implement to_string method, prevent memory index overwrite on validation step

src/profile/bookmark/memory.rs

@@ -23,8 +23,17 @@ impl Memory {
     /// Add new record with `request` as key and `id` as value
     /// * validate record with same key does not exist yet
     pub fn add(&self, request: String, id: i64) -> Result<(), Error> {
-        match self.index.borrow_mut().insert(request, id) {
-            Some(key) => Err(Error::Overwrite(key)), // @TODO prevent?
+        // Borrow shared index access
+        let mut index = self.index.borrow_mut();
+
+        // Prevent existing key overwrite
+        if index.contains_key(&request) {
+            return Err(Error::Overwrite(request));
+        }
+
+        // Slot should be free, let check it twice
+        match index.insert(request, id) {
+            Some(_) => return Err(Error::Unexpected),
             None => Ok(()),
         }
     }
@@ -34,15 +43,15 @@ impl Memory {
     pub fn delete(&self, request: &str) -> Result<(), Error> {
         match self.index.borrow_mut().remove(request) {
             Some(_) => Ok(()),
-            None => Err(Error::NotFound),
+            None => Err(Error::Unexpected), // @TODO
         }
     }
 
     /// Get `id` by `request` from memory index
     pub fn get(&self, request: &str) -> Result<i64, Error> {
         match self.index.borrow().get(request) {
-            Some(&id) => Ok(id),
-            None => Err(Error::NotFound),
+            Some(&value) => Ok(value),
+            None => Err(Error::Unexpected), // @TODO
         }
     }
 }