[IC-165] storage stats endpoint (#94)

This change introduces a new canister query method returning storage
stats: the number of registered users and the range of user numbers
assigned to this canister.

Author: roman-kashitsyn

src/idp_service/idp_service.did

@@ -63,6 +63,11 @@ type GetDelegationResponse = variant {
   no_such_delegation
 };
 
+type InternetIdentityStats = record {
+  users_registered: nat64;
+  assigned_user_number_range: record { nat64; nat64; };
+};
+
 type InternetIdentityInit = record {
   assigned_user_number_range : record { nat64; nat64; };
 };
@@ -72,6 +77,7 @@ service : (opt InternetIdentityInit) -> {
   add : (UserNumber, DeviceData) -> ();
   remove : (UserNumber, DeviceKey) -> ();
   lookup : (UserNumber) -> (vec DeviceData) query;
+  stats : () -> (InternetIdentityStats) query;
 
   prepare_delegation : (UserNumber, FrontendHostname, SessionKey) -> (UserKey, Timestamp);
   get_delegation: (UserNumber, FrontendHostname, SessionKey, Timestamp) -> (GetDelegationResponse) query;

src/idp_service/src/main.rs

@@ -88,6 +88,12 @@ struct StreamingCallbackHttpResponse {
     token: Option<Token>,
 }
 
+#[derive(Clone, Debug, CandidType, Deserialize)]
+struct InternetIdentityStats {
+    assigned_user_number_range: (UserNumber, UserNumber),
+    users_registered: u64,
+}
+
 #[derive(Clone, Debug, CandidType, Deserialize)]
 struct InternetIdentityInit {
     assigned_user_number_range: (UserNumber, UserNumber),
@@ -295,6 +301,17 @@ fn http_request(req: HttpRequest) -> HttpResponse {
     })
 }
 
+#[query]
+fn stats() -> InternetIdentityStats {
+    STATE.with(|state| {
+        let storage = state.storage.borrow();
+        InternetIdentityStats {
+            assigned_user_number_range: storage.assigned_user_number_range(),
+            users_registered: storage.user_count() as u64,
+        }
+    })
+}
+
 // used both in init and post_upgrade
 fn init_assets() {
     ASSETS.with(|a| {

src/idp_service/src/storage.rs

@@ -154,6 +154,14 @@ impl<T: candid::CandidType + serde::de::DeserializeOwned> Storage<T> {
         stable_write(0, &buf);
     }
 
+    pub fn user_count(&self) -> usize {
+        self.num_users as usize
+    }
+
+    pub fn assigned_user_number_range(&self) -> (UserNumber, UserNumber) {
+        self.user_number_range
+    }
+
     fn value_size_limit(&self) -> usize {
         self.entry_size as usize - std::mem::size_of::<u16>()
     }