwrap();
         let init_operation = Operation::new(op_store.clone(), init_operation_id, init_operation);
         let op_heads_store = op_heads_store_initializer(user_settings, &op_heads_path);
-        op_heads_store.add_op_head(init_operation.id());
+        op_heads_store.update_op_heads(&[], init_operation.id());
         let op_heads_type_path = op_heads_path.join("type");
         fs::write(&op_heads_type_path, op_heads_store.name()).context(&op_heads_type_path)?;
         let op_heads_store = Arc::from(op_heads_store);

lib/src/simple_op_heads_store.rs

@@ -72,6 +72,13 @@ impl OpHeadsStore for SimpleOpHeadsStore {
         Self::name()
     }
 
+    fn update_op_heads(&self, old_ids: &[OperationId], new_id: &OperationId) {
+        self.add_op_head(new_id);
+        for old_id in old_ids {
+            self.remove_op_head(old_id)
+        }
+    }
+
     fn add_op_head(&self, id: &OperationId) {
         std::fs::write(self.dir.join(id.hex()), "").unwrap();
     }