Add an iterator over values (#3247)

# Motivation
The accounts store tests include an iterator over values. We will need
that in account databases, if we are to keep that test in the current
form. There seems no harm in having such an iterator so just add it.

# Changes
* Add an iterator over accounts.

# Tests
- This PR includes a simple test that inserts a few values and verifies
that the account IDs from the iterator are as expected.
- The iterator will be used in this test:
https://github.com/dfinity/nns-dapp/blob/main/rs/backend/src/accounts_store/tests.rs#L1378

# Todos

- [ ] Add entry to changelog (if necessary).
I think this is not necessary; this is a small PR currently affecting
tests only and has no user-facing changes.

---------

Co-authored-by: Max Murphy-Skvorzov <[email protected]>

rs/backend/src/accounts_store/schema.rs

@@ -77,4 +77,7 @@ pub trait AccountsDbTrait {
             None
         }
     }
+
+    /// Iterates over accounts in the data store.
+    fn values(&self) -> Box<dyn Iterator<Item = Account> + '_>;
 }

rs/backend/src/accounts_store/schema/map.rs

@@ -24,6 +24,10 @@ impl AccountsDbTrait for AccountsDbAsMap {
     fn db_accounts_len(&self) -> u64 {
         self.accounts.len() as u64
     }
+    fn values(&self) -> Box<dyn Iterator<Item = Account> + '_> {
+        let iterator = self.accounts.values().cloned();
+        Box::new(iterator)
+    }
 }
 
 #[cfg(test)]
@@ -55,4 +59,9 @@ mod tests {
     fn map_account_counts_should_be_correct() {
         generic_tests::assert_account_count_is_correct(AccountsDbAsMap::default());
     }
+
+    #[test]
+    fn map_accounts_db_should_iterate_over_values() {
+        generic_tests::assert_iterates_over_values(AccountsDbAsMap::default());
+    }
 }

rs/backend/src/accounts_store/schema/tests.rs

@@ -2,7 +2,7 @@
 
 use super::super::{AccountIdentifier, CanisterId, NamedCanister, PrincipalId};
 use super::*;
-use std::collections::HashMap;
+use std::collections::{BTreeSet, HashMap};
 
 /// Creates a toy canister.
 pub fn toy_canister(account_index: u64, canister_index: u64) -> NamedCanister {
@@ -265,3 +265,25 @@ where
         "Deleting a non-existent canister should not affect the count."
     );
 }
+
+/// Verifies that the `values()` iterator works corerctly.
+pub fn assert_iterates_over_values<D>(mut storage: D)
+where
+    D: AccountsDbTrait,
+{
+    // To store accounts in say a BTreeMap requires adding comparison operators to accounts.
+    // These are not generally needed or wanted.  What we will do instead is to compare
+    // account IDs.
+    let mut expected_values = BTreeSet::new();
+    for account_id in 0..10 {
+        let account_key = vec![account_id as u8];
+        let account = toy_account(account_id, 5);
+        expected_values.insert(account.principal.unwrap());
+        storage.db_insert_account(&account_key, account.clone());
+    }
+    let actual_values = storage
+        .values()
+        .map(|account| account.principal.unwrap())
+        .collect::<BTreeSet<_>>();
+    assert_eq!(expected_values, actual_values);
+}