Avoid empty HTTP requests to load Enumerations (#4369)

Previously, we weren't checking if we actually had any enumerations to load which resulted in REST requests for an empty list of enumerations. That just wastes time and resources for no reason so this change avoids the request whn there's no work to do.

---
TYPE: IMPROVEMENT
DESC: Avoid empty Enumeration REST requests.

tiledb/sm/array/array.cc

@@ -599,43 +599,47 @@ std::vector<shared_ptr<const Enumeration>> Array::get_enumerations(
     enmrs_to_load.insert(enmr_name);
   }
 
-  std::vector<shared_ptr<const Enumeration>> loaded;
+  // Only attempt to load enumerations if we have at least one Enumeration
+  // to load.
+  if (enmrs_to_load.size() > 0) {
+    std::vector<shared_ptr<const Enumeration>> loaded;
 
-  if (remote_) {
-    auto rest_client = resources_.rest_client();
-    if (rest_client == nullptr) {
-      throw ArrayException(
-          "Error loading enumerations; "
-          "Remote array with no REST client.");
-    }
+    if (remote_) {
+      auto rest_client = resources_.rest_client();
+      if (rest_client == nullptr) {
+        throw ArrayException(
+            "Error loading enumerations; "
+            "Remote array with no REST client.");
+      }
 
-    std::vector<std::string> names_to_load;
-    for (auto& enmr_name : enmrs_to_load) {
-      names_to_load.push_back(enmr_name);
-    }
+      std::vector<std::string> names_to_load;
+      for (auto& enmr_name : enmrs_to_load) {
+        names_to_load.push_back(enmr_name);
+      }
 
-    loaded = rest_client->post_enumerations_from_rest(
-        array_uri_,
-        timestamp_start_,
-        timestamp_end_opened_at_,
-        this,
-        names_to_load);
-  } else {
-    // Create a vector of paths to be loaded.
-    std::vector<std::string> paths_to_load;
-    for (auto& enmr_name : enmrs_to_load) {
-      auto path = array_schema_latest_->get_enumeration_path_name(enmr_name);
-      paths_to_load.push_back(path);
-    }
+      loaded = rest_client->post_enumerations_from_rest(
+          array_uri_,
+          timestamp_start_,
+          timestamp_end_opened_at_,
+          this,
+          names_to_load);
+    } else {
+      // Create a vector of paths to be loaded.
+      std::vector<std::string> paths_to_load;
+      for (auto& enmr_name : enmrs_to_load) {
+        auto path = array_schema_latest_->get_enumeration_path_name(enmr_name);
+        paths_to_load.push_back(path);
+      }
 
-    // Load the enumerations from storage
-    loaded = array_dir_.load_enumerations_from_paths(
-        paths_to_load, get_encryption_key());
-  }
+      // Load the enumerations from storage
+      loaded = array_dir_.load_enumerations_from_paths(
+          paths_to_load, get_encryption_key());
+    }
 
-  // Store the loaded enumerations in the schema
-  for (auto& enmr : loaded) {
-    array_schema_latest_->store_enumeration(enmr);
+    // Store the loaded enumerations in the schema
+    for (auto& enmr : loaded) {
+      array_schema_latest_->store_enumeration(enmr);
+    }
   }
 
   // Return the requested list of enumerations

tiledb/sm/array/array_directory.cc

@@ -192,6 +192,13 @@ std::vector<shared_ptr<const Enumeration>>
 ArrayDirectory::load_enumerations_from_paths(
     const std::vector<std::string>& enumeration_paths,
     const EncryptionKey& encryption_key) const {
+  // This should never be called with an empty list of enumeration paths, but
+  // there's no reason to not check an early return case here given that code
+  // changes.
+  if (enumeration_paths.size() == 0) {
+    return {};
+  }
+
   std::vector<shared_ptr<const Enumeration>> ret(enumeration_paths.size());
   auto& tp = resources_.get().io_tp();
   throw_if_not_ok(parallel_for(&tp, 0, enumeration_paths.size(), [&](size_t i) {

tiledb/sm/rest/rest_client.cc

@@ -524,6 +524,13 @@ RestClient::post_enumerations_from_rest(
         "Error getting enumerations from REST; array is null.");
   }
 
+  // This should never be called with an empty list of enumeration names, but
+  // there's no reason to not check an early return case here given that code
+  // changes.
+  if (enumeration_names.size() == 0) {
+    return {};
+  }
+
   Buffer buf;
   serialization::serialize_load_enumerations_request(
       array->config(), enumeration_names, serialization_type_, buf);