Fix Galaxy paging review findings

clients/rust/src/galaxy.rs

@@ -144,6 +144,7 @@ impl GalaxyClient {
     /// the object's identifying names plus its dynamic attributes.
     pub async fn discover_hierarchy(&mut self) -> Result<Vec<GalaxyObject>, Error> {
         let mut objects = Vec::new();
+        let mut seen_page_tokens = std::collections::HashSet::new();
         let mut page_token = String::new();
         loop {
             let response = self
@@ -159,6 +160,14 @@ impl GalaxyClient {
             if page_token.is_empty() {
                 return Ok(objects);
             }
+            if !seen_page_tokens.insert(page_token.clone()) {
+                return Err(Error::InvalidArgument {
+                    name: "page_token".to_owned(),
+                    detail: format!(
+                        "galaxy discover hierarchy returned repeated page token `{page_token}`"
+                    ),
+                });
+            }
         }
     }
 
@@ -517,6 +526,37 @@ mod tests {
         );
     }
 
+    #[tokio::test]
+    async fn discover_hierarchy_rejects_repeated_page_token() {
+        let state = Arc::new(FakeState::default());
+        state
+            .discover_replies
+            .lock()
+            .unwrap()
+            .push_back(DiscoverHierarchyReply {
+                objects: Vec::new(),
+                next_page_token: "7:1".to_owned(),
+                total_object_count: 1,
+            });
+        state
+            .discover_replies
+            .lock()
+            .unwrap()
+            .push_back(DiscoverHierarchyReply {
+                objects: Vec::new(),
+                next_page_token: "7:1".to_owned(),
+                total_object_count: 1,
+            });
+        let endpoint = spawn_fake(state).await;
+        let mut client = GalaxyClient::connect(ClientOptions::new(endpoint))
+            .await
+            .unwrap();
+
+        let error = client.discover_hierarchy().await.unwrap_err();
+
+        assert!(error.to_string().contains("repeated page token"));
+    }
+
     #[tokio::test]
     async fn watch_deploy_events_yields_events_in_order() {
         let state = Arc::new(FakeState::default());